Method for depositing ceramic layers using liquid organometallic precursors - Patents.com

Abstract

Metallic or ceramic layers may be deposited on the nuclear material by chemical vapor deposition using non-halogenated liquid organometallic precursors. Chemical vapor deposition is performed by a method including the steps of introducing nuclear fuel particles into a fluidized bed reactor and heating the fluidized bed reactor to a desired operating temperature T1. A carrier gas supply is started through a vaporizer and a non-halogenated liquid organometallic precursor is injected into the vaporizer and vaporized. The first mixture of carrier gas and vaporized non-halogenated liquid organometallic precursor may be mixed with a gaseous carbon source, a gaseous nitrogen source, a gaseous oxygen source, or a mixture thereof to produce a second mixture. The second mixture flows into the fluidized bed reactor at the operating temperature T1 to deposit the desired ceramic coating on the particles. The non-halogenated liquid organometallic precursor may be a compound of Zr, Hf, Nb, Ta, W, V, Ti, or a mixture thereof.

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This nonprovisional application claims the benefit of U.S. Provisional Application No. 63 / 301,656, filed January 21, 2022. The disclosure of the prior application is incorporated by reference in its entirety.

[0002] Various exemplary embodiments disclosed herein generally relate to the deposition of ceramic materials using non-halogenated liquid organometallic compounds as metal precursors. In particular, embodiments disclosed herein relate to the chemical vapor deposition of ceramic layers on solid substrates using non-halogenated liquid organometallic compounds as metal precursors. Also, embodiments disclosed herein relate to the chemical vapor infiltration of ceramic materials into porous or particulate substrates using non-halogenated liquid organometallic compounds as metal precursors. [Background technology]

[0003] Chemical vapor deposition (CVD) of metal-containing ceramics for nuclear fuel particles has traditionally been carried out using metal halides in fluidized bed reactors. Some halogen-containing metal or metalloid compounds are liquids that are easily fed into CVD growth systems. For example, Si(CH3) (4-n) Cl n Silanes such as silanes are liquids that can be injected into the carrier gas lines of CVD growth systems. However, many metal halides are high-melting solids, i.e., solids that melt at temperatures greater than 200 °C, and cannot be easily delivered to CVD systems. Also, metal halide deposition sources can produce toxic halogen-containing gases as by-products.

[0004] Chemical vapor deposition is used to fabricate nuclear fuel particles. For example, tristructural isotropic (TRISO) nuclear fuel particles contain two layers of carbon deposited on a nuclear fuel kernel, followed by a layer of silicon carbide (SiC) and an outer layer of high density pyrolytic carbon (PyC). The SiC layer is typically deposited using methyltrichlorosilane (MTS) or another liquid chlorosilane that is introduced into the chemical vapor deposition reactor using a carrier gas. While this method is effective for depositing silicon-based ceramic layers, the use of other metals, such as chlorides of zirconium, hafnium, or tungsten, in chemical vapor deposition is inconvenient because these chlorides are usually solids with high melting points.

[0005] In view of the above, it would be desirable to develop techniques for depositing ceramic layers using precursor gases containing liquid or low melting point metals, and in particular, for depositing ceramic layers using precursor gases containing non-halogenated metals.

[0006] The present disclosure relates to novel CVD and chemical vapor impregnation (CVI) systems and methods that allow for mixing gaseous carbon, nitrogen, or oxygen precursors with liquid organometallic precursors, which can be fed into a CVD or CVI reactor and used to deposit ceramic layers. The liquid organometallic precursor can be fed into a vaporizer unit through a liquid mass flow controller, such as a valve, for example a ball valve or needle valve, at a constant liquid flow rate. As the constant liquid flow enters the vaporizer, a carrier gas mixes with the liquid and is heated in the vaporizer unit above the boiling temperature of the liquid. The liquid is evaporated and pumped into a CVD or CVI reaction chamber. The systems and methods disclosed herein can be used to grow ceramic materials directly on nuclear fuel. The systems and methods also allow for chemical metal impregnation of ceramic matrix materials into porous fuel forms or surface coating of solid fuel forms. Thus, the systems and methods disclosed herein allow for the production of structural metal or ceramic nuclear fuels.

[0007] Novel organometallic precursors, such as liquid organometallic compounds of zirconium, hafnium, niobium, tantalum, tungsten, titanium and vanadium, have been identified as useful CVD or CVI metal precursors that can be used to deposit ceramic materials using the systems and methods disclosed herein. These liquid organometallic compounds can be introduced into a CVD or CVI system or reactor without the need for a complex delivery system. Techniques have been developed to deliver liquid organometallic precursors to a CVD or CVI system using a liquid vaporizer, which saturates a carrier gas for delivery to a chemical reactor. This saturated gas mixture can be fed directly to the reactor for deposition of the metal layer or can be mixed with additional gases, such as, for example, a gaseous carbon source, a gaseous nitrogen source, and / or a gaseous oxygen source, to produce various ceramic coatings or to deposit ceramic substrate materials on porous materials by CVI.

[0008] Additionally, the systems and methods disclosed herein significantly reduce and / or eliminate toxicity and corrosion during deposition of ceramic layers on particulate substrates, including nuclear fuel kernels, by using halogen-free liquid organometallic compounds as metal precursors for CVD or CVI deposition of ceramic materials. The systems and methods disclosed herein allow for the production of metal ceramics directly on nuclear fuel, while increasing safety and preventing the release of toxic halogen gases into the environment.

[0009] Various embodiments disclosed herein relate to a method for depositing a metal or ceramic layer by chemical vapor deposition by introducing particles into a fluidized bed reactor. The fluidized bed reactor is heated to a predetermined operating temperature T1, a carrier gas supply is started through a vaporizer, and a non-halogenated liquid organometallic precursor is introduced into the vaporizer for vaporization. A first mixture of the carrier gas and the vaporized non-halogenated liquid organometallic precursor leaves the vaporizer and is mixed with a gaseous carbon source, a gaseous nitrogen source, a gaseous oxygen source, or a mixture thereof, as required, to produce a second mixture. If a gaseous carbon source, a gaseous nitrogen source, or a gaseous oxygen source is not used, the first mixture flows into the fluidized bed reactor at the operating temperature T1 to deposit a desired metal coating on the particles. Alternatively, a second mixture containing a carbon, nitrogen, or oxygen source flows into the fluidized bed reactor at the operating temperature T1 to deposit a desired ceramic coating on the particles.

[0010] In various embodiments disclosed herein, the step of introducing particles into the fluidized bed reactor includes introducing particles that are nuclear fuel kernels. The particles may be nuclear fuel kernels selected from the group consisting of oxides, carbides, oxycarbides, and nitrides of metals selected from the group consisting of uranium, plutonium, and thorium. The particles may be nuclear fuel kernels selected from the group consisting of uranium oxide, uranium carbide, uranium nitride, uranium oxycarbide, and mixtures thereof.

[0011] In various embodiments, each particle includes a nuclear fuel kernel and at least one carbon layer on the kernel. Each nuclear fuel kernel can have multiple carbon layers including, for example, a porous carbon buffer layer on the kernel and a dense layer of pyrolytic carbon on the buffer layer.

[0012] In various embodiments, each particle includes a nuclear fuel kernel and at least one ceramic layer on the kernel. Each nuclear fuel kernel can have multiple ceramic layers, such as layers of metal oxides, metal carbides, metal nitrides, metal oxycarbides, or alloys thereof.

[0013] In various embodiments of the method for depositing a metal or ceramic layer by chemical vapor deposition, a second mixture containing a carbon, nitrogen, or oxygen source can be flowed into the fluidized bed reactor at an operating temperature T1 to deposit a desired ceramic coating on the particles. Such a method can further include the steps of:

[0014] Prior to introducing the non-halogenated liquid organometallic precursor into the vaporizer, an inner carbon layer is deposited on the nuclear fuel kernels, and an outer carbon layer is deposited on the desired ceramic coating.

[0015] The non-halogenated liquid organometallic precursor may be a compound of a metal selected from the group consisting of Zr, Hf, Nb, Ta, W, V, Ti, and mixtures thereof. The metal in the non-halogenated liquid organometallic precursor may be bound to an alkoxide group, an amino group, an imino group, or a combination thereof.

[0016] In various embodiments, the non-halogenated liquid organometallic precursor has the formula M +n L n , M +n L 1 (n-m) L 2 (m) Or, (L 3 =) p M +n L (n-2p) In the formula: M is Zr, Hf, Nb, Ta, W, V, Ti, or a mixture thereof; L and L 1 are independently selected from a C1-C4 alkyl group, a C1-C4 alkoxy group, a bis(C1-C4 dialkyl)amide group, or a combination thereof; L 2 is the cyclopentadienyl anion, L 3 is O= or RN=, where R is an alkyl group; M is 1 or 2, p is 1 or 2, and The valence of the metal M is n.

[0017] The non-halogenated liquid organometallic precursors are a metal alkoxide selected from the group consisting of titanium(IV) isopropoxide, titanium(IV) isobutoxide, hafnium(IV) butoxide, niobium(V) t-butoxide, niobium(III) t-butoxide, and zirconium(IV) butoxide; or A metal amine selected from the group consisting of: (t-butylimino)tris(diethylamino)tantalum(V), (t-butylimino)tris(diethylamino)niobium(V), (t-butylimino)tris(methylethylamino)niobium(V), Tris(diethylamino)niobium(III), Tris(methylethylamino)niobium(III), Tris(diethylamino)niobium(III), Tris(dimethylamino)cyclopentadienylzirconium(IV), Tetrakis(isopropylmethylamino)zirconium(IV), Tetrakis(ethylmethylamino)zirconium(IV), Tetrakis(diethylamino)zirconium(IV), Tris(dimethylamino)cyclopentadienylhafnium(IV), Bis(methylimino)bis(dimethylamino)tungsten(VI), and Bis(t-butylimino)bis(dimethylamino)tungsten(VI).

