Manufacturing method for high melting point metal thin films

Abstract

To provide a method for manufacturing a high-melting-point metal thin film that can manufacture a smooth metal thin film with a uniform thickness at low cost and, compared with a conventional manufacturing method, does not require melting and rolling of a target metal.SOLUTION: A method for manufacturing a high-melting-point metal thin film, comprising: melting a metal halide, a molten salt and a rare earth metal with the metal halide of a metal to be a thin film, the molten salt having specific gravity smaller than the metal, and the rare earth metal having the specific gravity larger than the metal; reducing the metal halide with the rare earth metal; and thereafter holding for a predetermined time at a temperature lower than a cooling or reducing temperature to segregate the metal to be the thin film on a surface of the rare earth metal, thereby a thin film of the metal is formed on a boundary surface between the molten salt and the rare earth metal.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a high melting point metal thin film.

Background Art

[0002] Titanium and its alloys are excellent corrosion prevention materials. In particular, industrial pure titanium has a very low corrosion rate even in seawater. It also has corrosion resistance to various chemicals. Therefore, it has been used in chemical plants, power plants, ocean ships, buildings near the ocean, etc.

[0003] In recent years, titanium has also been used for reinforcing the corrosion resistance of bridge coatings and as a separator for in-vehicle fuel cells. In these applications, thin film or plate-like titanium is required. These titanium thin films and titanium plates are usually produced by the Kroll method in which titanium ore is reacted with chlorine together with calcined coke and then the obtained titanium chloride is reduced with magnesium to produce metallic titanium, and then rolling the titanium ingot produced by subsequent vacuum arc melting. However, the extraction of pure titanium from titanium ore using the above conventional Kroll method is very costly, and there is also a problem that the melting and rolling of titanium are also costly.

[0004] To address such problems, a method has been proposed in which titanium is smoothly electrodeposited on a molybdenum or silicon substrate by performing pulse electrolysis of a titanium raw material in a molten salt and then mechanically peeling it to obtain a titanium foil (Patent Document 1), and a method of producing a titanium sheet by rolling powder has also been proposed (Non-Patent Document 1).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Non-Patent Documents

[0006] [Non-Patent Document 1] WHPETER,T.MUTH,W.CHEN,Y.YAMAMOTO,BRIAN JOLLY,NASTONE,GMDCANTIN,J.BARNES,M.PALIWAL,R.SMITH,J.CAPONE,A.LIBY,J.WILLIAMS, and C.BLUE "Titanium Sheet Fabricated from Powder for Industrial Applications" JOM, Vol.64, No.5, 2012 p.566-571 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the proposal in Patent Document 1 utilizes the property that titanium is electrodeposited smoothly only on specific metal substrates. To grow titanium under conditions that prevent nucleation of titanium on the titanium during electrodeposition, complex control of applied current and voltage is required, which presents the problem that it is extremely difficult to grow a uniform, smooth, and thick titanium film. Furthermore, even in the method of manufacturing titanium sheets by rolling, as proposed in Non-Patent Document 1, it is difficult to obtain titanium sheets with uniform thickness and smoothness.

[0008] This invention has been made in view of the above circumstances, and aims to provide a method for manufacturing high-melting-point metal thin films that can be produced inexpensively and have a uniform thickness and smooth surface, and that does not require the melting and rolling processes of the target metal, compared to conventional manufacturing methods. [Means for solving the problem]

[0009] The present invention provides a method for manufacturing high-melting-point metal thin films, which was developed to solve the above-mentioned technical problems and is characterized by the following:

[0010] Firstly, the present invention provides a method for producing a high-melting-point metal thin film, comprising: a melting step of melting the metal halide, the molten salt, and the rare earth metal, using a metal halide to be used as the thin film, a molten salt having a lower specific gravity than the metal, and a rare earth metal having a higher specific gravity than the metal; a reduction step of reducing the metal halide with the rare earth metal; and a segregation step of cooling or holding at a temperature lower than the reduction temperature for a certain period of time to segregate the metal to be used as the thin film onto the surface of the rare earth metal, wherein a thin film of the metal is formed at the interface between the molten salt and the rare earth metal. Secondly, in the method for producing a high-melting-point metal thin film according to the first invention described above, it is preferable that the molten salt is a halide that is stable with respect to the rare earth metal. Thirdly, in the method for producing a high-melting-point metal thin film according to the first or second invention, it is preferable that the reduction step includes an alloying step in which the metal halide is alloyed with the metal or the rare earth metal. Fourth, in the method for producing a high-melting-point metal thin film according to the first to third inventions, it is preferable that the high-melting-point metal thin film is a titanium thin film, the metal halide is titanium halide, and the melting points of the molten salt and the rare earth metal are 1000°C or lower. Fifth, in the method for producing high-melting-point metal thin films according to the first to fourth inventions described above, it is preferable that the metal halide is any of titanium fluoride, lithium titanate, titanium chloride, titanium bromide, or titanium iodide; the molten salt is any of lithium fluoride, calcium fluoride, or a mixed salt of these fluorides; lithium chloride, sodium chloride, potassium chloride, calcium chloride, or a mixed salt of these chlorides; calcium bromide, lithium bromide, sodium bromide, potassium bromide, or a mixed salt of these bromides; calcium iodide, lithium iodide, or a mixed salt of these iodides; and the rare earth metal is any of cerium, lanthanum, praseodymium, europium, or ytterbium. [Effects of the Invention]

