High-thermal-conductivity aluminum nitride ceramic as well as preparation method and application thereof

Abstract

The invention discloses high-thermal-conductivity aluminum nitride ceramic as well as a preparation method and application thereof. The method comprises the following steps: taking AlN powder as a ceramic matrix, adding a quaternary sintering aid composed of Y2O3, CaCO3, MgO and Li2CO3, carrying out mixing and ball milling to prepare ceramic slurry, then carrying out drying, sieving and aging to obtain complex-phase powder, and then carrying out dry pressing and cold isostatic pressing to obtain a biscuit; after the green body is subjected to stepped heating and glue discharging, two-stage secondary sintering is carried out in a nitrogen atmosphere, the temperature is raised to 1500 DEG C at the first stage, heat preservation is carried out, the temperature is raised to 1750 DEG C at the second stage, and a crystal boundary phase formed by a sintering aid is promoted to shrink at the trident crystal boundary of AlN crystal grains and is distributed in an island shape by accurately controlling a heating system, so that the heat-conducting property of the material is greatly improved. The heat conductivity of the prepared aluminum nitride ceramic is not lower than 199 W / (m.K), the bending strength is not lower than 269 MPa, the relative density is higher than 98%, and the aluminum nitride ceramic is suitable for the field of heat dissipation of electronic devices.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of material preparation. Specifically, it is a high-thermal-conductivity aluminum nitride ceramic and its preparation method and application. BACKGROUND

[0002] At present, with the rapid development of 5G communication, microwave components, heat dissipation substrates and electronic device packaging and other high-end equipment, the requirements of electronic systems in terms of functional integrity and reliability are becoming higher and higher. Power electronic devices are increasingly developing towards high integration and miniaturization, and heat dissipation materials play a crucial role in realizing safe and efficient operation of devices. Compared with other heat dissipation materials, BeO is toxic, SiC is difficult to prepare, and c-BN is expensive. Replacing traditional Al2O3 ceramic with high-thermal-conductivity AlN ceramic as electronic substrate material is currently the dominant solution to realize rapid heat dissipation of power electronic devices.

[0003] AlN ceramic has become the first choice for microwave power device and large-scale integrated circuit packaging material due to its high thermal conductivity (320 W / m·K -1 ), low dielectric constant (about 8.9 at 1 MHz) and low thermal expansion coefficient. However, due to the interaction of multiple factors such as grain boundary phase, impurity phase and pores, the thermal conductivity of traditional AlN ceramic substrate is only in the range of 150~180 W / (m·K), which is far lower than its theoretical value. AlN is a covalent compound and has a small self-diffusion coefficient, so it cannot generate a liquid phase during sintering. Its densification process mainly relies on the Al2O3 nanolayer on the surface of AlN particles to form a liquid phase.

[0004] The main factors limiting thermal conductivity include pores, oxygen content, grain boundary phase and grain boundary. The grain boundary phase is caused by the residual of sintering aids in the AlN matrix after high-temperature densification, which hinders the heat transfer between the grains. Therefore, to further improve the thermal conductivity of AlN ceramic, one way is to reduce the oxygen impurities in the AlN lattice to increase the average free path of phonons. However, it is usually difficult to introduce new elements to complete the lattice during the late sintering stage, which not only causes lattice distortion but also brings other impurity elements, making it difficult to ensure the integrity of the lattice. The second way is to add rare earth or alkaline oxides as sintering aids to promote the formation of a liquid phase and achieve densification by controlling the microstructure. However, the sintering aids react with Al2O3 on the surface of AlN at high temperature to form a liquid phase that can flow. At high temperature, this liquid phase gradually wraps and wets the entire grain. During cooling, due to the high speed, this liquid phase cannot be discharged and changes from liquid to solid, forming an amorphous phase that wraps around the grain, forming an insulating layer that affects the thermal conductivity. Therefore, if low-temperature sintering is to be achieved, how to reduce the content of the liquid phase and discharge it during the late sintering stage to prevent the formation of amorphous phase during the cooling process is a key problem in improving thermal conductivity. Moreover, in the case of multi-phase co-sintering, a single component of sintering aid cannot meet the current situation of low-temperature sintering. SUMMARY

[0005] To this end, the technical problem to be solved by the present application is to provide a high-thermal-conductivity aluminum nitride ceramic and a preparation method and application thereof, which reduce liquid phase content while reducing sintering temperature.