[0018] In various embodiments, the second mixture includes a carbon, nitrogen, or oxygen source. The second mixture may include a gaseous carbon source. The second mixture may flow into a fluidized bed reactor to deposit a desired metal carbide coating on the particles. The second mixture may include a gaseous nitrogen source. The second mixture may flow into a fluidized bed reactor to deposit a desired metal nitride coating on the particles. The second mixture may include a gaseous oxygen source. The second mixture may flow into a fluidized bed reactor to deposit a desired metal oxide coating on the particles.

[0019] Various embodiments disclosed herein relate to a method for providing a porous body having a metal or ceramic substrate by chemical vapor infiltration, the method comprising the steps of: Introducing the porous body into a chemical vapor impregnation reactor; Heating the chemical vapor impregnation reactor to a predetermined operating temperature T1; Start the carrier gas supply through the vaporizer. Injecting a non-halogenated liquid organometallic precursor into a vaporizer and vaporizing the non-halogenated liquid organometallic precursor; allowing a first mixture of carrier gas and vaporized non-halogenated liquid organometallic precursor to leave the vaporizer; optionally mixing the first mixture with a gaseous carbon source, a gaseous nitrogen source, a gaseous oxygen source, or a mixture thereof to form a second mixture; Flowing the first mixture through a chemical vapor impregnation reactor at an operating temperature T1 and passing the first mixture through the porous body to deposit the desired metal matrix within the voids in the porous body; or The second mixture is flowed into a chemical vapor impregnation reactor at an operating temperature T1 and passes through the porous body to deposit the desired ceramic matrix within the voids in the porous body.

[0020] In the chemical vapor impregnation method, the non-halogenated liquid organometallic precursor may be a compound of a metal selected from the group consisting of Zr, Hf, Nb, Ta, W, V, Ti, and mixtures thereof. The metal in the non-halogenated liquid organometallic precursor may be bound to an alkoxide group, an amino group, an imino group, or a combination thereof. The non-halogenated liquid organometallic precursor may be represented by the formula M +n L n , M +n L 1 (n-m) L 2 (m) Or, (L3=) p M +n L (n-2p) wherein M is Zr, Hf, Nb, Ta, W, V, Ti, or a mixture thereof; L and L 1 are independently selected from a C1-C4 alkyl group, a C1-C4 alkoxy group, a bis(C1-C4 dialkyl)amide group, or a combination thereof; L 2 is the cyclopentadienyl anion, L 3 is O= or RN=, where R is an alkyl group; m is 1 or 2, p is 1 or 2, n is the valence of the metal M.

[0021] Various embodiments disclosed herein relate to a method for coating a metal or ceramic layer on a solid material by chemical vapor deposition, comprising the following steps: Introducing a solid material into a chemical vapor deposition reactor; Heating the chemical vapor deposition reactor to a desired operating temperature T1; Start the carrier gas supply through the vaporizer. Injecting a non-halogenated liquid organometallic precursor into a vaporizer and vaporizing the non-halogenated liquid organometallic precursor; allowing a first mixture of carrier gas and vaporized non-halogenated liquid organometallic precursor to leave the vaporizer; optionally mixing the first mixture with a gaseous carbon source, a gaseous nitrogen source, a gaseous oxygen source, or a mixture thereof to form a second mixture; Flowing the first mixture into a chemical vapor deposition reactor at an operating temperature T1 to deposit a desired metal coating on the solid material; or The second mixture is flowed into a chemical vapor deposition reactor at an operating temperature T1 to deposit a desired ceramic coating on the solid material.

[0022] In a method for coating a metal or ceramic layer on a solid material by chemical vapor deposition, the non-halogenated liquid organometallic precursor may be a compound of a metal selected from the group consisting of Zr, Hf, Nb, Ta, W, V, Ti, and mixtures thereof. The metal in the non-halogenated liquid organometallic precursor may be bonded with an alkoxide group, an amino group, an imino group, or a combination thereof. [Brief description of the drawings]

[0023] For a better understanding of various exemplary embodiments, reference is made to the following drawings. [Figure 1] 1 shows a prior art system for fluidized bed CVD using solid metal halides as metal precursors for the deposition of ceramic layers. [Figure 2a] 2 shows a particle having a ceramic layer deposited by the system of FIG. 1. [Figure 2b] 2 shows a particle having a ceramic layer deposited by the system of FIG. 1. [Diagram 3] 2 shows a nuclear fuel particle having a ceramic layer deposited by the system of FIG. 1. [Figure 4] 2 shows a variation of the system of FIG. 1 that allows for the deposition of a ceramic layer that includes a mixture of metals. [Diagram 5] 2 shows a variation of the system of FIG. 1 that allows for the deposition of a ceramic layer that includes a mixture of metals. [Figure 6] 1 shows a system for fluidized bed CVD using liquid organometallic compounds as metal precursors for the deposition of ceramic layers. [Figure 7] 1 is a flow chart illustrating a method of fluidized bed CVD using liquid organometallic compounds as metal precursors for the deposition of ceramic layers. [Figure 8] 1 illustrates a system for vaporizing liquid organometallic compounds to produce gaseous metal precursors for the deposition of ceramic layers. [Figure 9] 1 illustrates a system for vaporizing liquid organometallic compounds to produce gaseous metal precursors for the deposition of ceramic layers. [Figure 10a] FIG. 7 shows a first variant of the vaporizer in the system of FIG. 6, which allows the deposition of a ceramic layer containing a mixture of metals. [Figure 10b] FIG. 7 shows a second variant of a vaporizer system for use in the system of FIG. 6, which allows the deposition of a ceramic layer containing a mixture of metals. [Figure 11] 1 is a flow chart illustrating a method of fluidized bed CVD using a mixture of liquid organometallic compounds as metal precursors for the deposition of ceramic layers containing mixed metals. [Figure 12a] 1 shows an isobaric chemical vapor impregnation (CVI) reactor. [Figure 12b] 1 shows an isobaric chemical vapor impregnation (CVI) reactor. [Figure 13] 1 shows the products produced by chemical vapor impregnation (CVI). [Figure 14] 1 shows the product produced by chemical vapor deposition (CVD) on a bulk material. [Figure 15] 1 shows a forced-flow chemical vapor impregnation (CVI) reactor. [Figure 16] 1 shows a tubular chemical vapor deposition (CVD) reactor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Referring now to the drawings, in which like numbers indicate like components or steps, broad aspects of various exemplary embodiments are disclosed.

[0025] As used herein, a TRISO particle comprises a uranium-based ceramic nuclear fuel kernel as a core, coated in sequence with the following layers: An inner porous carbon layer, A dense pyrolytic carbon layer on the inside, Ceramic layers such as metal carbides, nitrides, oxides, and oxycarbides; Dense outer pyrolytic carbon layer.

[0026] As used herein, a TRISO-like particle comprises a ceramic nuclear fuel kernel, e.g., an oxide, carbide, nitride or oxycarbide of uranium, plutonium, thorium or mixtures thereof, as a core, coated with at least one carbon layer and at least one ceramic layer, e.g., a carbide, nitride, oxide or oxycarbide of a metal. The carbon and ceramic layers may be deposited in any order.

[0027] Oxides, carbides, nitrides, and borides of metals such as zirconium, hafnium, vanadium, niobium, tantalum, and titanium are valued for their chemical and mechanical properties. These properties include high melting points, density, and hardness. This makes thin ceramic layers of zirconium, hafnium, niobium, tantalum, or mixtures thereof, highly valuable in nuclear applications, both terrestrial and space. However, the chlorides and other halides of these metals are high melting solids, as seen in Table 1. These compounds vaporize by sublimation, often at temperatures in excess of 300 °C, as shown in Table 1.

[0028] Chemical vapor deposition (CVD) and chemical vapor impregnation (CVI) of transition metal oxides, carbides, borides, and nitrides have historically been difficult due to the utilization of halide-based metal precursors, such as the metal halide salts in Table 1. The utilization of these halide-based precursors is challenging for two reasons: the presence of halides, which pose toxicity and corrosion concerns, and the non-ideal chemical nature of the halides. TIFF2025503023000001.tif55161Table 1 Thermal properties of metal halide CVD precursor compounds

[0029] The halides in Table 1 are not ideal for CVD / CVI because they exist as solids at standard temperature and pressure (STP) and / or have low vapor pressures at STP. Because of these non-ideal chemical properties, delivery of precursors such as those in Table 1 has required complex delivery systems such as pneumatic delivery systems, metal chlorination systems, or salt sublimation furnaces. This makes CVD and CVI production of metal carbides very difficult and impossible to perform beyond laboratory-scale operations.

[0030] Metal halide precursors such as vanadium(IV) chloride and titanium(IV) chloride are liquids at STP. This makes them seem like useful CVD / CVI metal deposition materials, but they are not without problems. They contain halides and can generate halogen and / or hydrogen halide gases at deposition temperatures. For example, heating vanadium(IV) chloride can generate toxic vapors of vanadium oxide and chlorine-containing gases.