[0011] According to the method for manufacturing a high melting point metal thin film of the present invention, it is possible to manufacture a metal thin film with uniform thickness and smoothness at low cost. Further, compared with the conventional manufacturing method, since the steps of melting and rolling the target metal are not required, the costs associated with these can be significantly reduced.

Brief Description of the Drawings

[0012] [Figure 1] (A) is a phase diagram of Ce and Ti, and (B) is a phase diagram of Yb and Ti. [Figure 2] It is an Ellingham diagram showing the reactivity of fluorides with respect to Ce. [Figure 3] It is an Ellingham diagram showing the reactivity of chlorides with respect to Ce. [Figure 4] It is an Ellingham diagram showing the reactivity of bromides with respect to Ce. [Figure 5] It is an Ellingham diagram showing the reactivity of iodides with respect to Ce. [Figure 6] It is an explanatory diagram showing the formation mechanism of a titanium thin film using Ce. [Figure 7] It is an explanatory diagram showing the formation mechanism of a titanium thin film using Yb. [Figure 8] It is a schematic diagram showing the film-forming apparatus used in the examples. [Figure 9] It is an electron micrograph of the sample cross-section obtained in the examples. [Figure 10] It is an elemental map showing the Ti, Ce, F, and O distributions of the sample cross-section obtained in the examples. [Figure 11] It is an XRD pattern of the Ti thin film obtained in the examples.

Embodiments for Carrying Out the Invention

[0013] The method for manufacturing a high melting point metal thin film of the present invention uses a metal halide of the metal to be formed into a thin film, a molten salt, and a rare earth metal, and includes a melting step of melting the metal halide, the molten salt, and the rare earth metal, a reduction step of reducing the metal halide with the rare earth metal, and then a segregation step of cooling or holding at a temperature lower than the reduction temperature for a certain period of time to segregate the metal to be formed into a thin film on the surface of the rare earth metal, and is characterized by forming a thin film of the metal at the interface between the molten salt and the rare earth metal. Further, it is a condition that the specific gravity of the molten salt is smaller than that of the solid metal of the thin film metal, and the specific gravity of the rare earth metal is larger than that of the solid metal.

[0014] The specific mechanism for generating a metal thin film by the method for manufacturing a high melting point metal thin film of the present invention is as follows. First, when the metal halide, the molten salt, and the rare earth metal are melted and brought into contact with each other, the metal ions in the metal halide are reduced to the rare earth metal itself, and an alloy of the rare earth metal and the metal is formed. Then, when slowly cooled or held at a temperature lower than the reduction temperature for a certain period of time, the metal segregates from the alloy. At this time, since the specific gravity of the segregating metal is intermediate between the molten salt and the rare earth metal, a thin film is formed at the interface between the molten salt and the rare earth metal.

[0015] The metal of the metal thin film that can be manufactured in the present invention is a high melting point metal. Specifically, for example, titanium, vanadium, and titanium alloys can be exemplified. Among these, a thin film of titanium can be preferably manufactured. In the present invention, the high melting point metal refers to a metal having a melting point higher than 1000°C.

[0016] Examples of the metal halide used for manufacturing the thin film of the metal include fluorides, chlorides, bromides, and iodides of the thin film metal, and among these, metal fluorides can be preferably used.

[0017] Furthermore, the molten salt mixed with the metal halide is preferably a halide with a specific gravity lower than the solid metal of the metal thin film, and is stable with respect to rare earth metals. Specifically, examples include fluorides, chlorides, bromides, and iodides, and in particular, alkali metal halides and alkaline earth metal halides with a melting point of 1000°C or lower can be suitably used.