[0006] To solve the above technical problem, the present application provides the following technical solution:

[0007] A preparation method of a high-thermal-conductivity aluminum nitride ceramic, characterized in that it comprises the following steps:

[0008] Step one, ceramic base powder and quaternary sintering aids are provided respectively; the ceramic base powder is AlN powder, and the quaternary sintering aids include Y2O3, CaCO3, MgO and Li2CO3;

[0009] Step two, the ceramic base powder and the quaternary sintering aids are mixed to obtain a mixture, and then a forming aid is added to obtain a ceramic slurry;

[0010] Step three, the ceramic slurry is dried, ground and sieved to obtain a composite powder, and the composite powder is pressed to obtain a ceramic green body;

[0011] Step four, the ceramic green body is subjected to degassing treatment to obtain a ceramic body, and then the ceramic body is subjected to secondary sintering to obtain an aluminum nitride ceramic with high thermal conductivity.

[0012] The application takes in-situ synthesized AlN powder as the base powder for ceramic sintering, introduces Y2O3, CaCO3, MgO and Li2CO3 as the quaternary sintering aids, Y2O3 and CaCO3 powders are used to prepare liquid phase, Y2O3 has high affinity to Al2O3, by controlling the adding amount of the quaternary sintering aids and the ratio among Y2O3, CaCO3, MgO and Li2CO3, and mixing with specific forming aids, then selecting appropriate ball milling process, finally selecting reasonable pressing process and sintering procedure (sintering in first stage and second stage in turn and controlling sintering temperature, heating rate and holding time), the eutectic temperature of Y2O3 and Al2O3 during solid phase reaction is also near the sintering temperature of AlN ceramic, the low-activity yttrate liquid phase fills the pores, improves the density of the ceramic and increases the oxygen removal driving force, which can effectively purify the AlN lattice and improve the thermal conductivity of AlN ceramic; MgO and Li2CO3 are used as adjusting components to adjust the liquid phase distribution, and the residual liquid phase is effectively volatilized during sintering. According to the theory of flow, dissolution, precipitation and volatilization, the composition and interface structure of the composite ceramic are designed, and through reasonable sintering procedure, the sintering aids are transformed from dispersing on the grain boundary to retreating in the three-pronged grain boundary, while reducing the amorphous layer, and trace doping of Li and Mg is beneficial to the volatilization of the sintering aids in the later stage, so as to greatly control the specific heat of the material, optimize the thermal conductivity, and it is expected to become a new ideal thermal diffusion and thermal conduction material.

[0013] In the step one of the above preparation method of high-thermal-conductivity aluminum nitride ceramic, the average particle size of the AlN powder is 1-3 μm, the total adding amount of the quaternary sintering aids is 2-4 wt% of the total mass of the mixture (the mixture of the ceramic base powder and the quaternary sintering aids), and the mass of the AlN powder is 96-98 wt% of the total mass of the mixture, that is, the overall proportion of the quaternary sintering aids should not exceed 4% of the total mass of the mixture, and too much sintering aid will form more liquid phase, although it improves the sintering activity, but the formed liquid phase cannot be discharged at high temperature, which will deteriorate the performance; the molar ratio of Y2O3 to CaCO3 is 2:1, the adding amount of MgO is 0.1-0.2 wt% of the total mass of the mixture, and if too much is added, it will introduce additional oxygen vacancies and deteriorate the thermal conductivity; the adding amount of Li2CO3 is 0.2-1 wt% of the total mass of the mixture, and a small amount of Li has a certain oxygen driving effect, and volatilization will take away a little oxide impurities, but when the amount exceeds a certain value, due to excessive volatilization, the sintered ceramic will be loose as a whole, and some micro-pores will be generated on the surface of the ceramic.