[0031] FIG. 1 shows a prior art system for depositing ceramic layers using solid metal halide MX4 as the metal source 11. A carrier gas enters a sublimation furnace 10 through a gas inlet 12. The carrier gas leaves the sublimation furnace 10 through an outlet 13 to deliver vaporized metal halide MX 4(vap) A valve or other mass flow controller 14 controls the rate at which the carrier gas passes through the exhaust 13. The valve 14 controls the MX 4(vap) The flow rate of the carrier gas can be adjusted in response to a signal from a sensor (not shown) that measures the concentration of metal in the carrier gas. If the metal concentration in the carrier gas is lower than desired, the valve 14 can be used to increase the carrier gas flow rate. If the metal concentration in the carrier gas is higher than desired, the valve 14 can be used to decrease the carrier gas flow rate.

[0032] The system of FIG. 1 includes at least one of a gaseous oxygen source 16a, a gaseous nitrogen source 16b, and a gaseous carbon source 16c. The oxygen, nitrogen, and carbon sources flow through gas inlets 15a, 15b, and 15c, respectively. Valves 17a, 17b, and 17c can be used to control the flow rates of the oxygen, nitrogen, and carbon sources, respectively. If a carbide ceramic is desired, for example, valve 17c can be opened and valves 17a and 17b can be closed. If a nitride ceramic is desired, valve 17b can be opened and valves 17a and 17c can be closed. If a carbonitride ceramic is desired, valves 17b and 17c can be opened and valve 17a can be closed. The oxygen, nitrogen, and carbon sources flow through gas passage 15, which meets exhaust 13 from sublimation furnace 10. Next, the carrier gas in the exhaust section 13 enters the flow path 18 and flows into the fluidized bed reactor 19, and the vaporized metal halide MX 4(vap) and oxygen, nitrogen, and / or carbon source gas. In the fluidized bed reactor 19, the vaporized metal halide MX 4(vap) The fluidized bed reactor 19 is heated by a heat source 19a, which may be a resistive heat source 19a configured to flow electrical energy through the reactor 19.

[0033] Figure 2a shows a core particle 1 having a ceramic coating 4 produced by the system of Figure 1. The particle 1 may be a solid metal or ceramic that does not melt at the deposition temperatures in the fluidized bed reactor 19 of Figure 1. In various embodiments, the core particle 1 is a nuclear fuel kernel. As used herein, a "fuel kernel 1" is a core particle 1 made of a radioactive ceramic material.

[0034] FIG. 2b shows a particle 1 with a ceramic coating 4 produced by the system of FIG. 1, where the coating 4 can have a variable structure. The particle 1 can be a solid metal or ceramic. The coating 4 has an inner layer 4a, a middle layer 4b, and an outer layer 4c, which can be formed by adjusting the flow rate of the deposition gas during deposition in a fluidized bed reactor 19. For example, in one embodiment, the inner layers 4a-4c are metal carbide layers M with varying metal to carbon ratios. n C (2-n) During the deposition of the inner layer 4a, the vaporized metal halide MX 4(vap) If a high concentration of carbon source gas is used for M n C (2-n) During deposition of the intermediate layer 4b, the concentration of the carbon source gas is increased to 100% by weight of the evaporated metal halide MX 4(vap) If it is lower than M n C (2-n) During deposition of the outer layer 4c, the concentration of the carbon source gas is increased to 1 / 2 of the vaporized metal halide MX 4(vap) can be further reduced to M n C (2-n) In this case, n>1 and the metal carbide layer 4c is carbon deficient.

[0035] The system of Figure 1 can be used to produce tristructural isotropic (TRISO) nuclear fuel particles as shown in Figure 3. The TRISO particle contains a core particle 1, which is a fuel kernel composed of uranium ceramic material, e.g., uranium oxide (e.g., UO2 or U3O8), uranium carbide (UC), uranium nitride (e.g., UN2 or U2N3), or uranium oxycarbide (UCO), at the core, coated with three isotropic materials deposited by chemical vapor deposition in a fluidized bed reactor 19. The fuel kernel 1 is coated with a porous buffer layer 2 of carbon, then a dense inner layer 3 of pyrolytic carbon (PyC), then a ceramic layer 4 to enhance the structural integrity of the TRISO particle, and then a dense outer layer 5 of PyC.

[0036] In various embodiments, the system of FIG. 1 can be used to produce TRISO-like nuclear fuel particles similar to those shown in FIG. 3. The TRISO-like particle includes a fuel kernel 1 made of a radioactive ceramic material such as an oxide, carbide, oxycarbide or nitride of plutonium or thorium at its core. The fuel kernel 1 is coated with an isotropic material deposited by chemical vapor deposition in a fluidized bed reactor 19. The fuel kernel 1 may be coated with at least one of the following: a porous buffer layer 2 made of carbon deposited on the fuel kernel; a dense inner layer of pyrolytic carbon (PyC) deposited on the fuel kernel; and This is the ceramic layer 4.

[0037] The kernel 1 may be coated with both a porous carbon buffer layer and an inner layer of pyrolytic carbon, which are sequentially deposited on the kernel 1. The carbon coated kernel may then be coated with a ceramic layer 4 to improve the structural integrity of the TRISO-like particles, and optionally a dense outer layer 5 of PyC. In various embodiments, the kernel 1 may be coated with a ceramic layer without depositing an intervening carbon layer on the kernel 1.

[0038] In various embodiments, the system of Figure 1 can be used to produce TRISO-like nuclear fuel particles that include a core of a fuel kernel 1 made of a radioactive ceramic material, such as an oxide, carbide, oxycarbide, or nitride of uranium, plutonium, or thorium. The kernel can then be directly coated with a ceramic layer 4 without an intervening carbon layer. Optionally, the ceramic-coated fuel kernel can be further coated with an outer layer 5 of PyC.

[0039] One problem with working with solid metal halides as metal sources in the system of FIG. 1 is that it limits the options for preparing ceramic layers containing mixed metals by CVD. For example, FIG. 4 shows the deposition of a first solid metal halide 11a (M 1X4), and the second solid metal halide 11b (M 2 1 to illustrate the use of a carrier gas, X4, discharged through exhaust 13, which carries the vaporized metal halide M from sublimation furnace 10. 1 X 4(vap) and M. 2 X 4(vap) However, in the sublimation furnace, both halides 11a and 11b must be solid at the sublimation temperature and both must have suitable vapor pressures at the desired sublimation temperature. Referring to Table 1, ZrCl4 and HfCl4 have similar melting points and sublimation temperatures and can be co-deposited by CVD from the sublimation furnace 10 as shown in Figure 4. However, in the sublimation furnace 10, the vaporized metal halide M 1 X 4(vap) and M. 2 X 4(vap) exists in equilibrium, so M 1 and M. 2 A mixed metal ceramic was deposited with a constant ratio of Zn and H, making the final product difficult to control. Furthermore, it may be desirable to simultaneously deposit a first metal halide with a high sublimation point, e.g., ZrCl4, and a second metal halide with a low sublimation point, e.g., TaCl5, which cannot be conveniently done in a single sublimation furnace 10, as shown in FIG.

[0040] These problems can be addressed by using multiple sublimation furnaces, as shown in Figure 5. The sublimation furnace 10a receives a carrier gas via a gas inlet 12a and produces a gas of formula M having a first sublimation temperature. 1 The carrier gas is a metal halide M vaporized from the sublimation furnace 10a through the exhaust 13a. 1 X 4(vap) The sublimation furnace 10b receives a carrier gas through the gas inlet 12b and carries a gas having a second sublimation temperature of formula M 2 The carrier gas is a metal halide M vaporized from the sublimation furnace 10b through the exhaust 13b. 2 X4(vap) Each of the outlets 13a and 13b has a valve (14a and 14b, respectively) that controls the gas flow rate from the sublimation furnaces 10a and 10b. The outlets 13a and 13b can be combined to form a common gas flow outlet 13 that leads generally to a fluidized bed reactor as shown in FIG.

[0041] The system of FIG. 5 allows for the simultaneous deposition of a first metal from a first metal halide sublimation source 11a having a high sublimation point, such as ZrCl4, and a second metal from a second metal halide sublimation source 11b having a low sublimation point, such as TaCl5. This can be done by heating the furnace 10a to a temperature at which the metal halide sublimation source 11a sublimes, such as 331° C. for ZrCl4, and heating the furnace 10b to a temperature at which the metal halide sublimation source 11b sublimes, such as ∼200° C. for TaCl5. The system of FIG. 5 also allows for the simultaneous deposition of a first metal from a first metal halide sublimation source 11a and a second metal from a second metal halide sublimation source 11b, where 11a and 11b have similar sublimation points, such as ZrCl4 and HfCl4. This can be done by heating the furnaces 10a and 10b to the sublimation temperature. The M in the final deposition product can be obtained by heating the furnaces 10a and 10b to the sublimation temperature. 1 Against M 2 The ratio of can be controlled by adjusting the relative flow rates of the metal-containing carrier gas from furnace 10a and the metal-containing carrier gas from furnace 10b using valves 22a and 22b. However, while the system of Figure 5 works, the use of two separate sublimation furnaces complicates the system. Moreover, it does not avoid the production of toxic halogen and hydrogen halide gases.