[0018] Furthermore, the rare earth metals to be melted and mixed with the above-mentioned metal halides and molten salts are those that have a higher specific gravity than the solid metal of the metal thin film and have the property of melting and completely separating from the metal to form compounds. Examples include lanthanides, specifically lanthanum, cerium, praseodymium, europium, and ytterbium. In addition, these rare earth metals are preferably those with a melting point of 1000°C or lower from the viewpoint of ease of handling.

[0019] In the method for producing high-melting-point metal thin films of the present invention, a metal halide, a molten salt, and a rare earth metal are mixed under the above conditions, heated, melted, reduced, and then cooled or held at a temperature lower than the reduction temperature for a certain period of time, thereby segregating a thin metal film at the interface between the molten salt and the rare earth metal to form a thin metal film. Furthermore, since the surface of the molten, liquid rare earth metal becomes very smooth, the thin metal film formed at the interface between the liquid metal and the molten salt is also formed to be smooth and of uniform thickness.

[0020] The following describes in detail an embodiment of the present invention for manufacturing high-melting-point metal thin films, specifically a method for manufacturing titanium thin films. In the method for manufacturing titanium thin films, titanium fluoride, lithium titanate, titanium chloride, titanium bromide, titanium iodide, and the like can be used as the metal halide, i.e., titanium halide.

[0021] Furthermore, the halides of the molten salt have the property of melting and separating from the rare earth metal during film formation. Therefore, they need to be stable with respect to the rare earth metal so as not to form compounds with it. On the other hand, as rare earth metals used in the production of titanium thin films, cerium (Ce) (melting point: 798°C) and ytterbium (Yb) (melting point: 819°C) can be suitably used, as they have a higher specific gravity than titanium and a melting point of 1000°C or less.

[0022] Figure 1(A) shows the phase diagram of Ce and Ti, and Figure 1(B) shows the phase diagram of Yb and Ti. From the phase diagram of Ce and Ti in Figure 1(A), it can be seen that a uniform molten state is formed at high temperatures, but as the temperature decreases, it separates into two phases: βTi and liquid Ce. That is, due to the relationship of specific gravity, Ti remains on the surface of liquid Ce. Also, from the phase diagram of Yb and Ti in Figure 1(B), it can be seen that liquid Yb and solid Ti separate without completely melting. That is, Ti remains on the surface of liquid Yb. In addition to Ce and Yb mentioned above, other rare earth metals such as lanthanum (La: melting point 918℃), praseodymium (Pr: melting point 935℃), and europium (Eu: melting point 822℃) can also be used.

[0023] Here, the halide used as the molten salt is one that is stable with respect to the rare earth metal. For example, when using a fluoride as the halide and Ce as the rare earth metal, it is considered that the halide used has a standard Gibbs free energy of formation on the vertical axis that is lower than Ce's in the Ellingham diagram for various fluorination reactions shown in Figure 2. Specifically, lithium fluoride (LiF), calcium fluoride (CaF2), and molten salts of these fluorides can be used, and among these, LiF can be preferably used.

[0024] Furthermore, when using chlorides as halides in the molten salt, lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl2), and mixed salts of these chlorides can be used, as shown in the Ellingham diagram for various chloride reactions in Figure 3, where the standard Gibbs free energy of formation on the vertical axis is lower than that of Ce.

[0025] Furthermore, when using bromides as halides in the molten salt, calcium bromide (CaBr), lithium bromide (LiBr), sodium bromide (NaBr), potassium bromide (KBr), and mixed salts of these bromides can be used, as shown in the Ellingham diagram for various bromide reactions in Figure 4.

[0026] Furthermore, when using iodides as halides in the molten salt, calcium iodide (CaI), lithium iodide (LiI), and mixed salts of these iodides can be used, as shown in the Ellingham diagram for various iodation reactions in Figure 5.

[0027] The following describes the formation mechanism of a titanium thin film using titanium(III) fluoride (TiF3) as the metal halide, lithium fluoride (LiF) as the halide of the molten salt, and cerium (Ce) as the rare earth metal, using the diagram illustrating the formation mechanism of a titanium thin film using Ce in Figure 6. First, in the melting step, a molten salt containing TiF3 and LiF is melted and brought into contact with Ce. In the reduction step, Ti ions in the molten salt are reduced to Ce itself, and in the alloying step, an alloy of Ce and Ti is formed by an interfacial reaction (Figure 6(A)). At this time, CeF3 is generated as a byproduct according to the following reaction equation (1), but it is dissolved and removed from the molten salt. TiF3 + Ce = Ti + CeF3 ... (1) The heating temperature in the melting process is determined according to the melting points of the selected rare earth metal and molten salt, but typically a temperature of around 900 to 1200°C is considered.