[0042] FIG. 6 shows the liquid non-halogenated metal compound ML as the metal source. 4(l)6 shows a system for depositing ceramic layers using a liquid non-halogenated metal compound. The use of a liquid non-halogenated metal compound avoids the need for a sublimation furnace and prevents the production of toxic halogen content by the products during CVD / CVI. In the system of FIG. 6, a carrier gas enters the vaporizer 23 through gas inlet 12, and the flow rate of the carrier gas is controlled by valve 30. The liquid non-halogenated metal compound is injected into the vaporizer 23 through inlet 32. The metal compound is vaporized, and the carrier gas is mixed with the vaporized metal compound ML 4(vap) 6 conveys the carrier gas from the vaporizer 23 through the outlet 13. A valve or other mass flow controller 14 controls the rate at which the carrier gas passes through the outlet 13. The system of FIG. 6 includes at least one of a gaseous oxygen source 16a, a gaseous nitrogen source 16b, and a gaseous carbon source 16c. The oxygen, nitrogen, and carbon sources flow through gas inlets 15a, 15b, and 15c, respectively, in a manner similar to that shown in FIG. 1. The oxygen, nitrogen, and carbon sources flow through valves 17a, 17b, and 17c, respectively, into a gas flow passage 15 that meets the outlet 13 from the vaporizer 23. The carrier gas in the outlet 13 then enters flow passage 18 and flows to a fluidized bed reactor 19, carrying the vaporized metal compound and the oxygen, nitrogen, and / or carbon source gas from the gas flow passage 15. The flow rate of the carrier gas from the outlet 13 to the reactor 19 through flow passage 18 can be controlled by a valve 14. The control valve used in the system of FIG. 6 can be a needle valve. The needle valves can restrict the flow of the fluid or gas stream in a graduated amount, and can regulate the flow rate of the carrier gas, the carrier gas carrying the vaporized metal precursor, and the carbon, nitrogen, and / or oxygen source streams to the reactor 19. The valves used in the system of FIG. 6 can be ball valves. However, ball valves are less preferred because they do not allow for graduated adjustment of the flow rate. Other types of valves that can be used include diaphragm valves, globe valves, pinch valves, bellows seals, and butterfly or throttle valves. Weir-type diaphragm valves in particular can be used to control the flow rate through the valve by graduatedly restricting the flow of the fluid or gas stream.

[0043] The fluidized bed reactor 19 is maintained at a desired temperature T1 suitable for deposition of the coating layer on the particles 21 and at a pressure suitable for deposition. The fluidized bed reactor 19 is heated by a heat source 19a. The heat source may be a resistive heat source 19a configured to flow electrical energy through the reactor 19. The deposition pressure may be, for example, greater than atmospheric pressure, or may be atmospheric pressure or a partial vacuum, for example, less than atmospheric pressure. In the fluidized bed reactor 19, the vaporized metal compound reacts with oxygen, nitrogen, and / or carbon source gas to produce a ceramic coating on the particles 21, and the carrier gas exits the reactor through a passage 20. The metal compound precursor injected into the vaporizer is not halogenated, so that the ceramic coating is deposited on the particles 21 without producing halogen or hydrogen halide gas. In various embodiments, chemical vapor deposition of a ceramic layer using an organometallic compound as a metal precursor may be carried out in a fluidized bed reactor at atmospheric pressure or a partial vacuum. Deposition in the reactor can be carried out at a pressure of 1-760 Torr, 1-20 Torr, 10-200 Torr, 20-400 Torr, 20-300 Torr, 400-760 Torr, 400-550 Torr, 300-600 Torr, or 200-500 Torr.

[0044] 6, temperature measurements within the vaporizer 23 and fluidized bed reactor 19 are made using a combination of thermocouples and pyrometers (not shown) that can measure the temperature of the vaporizer, the reactor 19 housing, and the particles 21. This allows accurate measurements from as low as 10°C to as high as 3000°C.

[0045] The non-halogenated liquid organometallic precursor may be a compound of a metal selected from the group consisting of Zr, Hf, Nb, Ta, W, V, Ti, and mixtures thereof. The metal in the non-halogenated liquid organometallic precursor may be bound to an alkoxide group, an amino group, an imino group, or a combination thereof.

[0046] The methods disclosed herein include a method for preparing a compound of formula M +n L n , M+n L 1 (n-m) L 2 (m) and (L 3 =) p M +n L (n-2p) The present invention relates to the deposition of metal nitrides, carbides, and oxides using non-halogenated liquid metal compounds represented by the formula: During the ceremony, M is Zr, Hf, Nb, Ta, W, V, Ti, or a mixture thereof; L and L 1 are independently selected from a C1-C4 alkyl group, a C1-C4 alkoxy group, a bis(C1-C4 dialkyl)amide group, or a combination thereof; L 2 is the cyclopentadienyl anion, L 3 is a divalent radical, such as O= or RN=, where R is an alkyl group; m is 1 or 2, p is 1 or 2, n is the valence of the metal M. As an example of a liquid organometallic compound for use as a metal precursor herein, the compound ML4 can be mentioned, which should be understood as not excluding compounds of trivalent, pentavalent or hexavalent metals.

[0047] Liquid metal compounds useful as metal deposition sources for CVD include the following: Metal alkoxides, for example metal C1-C4 alkoxides such as titanium(IV) isopropoxide, titanium(IV) isobutoxide, hafnium(IV) butoxide, niobium(V) t-butoxide, niobium(III) t-butoxide, and zirconium(IV) butoxide; and The following metal amines: (t-Butylimino)tris(diethylamino)tantalum(V) t BuN = Ta(NEt2)3, TBTDET); (t-Butylimino)tris(diethylamino)niobium(V)t BuN = Nb(NEt2)3, TBTDEN); (t-Butylimino)tris(methylethylamino)niobium(V) t BuN = Nb(NMeEt)3), Tris(diethylamino)niobium(III) (Nb(NEt2)3), Tris(methylethylamino)niobium(III) (Nb(NMeEt)3), Tris(diethylamino)niobium(III) (Nb(NEt2)3), Tris(dimethylamino)cyclopentadienyl zirconium(IV) (CpZr(NMe2), ZyALD), Tetrakis(isopropylmethylamino)zirconium(IV) (Zr(N i PrMe)4, TMPAZ), Tetrakis(ethylmethylamino)zirconium(IV) (Zr(NEtMe)4TEMAZ), Tetrakis(diethylamino)zirconium(IV) (Zr(N i PrMe)4,TDEAZ) Tris(dimethylamino)cyclopentadienylhafnium(IV) (CpZr(NMe2)), Bis(methylimino)bis(dimethylamino)tungsten(VI), ((MeN=)2W(NMe2)2), and Bis(t-butylimino)bis(dimethylamino)tungsten(VI) ((tBuN=)2W(NMe2)2).

[0048] Suitable carrier gases include argon, hydrogen, helium, nitrogen, carbon dioxide, and other carrier gases known in the art. The metal deposition source may be combined with a gaseous carbon source to produce a metal carbide ceramic coating. Carbon sources suitable for use in CVD include gaseous hydrocarbons, such as methane, ethane, ethylene, propylene, acetylene, and natural gas. Alternatively, the metal deposition source may be combined with a gaseous oxygen source to produce a metal oxide ceramic coating. Oxygen sources suitable for use in CVD include ozone, oxygen, nitric oxide, and nitrous oxide. The metal deposition source may be combined with a gaseous nitrogen source to produce a metal nitride ceramic coating. Nitrogen sources suitable for use in CVD include nitrogen gas and ammonia. Also, if desired, the carrier gas containing the vaporized metal deposition source can be flowed from the vaporizer 23 to the fluidized bed reactor 19 without the introduction of a carbon, oxygen, or nitrogen source to deposit a metal coating layer instead of a ceramic coating layer. Using the system of FIG. 6, pure metal layers can be deposited by closing valves 17a, 17b, and 17c during CVD / CVI and, optionally, removing sources 16a, 16b, and 16c from the system.

[0049] FIG. 7 shows a flow chart for depositing ceramic on particles 21 using the system of FIG. 6. In step 100, particles 21 are added to a fluidized bed reactor 19 for coating. In step 110, the fluidized bed reactor 19 is heated to a desired operating temperature T1. In step 120, the supply of carrier gas is started through the vaporizer 23. In step 130, the liquid metal precursor ML 4(L) is injected into the vaporizer 23. In step 140, the carrier gas is mixed with the vaporized metal precursor ML 4(v) The gas leaves the vaporizer 23 through outlet 13 carrying the cations of argon and is mixed with a gaseous carbon, nitrogen, or oxygen source. In step 150, the mixed gas flows into the fluidized bed reactor 19 at operating temperature T1 to deposit the desired ceramic coating on the particles 21 in step 160.

[0050] FIG. 8 shows a first embodiment of an apparatus for vaporizing liquid organometallic compounds for use as metal deposition precursors. Liquid precursor compound 26 is pumped into vaporizer 23 at a defined liquid flow rate. Liquid supply cylinder 25 is pressurized by an inert gas, such as argon, helium, hydrogen or nitrogen, supplied via inlet 24. To control the pressure in cylinder 25, the flow of pressurized gas is controlled by a regulator or mass flow controller, such as valve 28. The pressure in cylinder 25 provides the power to drive liquid precursor compound 26 into wick 27. Wire 27 carries liquid precursor compound 26 to vaporizer 23. The rate at which liquid precursor compound 26 enters vaporizer 23 can be controlled by a liquid mass flow controller, such as valve 29, which controls the flow through wick 27. As liquid precursor compound 26 enters vaporizer 23, carrier gas flows into vaporizer 23 through gas inlet 12. The pressure of the carrier gas in the vaporizer can be controlled by adjusting the carrier gas flow rate by valve 30. In the vaporizer 23, the liquid precursor compound 26 is subjected to temperature and pressure conditions that vaporize the compound 26, and the vaporized compound 26 is mixed with a carrier gas. The vaporized precursor compound 26 is carried by the flow of the carrier gas to the outlet 13 which leads to the fluidized bed reactor 19 as shown in FIG.

[0051] If the liquid precursor compound 26 has a low boiling point, the vaporizer 23 can be maintained at atmospheric pressure and heated to a temperature above the normal boiling point of the liquid precursor compound 26, i.e., above its boiling point at 1 atmosphere, to vaporize the compound 26. Alternatively, for low boiling point precursor compounds 26, the vaporizer 23 can be maintained at ambient temperature, e.g., 25°C ± 5°C, and the pressure in the vaporizer 23 can be reduced until the liquid precursor compound 26 boils. If the liquid precursor compound 26 has a high boiling point, the vaporizer 23 can be maintained under reduced pressure to reduce the boiling point of the compound 26 and heated until the compound 26 vaporizes. For high boiling point compounds 26, boiling under reduced pressure reduces the possibility of thermal decomposition of the compound 26 in the vaporizer 23.

[0052] In various embodiments, the liquid precursor compound 26 is a liquid organometallic compound of zirconium, hafnium, niobium, tantalum, tungsten, vanadium, or titanium. When the liquid precursor compound 26 is a liquid organometallic compound, a ceramic layer containing the metal can be deposited in the fluidized bed reactor 19. For example, when the liquid precursor compound 26 is ZyALD, a layer of a zirconium ceramic, such as ZrC, Zr3N4, or ZrO2, can be deposited in the fluidized bed reactor 19.

[0053] Various embodiments disclosed herein include a bubble cylinder for saturating the flow of carrier gas with vapor from the liquid precursor compound 26. The carrier gas flows into the liquid supply cylinder 25 through the gas inlet 12. The flow rate of the carrier gas into the liquid supply cylinder 25 can be controlled using a mass flow controller, such as a valve 30. The carrier gas from the gas inlet 12 flows into bubbles 31 that are generated below the surface of the liquid precursor compound 26 in the liquid supply cylinder 25. As the carrier gas flows out of the bubbles 31 and passes through the liquid precursor compound 26 in the liquid supply cylinder 25, the liquid precursor compound 26 is entrained in the carrier gas as vapor. The carrier gas containing the vaporized precursor compound 26 is discharged from the liquid supply cylinder 25 through the outlet 13 and leads to the fluidized bed reactor 19, as shown in FIG. 6.

[0054] If the liquid precursor compound 26 has a high vapor pressure and / or a low boiling point, the vapor from the liquid precursor compound 26 can be entrained in the carrier gas using the system of Figure 9 without heating the liquid precursor compound 26 in the liquid supply cylinder 25. However, if the liquid precursor compound 26 has a low vapor pressure and / or a high boiling point, it is necessary to heat the liquid supply cylinder 25 in order to successfully entrain the vapor from the liquid precursor compound 26 in the carrier gas using the system of Figure 9.

[0055] In various embodiments, the liquid precursor compound 26 used in the system of FIG. 9 is a liquid organometallic compound of zirconium, hafnium, niobium, tantalum, tungsten, vanadium, or titanium.

[0056] The techniques disclosed herein can be used to produce ceramic alloys by co-depositing liquid organometallic precursors with each other and / or with additional gases, enabling the production of unique alloy metal ceramics for terrestrial and space nuclear applications.

[0057] Figure 10a shows a first variant of a vaporizer for use in the system of Figure 6. In the system of Figure 10a, a carrier gas enters the vaporizer 23 through a gas inlet 12, and the flow rate of the carrier gas is controlled by a valve 30. A first liquid non-halogenated metal compound M 1 L 4(L) is injected into the vaporizer 23 from the inlet 32a. 2 L 4(L) is injected into vaporizer 23 via inlet 32b. The flow rates of the first and second liquid non-halogenated metal compounds are controlled by valves 33a and 33b, respectively. The source of each liquid non-halogenated metal compound may be a system according to FIG. 8 or FIG. 9, as appropriate.

[0058] In the vaporizer 23, the metal compound M 1 L4 and M 2 L4 is vaporized and the carrier gas carries the vaporized metal compounds from the vaporizer 23 through the outlet 13. A valve or other mass flow controller 14 controls the rate at which the carrier gas passes through the outlet 13. The carrier gas in the outlet 13 then flows to the fluidized bed reactor 19, carrying the vaporized metal compounds, as shown in FIG. 6. In various embodiments, M 1 L4 and M 2 Valves 33a and 33b can be adjusted so that L4 flows into vaporizer 23 at a constant mass ratio to deposit a ceramic alloy of constant composition in reactor 19.

[0059] Or, M 1 L4 and M 2 Valves 33a and 33b may be adjusted to allow L4 to flow into vaporizer 23 in variable proportions to deposit ceramic alloys having variable compositions in reactor 19. For example, 1 L4 and M 2 If valves 33a and 33b are initially set so that L4 flows into vaporizer 23 at a molar ratio of 2:1, then M 1 The M-rich ceramic alloy layer is deposited first. 1 L4 and M 2 As the deposition was continued while adjusting the molar ratio of L4 to flow into the vaporizer 23 at 1:2, M 2 A second ceramic alloy layer rich in M ​​is subsequently deposited. When valves 33a and 33b are replaced, M 1 A third ceramic alloy layer rich in ZnO is deposited to produce a multi-layer ceramic coating. This is carried out in a fluidized bed reactor 19 without terminating the deposition process, resulting in multiple alternating layers in which two layers of the first ceramic material are separated by layers of the second ceramic material.

[0060] Alternatively, in a single deposition procedure, valves 33a and 33b may M 1 L4 and M 2 L4 was initially set to flow into the vaporizer 23 at a molar ratio of 2:1, and M 1 A first ceramic alloy layer rich in Zn is first deposited in the reactor 19; M 1 L4 and M 2 L4 is adjusted to flow into the vaporizer 23 at a molar ratio of 1:1, and M 1 and M. 2 A second ceramic alloy layer containing approximately equal amounts of M 1 L4 and M 2 L4 is adjusted to flow into the vaporizer 23 at a molar ratio of 1:2, and M 2 A copper-rich third ceramic alloy layer is deposited over the second ceramic alloy layer. Thus, the techniques disclosed herein allow for the deposition of multiple ceramic layers, where the composition of the layers varies in graded fashion.

[0061] Figure 10b shows a second variation of the vaporizer system for use in the system of Figure 6. In the system of Figure 10b, a carrier gas enters the first vaporizer 23a through the gas inlet 12a, and the flow rate of the carrier gas is controlled by the valve 30a. 1 L 4(L) The second liquid non-halogenated metal compound M is injected into the first vaporizer 23a through the inlet 32a. The carrier gas passes through the gas inlet 12b and enters the second vaporizer 23b, and the flow rate of the carrier gas is controlled by the valve 30b. 2 L 4(L) is injected into the second vaporizer 23b via inlet 32b. The flow rates of the first and second liquid non-halogenated metal compounds are controlled by valves 33a and 33b, respectively. The source of each liquid non-halogenated metal compound may be a system according to FIG. 8 or FIG. 9, as appropriate.

[0062] In the vaporizers 23a and 23b, the metal compound M 1 L4 and M 2 L4 is vaporized. The carrier gas is the vaporized metal compound M 1 L4 from the vaporizer 23a through the exhaust 13a. A valve or other mass flow controller 14a controls the rate at which the carrier gas passes through the exhaust 13a. Similarly, the carrier gas carries the vaporized metal compound M 2 L4 is conveyed from vaporizer 23b through outlet 13b, with valve 14b controlling the rate at which the carrier gas passes through outlet 13b. Outlets 13a and 13b may be coupled to outlet 13, where the carrier gas then flows to fluidized bed reactor 19, conveying the vaporized metal compound, generally as shown in FIG. 6. In various embodiments, valves 33a and 33b control the flow of M 1 L4 and M 2L4 may be adjusted so that a constant mass ratio flows into the vaporizers 23a and 23b, respectively, to deposit a ceramic alloy of a constant composition in the reactor 19. 1 L4 and M 2 Valves 33a and 33b can be adjusted to allow L4 to flow in variable mass ratios into vaporizers 23a and 23b, respectively, to deposit ceramic alloys of variable composition within reactor 19.

[0063] 11 shows a modified flow chart for depositing a ceramic alloy on particles 21 using the system of FIG. 6, where vaporizer 23 is the vaporizer shown in FIG. 10a. Steps 100 through 120 are substantially identical to steps 100 through 120 of the flow chart of FIG. 7. In step 120, a carrier gas is flowed into vaporizer 23. In step 131, a first liquid metal precursor M 1 L 4(L) is injected into the vaporizer 23. In step 132, a second liquid metal precursor M 2 L4 is injected into the carburetor 23. If desired, M 1 L 4(L) and M. 2 The relative flow rates of L4 can be adjusted using valves 33a and 33b in FIG. 10a, as shown in step 133. In step 141, a carrier gas is introduced into the vaporized metal precursor M 1 L 4(vap) and M. 2 L 4(vap) The gas mixture exits the vaporizer 23 through outlet 13 carrying a mixture of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 109, 108, 109, 109, 109, 109, 110, 111, 112, 113, 114 1 n M 2 (1-n) C is deposited.