[0028] Subsequently, as a segregation process, if the mixture is slowly cooled or held at a temperature lower than the reduction temperature for a certain period of time, Ti segregates from the alloy and a Ti film grows (Figure 6(B)). In this case, since the density of the segregated metal is intermediate between that of the molten salt and the rare earth metal, a Ti thin film can be formed at the interface between the molten salt and the rare earth metal. As for the slow cooling conditions in the segregation process, a cooling rate of about 1 to 0.1°C / min to a temperature about 50°C higher than the melting point of the rare earth metal used is usually considered. If the mixture is held at a temperature lower than the reduction temperature for a certain period of time, a temperature about 100°C lower than the reduction temperature and about 50°C higher than the melting point of the rare earth metal used is considered. For example, if cerium is used as the rare earth metal and the reduction temperature is 1000°C, then holding at a segregation temperature of 850 to 900°C for 3 to 12 hours is considered.

[0029] Furthermore, when using Ce as a rare earth metal, since interfacial reactions and segregation are utilized, it is possible to control the thickness of the Ti thin film by adjusting the difference between the reduction / alloying temperature and the segregation temperature, as well as the amount of Ti relative to Ce.

[0030] On the other hand, when Yb is used as the rare earth metal, as shown in the phase diagram of Yb and Ti in Figure 1(B), Ti and Yb hardly melt with each other. Therefore, as shown in the diagram illustrating the formation mechanism of a titanium thin film using Yb in Figure 7, when Ti ions in the molten salt are reduced by Yb, a Ti thin film grows through an interfacial reaction. When the Ti covers the surface of the liquid Yb, the reaction ends and a Ti thin film is formed.

[0031] In addition, in the method for manufacturing the titanium thin film of this embodiment, in addition to the method of reacting a molten salt made by mixing and solidifying TiF3 and LiF in advance with Ce, it is also possible to first create a state in which Ce and LiF are molten, and then directly add a compound such as Li2TiF6 to melt and cool to form a film. That is, a double salt containing a halide that is stable with respect to rare earth metals can also be used.

[0032] In the above embodiment for manufacturing a titanium thin film using Ce as the rare earth metal, the segregated titanium thin film can be recovered and subjected to heat treatment and surface treatment to produce a Ti thin film. Furthermore, in this embodiment, the segregated Ce can be converted back to Ti through reduction, and the molten salt can be used for hydrofluoric acid conversion as LiF and Ce by molten salt electrolysis, and as hydrogen fluoride (HF) via fluorine (F2), thus minimizing the amount of material waste. [Examples]

[0033] The present invention will now describe in more detail, with reference to examples, a method for producing a titanium thin film as a method for producing a high-melting-point metal thin film. However, the present invention is not limited to the following examples.

[0034] (Preparation of molten salt) First, a TiF3-containing molten salt was prepared using TiF3 as the metal halide and LiF as the halide of the molten salt. 60% H2TiF6(aq) and Li2CO3 were mixed in a molar ratio of Ti / Li, and HF was added while stirring, maintaining the mixture at 40°C for 3 hours. The chemical formulas for this procedure are as follows.

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[0035] Next, the mixture was evaporated to dryness by maintaining it at 100°C for 50 hours, and then the phase was identified by X-ray diffraction (XRD) using Cu-Kα radiation. From these results, it was confirmed that Li2TiF6 was prepared as the molten salt.

[0036] Next, the obtained Li2TiF6 was weighed and placed in a Ti crucible, sealed in a SUS304 container with Ar gas, and held at 900°C for 3 hours to melt. After furnace cooling, the sample was collected and chemically analyzed by hydrogen generation method and titration method to measure the TiF3 concentration in the molten salt sample. As a result, it was confirmed that the TiF3 concentration in the TiF3-LiF molten salt was 34.6% by mass.

[0037] (Deposition of high-melting-point metal thin films) Next, using the apparatus shown in Figure 8, the melting process involved placing Ce in a Mo crucible and setting it in an electric furnace, where the temperature was raised to 900°C to melt it. Subsequently, the manufactured TiF3-LiF molten salt was introduced through the sample inlet, and the reduction and alloying process was carried out at 900°C for 12 hours. Next, in the segregation process, the mixture was slowly cooled to 700°C at a cooling rate of 0.3°C / min to form a thin Ti film on the Ce surface.