[0064] 12a and 12b show metal deposition from vaporized metal-organic compounds into a porous body by chemical vapor impregnation. Chemical vapor impregnation uses a system similar to that of FIG. 6, except that a different reactor 34 is used. Reactor 34 contains vaporized metal-organic deposition precursor ML 4(vap) , a carrier gas, and optionally a carbon, nitrogen, or oxygen source to reactor 34. Reactor 34 can receive vaporized metal-organic deposition precursor from vaporizer 23, substantially as shown in Figure 6, and can receive a carbon, nitrogen, or oxygen source from sources 16a, 16b, and 16c, substantially as shown in Figure 6. Fluidized bed reactor 34 is heated with heat source 42, which may be a resistive heat source.

[0065] In Figures 12a and 12b, a sieve or grid 35 supports the porous body, which serves as a substrate for chemical vapor impregnation. As shown in Figure 12a, the porous body contains particles 37 supported by a mold 36. As shown in Figure 12a, organometallic precursor gases can be mixed with carbon, nitrogen, or oxygen sources and / or carrier gases and flow around the porous body at constant pressure surrounding the sample. This system provides isobaric chemical vapor impregnation and allows for consistent, high-quality growth of metal or ceramic layers, but requires long processing times and may not uniformly infiltrate the sample. In various embodiments, the porous body may be a fibrous material, such as a felt or mat of carbon or ceramic fibers.

[0066] As shown in Figure 12, the carrier gas is 4(vap) into the reactor and upwards through a porous body of particles or fibers 37. The metal or ceramic is deposited in the spaces between the particles or fibers 37 to form a solid. The carrier gas exits the reactor through exhaust 39. The product after separation from the mold 36 is shown in Figure 13. The particles or fibers 37 are surrounded by a matrix of deposited metal or ceramic 38. The system of Figure 12 can also be used to deposit a layer of metal or ceramic 40 onto a solid material.

[0067] In FIG. 12b, a sieve or grid 35 supports a porous body containing a free-standing solid 43 of particles or fibers 44. In various embodiments, the free-standing solid 43 can include a solid 44 of sintered particles or a non-woven solid 44 of fibers. The sintered particles 44 can be ceramic particles. The non-woven solid 44 of fibers can be a mass of ceramic fibers or carbon fibers. As shown in FIG. 12b, the organometallic precursor gas flows all around the porous body at a constant pressure surrounding the sample. This system provides isobaric chemical vapor infiltration into the solid 43, allowing for consistent, high-quality growth of a metal or ceramic layer. Because the solid 43 is free-standing, no mold is required.

[0068] The system of FIG. 6 can be used during deposition of a ceramic layer on a TRISO core particle. The system of FIG. 6 can be used in depositing a ceramic layer on an uncoated nuclear fuel material such as uranium oxide or uranium oxycarbide. The system of FIG. 12b can be used for depositing a metal or ceramic layer 40 directly on a solid structural nuclear material or fuel element 41 as shown in FIG. 14. The system of FIG. 12a can be used for impregnation of a ceramic or metal material into a porous nuclear material to produce fuel particles 37 dispersed in a metal or ceramic matrix 38 following removal of the mold 36 as shown in FIG. 13.

[0069] 15 illustrates the deposition of metals from vaporized metal-organic compounds onto a porous body by chemical vapor impregnation using a forced-flow reactor. In FIG. 15, reactor 34 contains vaporized metal-organic deposition precursor ML 4(vap) , a carrier gas, and an optional carbon, nitrogen, or oxygen source into reactor 34. Reactor 34 can receive vaporized metal-organic deposition precursor from vaporizer 23, substantially as shown in Figure 6, and can receive carbon, nitrogen, or oxygen sources from sources 16a, 16b, and 16c, substantially as shown in Figure 6. Fluidized bed reactor 34 is heated with heat source 42, which may be a resistive heat source.

[0070] Whereas the isobaric reactor of Figures 12a and 12b uses a sieve or grate 35 that allows gas to flow at a constant pressure through the reactor 34, the reactor 34 of Figure 15 is a forced-flow reactor in which the sample is partially enclosed relative to the volume in which the precursor gas is injected into the CVI system. As seen in Figure 15, a porous body of particles or fibers 37, which may be supported by a mold 36, is supported on a gas-permeable support 46. If the porous body is a free-standing solid, such as a solid of sintered particles or a non-woven material, the mold 36 is not required. The support 46 may be supported by a gas-impermeable support 45. This creates a pressure gradient in which the pressure P1 below the support 46 is greater than the pressure P2 above the support 46. This creates a pressure gradient between the initial volume of the CVI system and the exhaust volume of the CVI system. The resulting pressure gradient "forces" the precursor to move through the porous body of particles or fibers. The advantage of this system is increased penetration of the sample into the void volume / pores and increased growth rate of the metal or ceramic material. However, the use of a forced flow system may reduce the quality of the metal or ceramic material deposited within the porous body compared to the isobaric system of Figure 12a. In various embodiments, P1 is equal to or greater than atmospheric pressure, e.g., 1 atm to 5 atm, 1.1 atm to 4 atm, 1.2 atm to 3 atm, or 1.3 atm to 2.5 atm, or 1.35 atm to 2 atm. In various embodiments, P2 is equal to or less than P1.

[0071] A tube furnace chemical vapor deposition reactor can be used to deposit metal or ceramic coatings onto generally solid, possibly non-porous materials. This can be used for the production of metal or ceramic coatings on nuclear materials. FIG. 15 shows a tube furnace CVD reactor 48 capable of coating all surfaces of a sample 47 that are not in contact with the tubular wall of the reactor 48. The sample 47 can be a generally homogeneous material, such as, for example, a solid ceramic, metal, or graphite mass. The sample 47 can also be a heterogeneous material, such as solid particles or fibers dispersed or distributed within a ceramic, metal, or graphite matrix. The sample 47 can be a heterogeneous nuclear fuel element, such as a nuclear fuel element having TRISO particles dispersed within a graphite matrix. This can be advantageous for materials where only certain surfaces are desired to have a metal or ceramic coating layer. The reactor 48 is filled with vaporized metal-organic deposition precursor ML. 4(vap) 3 includes a gas inlet 38 that carries the vaporized metalorganic deposition precursor from vaporizer 23, and the carbon, nitrogen, or oxygen source from sources 16a, 16b, and 16c, substantially as shown in FIG. 6. Reactor 34 is heated by heat source 42, which may be a resistive heat source. Within reactor 48, exposed portions of the surface of sample 47 may be coated with a metal or ceramic layer, and carrier gas may be withdrawn through exhaust 39. If it is desired to coat all portions of the surface of sample 47, reactor 48 may be rotated about its axis by methods known in the art such that sample 47 rotates within reactor 48, thereby exposing all portions of the surface to the metal or ceramic precursor gas.

[0072] Deposition of metal-ceramic materials with metal-organic precursors can increase power density, increase burnup, and improve high temperature applications compared to conventional nuclear fuels. Metal-ceramic materials produced with metal-organic precursors have applications in materials testing reactors, isotope production reactors, and reactors developed for both power generation and propulsion in space. Chemical vapor infiltration of refractory ceramics prepared from metal-organic liquid precursors into ceramic nonwoven materials produces composites with high strength and heat resistance. [Example 1] Preparation of TRISO particles

[0073] As shown in Figure 3, TRISO particles were prepared using the system of Figure 6. TRISO particles were prepared using uranium dioxide cores as particles 21, which were placed in a fluidized bed reactor 19 that was resistively heated to the deposition temperature. Liquid ZyALD was used as the zirconium source. Argon was used as the carrier gas. Ethylene and propylene were used as the carbon sources.

[0074] The fluidized bed reactor was heated to an initial deposition temperature of 1400° C., and argon was used as a carrier gas, mixed with ethylene, and the resulting gas mixture was allowed to flow through passage 18 into the fluidized bed reactor 19. A porous carbon layer (layer 2 in FIG. 3) was deposited on the uranium dioxide core (core 1 in FIG. 3).

[0075] The temperature of the fluidized bed reactor was reduced from 1200° C. to a deposition temperature of 1350° C., argon was used as a carrier gas and mixed with a mixture of ethylene and propylene as a carbon source, and the resulting gas mixture was allowed to flow through passage 18 into the fluidized bed reactor 19. An inner pyrolytic carbon layer (layer 3 in FIG. 3 ) was deposited on the porous carbon layer 2.

[0076] The temperature of the fluidized bed reactor was increased to a deposition temperature of 1400°C to 1500°C. A flow of argon was used as a carrier gas and flowed into the vaporizer 23 of Figure 6. Liquid ZyALD was injected into the vaporizer 23 as a zirconium source, and a mixture of argon and vaporized ZyALD exits the vaporizer through outlet 13. The ZyALD / argon mixture is combined with ethylene as a carbon source. The resulting gas mixture containing ZyALD and ethylene flowed into the fluidized bed reactor 19 through passage 18. A ceramic zirconium carbide layer (layer 4 of Figure 3) was deposited on the inner pyrolytic carbon layer 3.

[0077] Following deposition of the ceramic layer, the temperature of the fluidized bed reactor was reduced to a deposition temperature of 1200° C. to 1350° C., and the ZyALD supply to reactor 19 was stopped. A carrier gas of argon was mixed with a mixture of ethylene and propylene as a carbon source, and the resulting gas mixture was allowed to flow through passage 18 into the fluidized bed reactor 19. An outer pyrolytic carbon layer (layer 5 in FIG. 3) was deposited on the ceramic zirconium carbide layer 4, completing the formation of the desired TRISO particles. [Example 2] Preparation of TRISO particles with ceramic multilayers

[0078] As shown in Figure 3, TRISO particles were prepared using the system of Figure 6. TRISO particles were prepared using uranium dioxide cores as particles 21, which were placed in a fluidized bed reactor 19 that was resistively heated to the deposition temperature. Liquid ZyALD was used as the zirconium source. Argon was used as the carrier gas. Ethylene and propylene were used as the carbon sources.

[0079] The fluidized bed reactor was heated to an initial deposition temperature of 1400° C., and argon was used as a carrier gas, mixed with ethylene, and the resulting gas mixture was allowed to flow through passage 18 into the fluidized bed reactor 19. A porous carbon layer (layer 2 in FIG. 3) was deposited on the uranium dioxide core (core 1 in FIG. 3).

[0080] The temperature of the fluidized bed reactor was reduced from 1200° C. to a deposition temperature of 1350° C., argon was used as a carrier gas and mixed with a mixture of ethylene and propylene as a carbon source, and the resulting gas mixture was allowed to flow through passage 18 into the fluidized bed reactor 19. An inner pyrolytic carbon layer (layer 3 in FIG. 3 ) was deposited on the porous carbon layer 2.

[0081] The temperature of the fluidized bed reactor was increased from 1400° C. to a deposition temperature of 1500° C. A flow of argon was used as a carrier gas and flowed into the vaporizer 23 of FIG. 6. Liquid ZyALD was injected into the vaporizer 23 as a zirconium source, and a mixture of argon and vaporized ZyALD was discharged from the vaporizer through the outlet 13. The ZyALD / argon mixture was combined with ethylene as a carbon source. The resulting gas mixture containing ZyALD and ethylene was flowed into the fluidized bed reactor 19 through the passage 18. A ceramic zirconium carbide layer (layer 4 of FIG. 3) was deposited on the inner pyrolytic carbon layer 3. The molar ratio of ZyALD to ethylene was adjusted using valves 17c and 14 of FIG. 6. The initial molar ratio of ZyALD to ethylene was 1:2. This was increased to a molar ratio of 1:1 and then to 2:1. The resulting ceramic zirconium carbide layer 4 was trilayer having a carbon-rich inner layer and a zirconium-rich outer layer.

[0082] Following deposition of the ceramic tri-layer, the temperature of the fluidized bed reactor was reduced to the deposition temperature of 1200° C. to 1350° C., and the flow of ZyALD to reactor 19 was stopped. A carrier gas of argon was mixed with a mixture of ethylene and propylene as a carbon source, and the resulting gas mixture was allowed to flow through passage 18 into the fluidized bed reactor 19. An outer pyrolytic carbon layer (layer 5 in FIG. 3) was deposited on the ceramic zirconium carbide layer 4, completing the formation of the desired TRISO particles. [Example 3] Preparation of TRISO particles with ceramic multilayers

[0083] As shown in Figure 3, TRISO particles were prepared using the system of Figure 6 modified to include a vaporizer according to Figure 10a. TRISO particles were prepared using uranium dioxide cores as particles 21, which were placed in a fluidized bed reactor 19 that was resistively heated to the deposition temperature. Liquid ZyALD was used as the zirconium source. Bis(methylimino)-bis(dimethylamino)tungsten(VI) was used as the tungsten source. Argon was used as the carrier gas. Ethylene and propylene were used as the carbon sources.

[0084] The fluidized bed reactor was heated to an initial deposition temperature of 1400° C., and argon was used as a carrier gas, mixed with ethylene, and the resulting gas mixture was allowed to flow through passage 18 into the fluidized bed reactor 19. A porous carbon layer (layer 2 in FIG. 3) was deposited on the uranium dioxide core (core 1 in FIG. 3).

[0085] The temperature of the fluidized bed reactor was reduced from 1200° C. to a deposition temperature of 1350° C., argon was used as a carrier gas and mixed with a mixture of ethylene and propylene as a carbon source, and the resulting gas mixture was allowed to flow through passage 18 into the fluidized bed reactor 19. An inner pyrolytic carbon layer (layer 3 in FIG. 3 ) was deposited on the porous carbon layer 2.

[0086] The temperature of the fluidized bed reactor was increased to a deposition temperature of 1400°C to 1500°C. A flow of argon was used as a carrier gas and flowed into the vaporizer 23 of FIG. 6. Liquid ZyALD was injected into the vaporizer 23 through inlet 32a as the zirconium source, and liquid bis(methylimino)bis(dimethylamino)tungsten(VI) was injected into the vaporizer 23 through inlet 32b as the tungsten source. A mixture of argon, vaporized ZyALD, and vaporized tungsten source exits the vaporizer through outlet 13. The ZyALD / tungsten source / argon mixture was combined with ethylene as the carbon source. The resulting gas mixture flowed into the fluidized bed reactor 19 through passage 18. A ceramic zirconium tungsten carbide layer (layer 4 of FIG. 3) was deposited on the inner pyrolytic carbon layer 3.

[0087] At the start of deposition of layer 4, valves 33a and 33b were used to adjust the relative flow rates of ZyALD and bis(methylimino)bis(dimethylamino)tungsten(VI) so that they entered the vaporizer in a molar ratio of 2:1 to deposit a zirconium-rich zirconium tungsten carbide layer on pyrolytic carbon layer 3. Valves 33a and 33b were then used to adjust the relative flow rates of ZyALD and bis(methylimino)bis(dimethylamino)tungsten(VI) so that they entered the vaporizer in a molar ratio of 1:2 to deposit a tungsten-rich zirconium tungsten carbide layer to form a ceramic bilayer.

[0088] Following deposition of the ceramic layer, the temperature of the fluidized bed reactor was reduced to the deposition temperature of 1200° C. to 1350° C. and the ZyALD supply to the reactor 19 was stopped. The carrier gas argon was mixed with a mixture of ethylene and propylene as a carbon source and the resulting gas mixture was allowed to flow through passage 18 into the fluidized bed reactor 19. An outer pyrolytic carbon layer (layer 5 in FIG. 3) was deposited on the ceramic zirconium carbide layer 4 to complete the formation of the desired TRISO particles. Example 4: Preparation of a composite material containing carbon fibers in a zirconium carbide matrix

[0089] Composites containing carbon fibers in a zirconium carbide matrix can be prepared using the system of FIG. 12a including a vaporizer according to FIG. 6. Carbon fiber felt containing fibers 37 can be placed on a sieve 35 in a chemical vapor impregnation reactor 34 that is resistively heated to the deposition temperature. Liquid ZyALD can be used as the zirconium source. Argon may be used as the carrier gas. Ethylene and propylene can be used as the carbon source.

[0090] The temperature of the chemical vapor impregnation reactor 34 is increased to a deposition temperature of 1400°C-1500°C. A flow of argon is used as a carrier gas and flows into the vaporizer 23 of FIG. 6. Liquid ZyALD is injected into the vaporizer 23 as a zirconium source through inlet 32. A mixture of argon and vaporized ZyALD exits the vaporizer through outlet 13. The ZyALD / argon mixture is combined with ethylene as a carbon source. The resulting gas mixture flows into the chemical vapor impregnation reactor 34 through gas inlet 38. Ceramic zirconium carbide is deposited in the spaces between the fibers 37 of the carbon fiber felt, forming a matrix 38 of FIG. 13 around the fibers 37. [Example 5] Deposition of zirconium carbide on a graphite substrate

[0091] The coated graphite mass can be prepared using the system of FIG. 16 including a vaporizer according to FIG. 6. A graphite ingot mass 47 having a size of 100 mm×50 mm×25 mm and a purity of 99.9% can be placed in a chemical vapor deposition reactor 48 that is resistively heated to the deposition temperature. The reactor 48 can be rotated about its axis to rotate the sample 47 within the reactor 48. Liquid ZyALD can be used as the zirconium source. Argon can be used as the carrier gas. Ethylene and propylene can be used as the carbon source.

[0092] The temperature of the chemical vapor deposition reactor 48 is increased to a deposition temperature of 1400°C-1500°C. A flow of argon is used as a carrier gas and flows into the vaporizer 23 of FIG. 6. Liquid ZyALD is injected into the vaporizer 23 as a zirconium source through inlet 32. A mixture of argon and vaporized ZyALD exits the vaporizer through outlet 13. The ZyALD / argon mixture is combined with ethylene as a carbon source. The resulting gas mixture flows into the chemical vapor deposition reactor 48 through gas inlet 38. A ceramic zirconium carbide layer is deposited on the outer surface of the graphite ingot chunk 47 to form a zirconium carbide layer on the ingot chunk 47. [Example 6] Deposition of zirconium carbide on nuclear fuel elements

[0093] The coated nuclear fuel element can be prepared using the system of FIG. 16 including a vaporizer according to FIG. 6. The nuclear fuel element 47 containing TRISO particles in a graphite matrix can be placed in a chemical vapor deposition reactor 48 that is resistively heated to a deposition temperature. The reactor 48 can be rotated about its axis to rotate the nuclear fuel element 47 within the reactor 48. Liquid ZyALD can be used as the zirconium source. Argon can be used as the carrier gas. Ethylene and propylene can be used as the carbon source.