[0038] (Observation by SEM-EDS) The extracted sample, along with the Mo crucible, was embedded in epoxy resin, then polished to expose the surface. The cross-section near the molten salt / Ce interface was observed using a scanning electron microscope (SEM-EDS) to confirm the formation of the thin film. The electron microscope image is shown in Figure 9, and the elemental map showing the distribution of Ti, Ce, F, and O in the same field of view is shown in Figure 10. From the electron microscope image in Figure 9, it was confirmed that a Ti thin film of approximately 40 μm was formed at the interface between the molten salt and Ce. In addition, the white area below the Ti thin film in Figure 9 was identified as containing Ce and O from the elemental map in Figure 10. This O is thought to be due to the oxidation of Ce during polishing, and this area was liquid Ce before polishing. Furthermore, it was confirmed that CeOF was dispersed in the molten salt phase. Notably, TiF3 was not observed in the molten salt after the reaction.

[0039] (XRD analysis) Next, the Ce in the obtained sample was dissolved in distilled water, and the Ti thin film was recovered and subjected to XRD analysis. The XRD pattern is shown in Figure 11. From this pattern, the Ti thin film was identified as α-Ti. The unknown peaks at 28° and 32° are thought to be due to cerium oxyfluoride, which is hardly soluble in water. These results indicate that the Ti thin film was formed by the reduction of TiF3 by liquid Ce and the formation of a Ti-Ce alloy, followed by segregation at the interface between the liquid Ce and the LiF-CeF3 molten salt.

[0040] These results confirm that the method for producing high-melting-point metal thin films according to the present invention does not require the processes of melting and rolling the metal, and makes it possible to produce uniform-thickness, smooth titanium thin films inexpensively and reliably.

Claims

1. A method for manufacturing a high melting point metal thin film, Using a metal halide of the metal to be used as a thin film, a molten salt having a lower specific gravity than the metal, and a rare earth metal having a higher specific gravity than the metal, A melting step of melting the metal halide, the molten salt, and the rare earth metal, A reduction step in which the metal halide is reduced by the rare earth metal, The process then includes a segregation step in which the metal to be formed into a thin film is segregated onto the surface of a rare earth metal by holding it at a temperature lower than the cooling or reduction temperature for a certain period of time. A thin film of the metal is formed at the interface between the molten salt and the rare earth metal. The aforementioned high-melting-point metal includes titanium, The aforementioned metal halide includes titanium halide, The molten salt has the property of melting and separating from the rare earth metal. A method for producing a high-melting-point metal thin film, characterized in that the rare earth metal and the molten salt have lower melting points than the high-melting-point metal.

2. The method for producing a high-melting-point metal thin film according to claim 1, characterized in that the molten salt is a metal halide whose Gibbs free energy change for the halogenation reaction per mole of halogen molecules is lower than that of the rare earth metal.

3. The method for producing a high-melting-point metal thin film according to claim 1, characterized in that the reduction step includes an alloying step in which the metal halide is alloyed with the metal or the rare earth metal.

4. The method for producing a high-melting-point metal thin film according to claim 1, characterized in that the high-melting-point metal thin film is a titanium thin film, the metal halide is titanium halide, and the melting points of the molten salt and the rare earth metal are 1000°C or less.

5. A method for manufacturing a high melting point metal thin film, Using a metal halide of the metal to be used as a thin film, a molten salt having a lower specific gravity than the metal, and a rare earth metal having a higher specific gravity than the metal, A melting step of melting the metal halide, the molten salt, and the rare earth metal, A reduction step in which the metal halide is reduced by the rare earth metal, The process then includes a segregation step in which the metal to be formed into a thin film is segregated onto the surface of a rare earth metal by holding it at a temperature lower than the cooling or reduction temperature for a certain period of time. A thin film of the metal is formed at the interface between the molten salt and the rare earth metal. A method for producing a high-melting-point metal thin film, characterized in that the metal halide is one of titanium fluoride, lithium titanate, titanium chloride, titanium bromide, or titanium iodide; the molten salt is one of lithium fluoride, calcium fluoride, or a mixed salt of these fluorides; lithium chloride, sodium chloride, potassium chloride, calcium chloride, or a mixed salt of these chlorides; calcium bromide, lithium bromide, sodium bromide, potassium bromide, or a mixed salt of these bromides; calcium iodide, lithium iodide, or a mixed salt of these iodides; and the rare earth metal is one of cerium, lanthanum, praseodymium, europium, or ytterbium.

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