[0094] The temperature of the chemical vapor deposition reactor 48 is increased to a deposition temperature of 1400°C-1500°C. A flow of argon is used as a carrier gas and flows into the vaporizer 23 of FIG. 6. Liquid ZyALD is injected into the vaporizer 23 as a zirconium source through the inlet 32. A mixture of argon and vaporized ZyALD leaves the vaporizer through the outlet 13. The ZyALD / argon mixture is combined with ethylene as a carbon source. The resulting gas mixture flows into the chemical vapor deposition reactor 48 through the gas inlet 38. A ceramic zirconium carbide layer is deposited on the outer surface of the nuclear fuel element 47 to form a zirconium carbide layer on the nuclear fuel element 47. The resulting nuclear fuel element has a structure generally corresponding to FIG. 14, with a zirconium carbide layer 40 deposited on a solid bulk material 41. In this example, the solid bulk material 41 includes a graphite matrix and TRISO particles dispersed within the solid graphite matrix.

[0095] Although various exemplary embodiments have been described in detail with particular reference to certain illustrative aspects thereof, it should be understood that the invention is capable of other embodiments and its details can be modified in various obvious respects. As will be readily apparent to those skilled in the art, variations and modifications can be effected while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not limit the invention, which is defined solely by the claims.

Claims

1. 1. A method for depositing a metal or ceramic layer by chemical vapor deposition, comprising the steps of: a) introducing particles into a fluidized bed reactor; b) The fluidized bed reactor is heated to a predetermined operating temperature T 1 and heating to c) starting the supply of carrier gas through the vaporizer; d) injecting a non-halogenated liquid organometallic precursor into a vaporizer and vaporizing the non-halogenated liquid organometallic precursor; e) allowing the first mixture of carrier gas and vaporized non-halogenated liquid organometallic precursor to exit the vaporizer; f) optionally mixing the first mixture with a gaseous carbon source, a gaseous nitrogen source, a gaseous oxygen source, or a mixture thereof to form a second mixture; g) subjecting the first mixture to an operating temperature T 1 or flowing said second mixture into a fluidized bed reactor at an operating temperature T 1 into a fluidized bed reactor to deposit a ceramic coating on said particles. The method includes:

2. The method of claim 1 , wherein the particles are nuclear fuel kernels.

3. 3. The method of claim 2, wherein the particles are nuclear fuel kernels selected from the group consisting of oxides, carbides, oxycarbides, and nitrides of metals selected from the group consisting of uranium, plutonium, and thorium, and mixtures thereof.

4. 3. The method of claim 2, wherein the particles are nuclear fuel kernels selected from the group consisting of uranium oxide, uranium carbide, uranium nitride, uranium oxycarbide, and mixtures thereof.

5. 3. The method of claim 2, The nuclear fuel kernels are Cores of ceramic uranium material, ceramic plutonium material, ceramic thorium material or mixtures thereof; and At least one coating of porous or non-porous pyrolytic carbon The method includes:

6. 10. The method of claim 1, wherein each particle comprises a nuclear fuel kernel, a porous carbon buffer layer on the kernel, and a dense layer of pyrolytic carbon on the buffer layer.

7. 3. The method of claim 2, step (g) flowing the second mixture into a fluidized bed reactor; The method further comprises: depositing an inner carbon layer on the nuclear fuel kernels prior to step (d), and depositing an outer carbon layer on said ceramic coating. The method of claim 2 , comprising:

8. 10. The method of claim 1, wherein the non-halogenated liquid organometallic precursor is a compound of a metal selected from the group consisting of Zr, Hf, Nb, Ta, W, V, Ti, and mixtures thereof.

9. The non-halogenated liquid organometallic precursor has the formula M ＋ｎ L ｎ , M. ＋ｎ L １ （ｎ－ｍ） L ２ （ｍ） Or, (L ３ =) ｐ M ＋ｎ L （ｎ－２ｐ） where: M is Zr, Hf, Nb, Ta, W, V, Ti, or a mixture thereof; L and L １ are independently selected from a C1-C4 alkyl group, a C1-C4 alkoxy group, a bis(C1-C4 dialkyl)amido group, or combinations thereof; L ２ is the cyclopentadienyl anion, L ３ is O= or RN=, where R is an alkyl group; m is 1 or 2, p is 1 or 2; n is the valence of the metal M; The method of claim 1.

10. said non-halogenated liquid organometallic precursor comprising a metal alkoxide selected from the group consisting of titanium(IV) isopropoxide, titanium(IV) isobutoxide, hafnium(IV) butoxide, niobium(V) t-butoxide, niobium(III) t-butoxide, and zirconium(IV) butoxide; or A metal amine selected from the group consisting of: (t-butylimino)tris(diethylamino)tantalum(V), (t-butylimino)tris(diethylamino)niobium(V), (t-butylimino)tris(methylethylamino)niobium(V), Tris(diethylamino)niobium(III), Tris(methylethylamino)niobium(III), Tris(diethylamino)niobium(III), Tris(dimethylamino)cyclopentadienyl zirconium(IV), tetrakis(isopropylmethylamino)zirconium(IV), tetrakis(ethylmethylamino)zirconium(IV), tetrakis(diethylamino)zirconium(IV), Tris(dimethylamino)cyclopentadienylhafnium(IV), Bis(methylimino)bis(dimethylamino)tungsten(VI), and Bis(t-butylimino)bis(dimethylamino)tungsten(VI) The method of claim 1, wherein

11. 10. The method of claim 1 , step (f) comprises mixing the first mixture with a gaseous carbon source to produce the second mixture; Step (g) comprises heating the second mixture to an operating temperature T 1 into a fluidized bed reactor at 100° C. to deposit a metal carbide coating on the particles. The method of claim 1.

12. 12. The method of claim 11, wherein the gaseous carbon source is methane, ethane, ethylene, propylene, acetylene, natural gas, or a mixture thereof.

13. 10. The method of claim 1 , Step (f) comprises mixing the first mixture with a gaseous nitrogen source to produce a second mixture; Step (g) comprises heating the second mixture to an operating temperature T 1 into a fluidized bed reactor at 100° C. to deposit a metal nitride coating on the particles. The method of claim 1.

14. 14. The method of claim 13, wherein the gaseous nitrogen source is nitrogen gas, ammonia, or a mixture thereof.

15. 10. The method of claim 1 , Step (f) comprises mixing the first mixture with a gaseous oxygen source to produce a second mixture; Step (g) comprises heating the second mixture to an operating temperature T 1 into a fluidized bed reactor at 100° C. to deposit a metal oxide coating on the particles. The method of claim 1.

16. 16. The method of claim 15, wherein the gaseous oxygen source is ozone, oxygen, nitric oxide, nitrous oxide, or a mixture thereof.

17. 1. A method for providing a porous body having a metal or ceramic matrix by chemical vapor impregnation, comprising the steps of: h) introducing the porous body into a chemical vapor impregnation reactor; i) heating the chemical vapor impregnation reactor to a predetermined operating temperature T 1 and heating to j) starting the supply of carrier gas through the vaporizer; k) injecting a non-halogenated liquid organometallic precursor into a vaporizer and vaporizing the non-halogenated liquid organometallic precursor; l) allowing the first mixture of carrier gas and vaporized non-halogenated liquid organometallic precursor to exit the vaporizer; m) optionally mixing the first mixture with a gaseous carbon source, a gaseous nitrogen source, a gaseous oxygen source, or a mixture thereof to form a second mixture; n) flowing the first mixture into a fluidized bed reactor and heating to an operating temperature T 1 or flowing the second mixture into a fluidized bed reactor and heating the second mixture to an operating temperature T 1 passing the porous body through the porous body to deposit a ceramic matrix within the voids in the porous body. The method includes:

18. 18. The method of claim 17, wherein the non-halogenated liquid organometallic precursor is a compound of a metal selected from the group consisting of Zr, Hf, Nb, Ta, W, V, Ti, and mixtures thereof.

19. 1. A method for coating a metallic or ceramic layer on a solid material by chemical vapor deposition, comprising the steps of: o) subjecting the solid material to a chemical vapor deposition reactor; p) heating the chemical vapor deposition reactor to a predetermined operating temperature T 1 and heating to q) starting the supply of carrier gas through the vaporizer; r) injecting a non-halogenated liquid organometallic precursor into a vaporizer and vaporizing the non-halogenated liquid organometallic precursor; s) allowing the first mixture of carrier gas and vaporized non-halogenated liquid organometallic precursor to exit the vaporizer; t) optionally mixing the first mixture with a gaseous carbon source, a gaseous nitrogen source, a gaseous oxygen source, or a mixture thereof to form a second mixture; u) heating the first mixture to an operating temperature T 1 or flowing the second mixture into a chemical vapor deposition reactor at an operating temperature T 1 into a chemical vapor deposition reactor to deposit a ceramic coating on said solid material. The method includes:

20. 20. The method of claim 19, wherein the non-halogenated liquid organometallic precursor is a compound of a metal selected from the group consisting of Zr, Hf, Nb, Ta, W, V, Ti, and mixtures thereof.

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