[0014] The specific surface area of the AlN powder is 4.5-6 m 2 / g, Y2O3 is a nano-level white powder, and CaCO3 and Li2CO3 are used to prevent the sintering aids from deliquescing in air.

[0015] In the above-mentioned method for preparing high thermal conductivity aluminum nitride ceramics, the amount of MgO added is 0.197~0.199 wt% of the total mass of the mixture, and the amount of Li2CO3 added is 0.298~0.988 wt% of the total mass of the mixture.

[0016] In the above-mentioned method for preparing high thermal conductivity aluminum nitride ceramics, in step two, the molding aid includes a binder, a dispersant, and a solvent. The molding aid is prepared by dissolving the binder in the solvent under water bath heating and shaking, and then adding the dispersant dropwise and stirring to prepare the molding aid.

[0017] In the above-mentioned method for preparing high thermal conductivity aluminum nitride ceramics, the binder is polyvinyl butyral, the dispersant is castor oil, and the solvent is anhydrous ethanol.

[0018] The total mass of polyvinyl butyral is 1-1.5% of the total mass of the mixture, and the total mass of castor oil is 0.2-0.5% of the total mass of the mixture. The volume ratio of the mixture to the molding aid is 1:1.2-1.5. The water bath heating temperature is 50-70 ℃. The viscosity of the molding aid is 1843.6±300 mPa·s. This viscosity is most suitable for dry pressing (the powder does not stick to the mold, does not break corners, is easy to demold, and does not separate). If the viscosity is too high, the AlN powder will agglomerate excessively. During pressing, uneven viscosity on the particle surface is likely to occur, often resulting in separation. Furthermore, it is difficult to remove the blank after dry pressing (demolding). If the viscosity is too low, it cannot play a binding role, and dry pressing cannot allow the powder to fully form into blocks. The overall quality of the blank is poor, and it is easy to break.

[0019] In the preparation method of the above-mentioned high thermal conductivity aluminum nitride ceramic, in step two, the mixture and molding aid are ball-milled to form a ceramic slurry. During ball milling, the mixture, molding aid, and ball milling media are placed in a ball milling jar and mixed to form a non-layered and non-clumping slurry. The slurry is then ball-milled in a planetary ball mill to form a ceramic slurry. The ball milling parameters are as follows: ball milling is performed by alternating forward and reverse directions, with a forward ball milling time of 0.5~1.5 h and a reverse ball milling time of 0.5~1.5 h, a ball milling speed of 250~400 r / min to ensure that the powder is fully wetted, and a ball milling time of 4~6 h. The ball milling media are zirconia balls, wherein the volume ratio of φ3 mm to φ4 mm zirconia balls is 2:1, and the volume ratio of the mixture, zirconia balls, and molding aid is always maintained at 1:2:(1.2~1.5).

[0020] In the preparation method of the above high thermal conductivity aluminum nitride ceramic, in step three, drying is carried out in a vacuum environment (the milling balls and the ball milling tank are placed in a vacuum drying oven together). The drying temperature is 50-70 °C, the drying time is 5-10 min. After taking out the dried block, it is first crushed and then ground, and finally passed through a 60-mesh sieve. The powder after sieving is also aged in a vacuum state to make a composite powder;

[0021] Pressing includes dry pressing and cold isostatic pressing processes carried out successively. During dry pressing, weigh the composite powder and pour it into the pressing mold, control the height of the mold wall to be flush with the powder filling surface, scrape the powder filling surface flat, and carry out dry pressing. The dry pressing pressure is 8-12 MPa, and the pressure holding time is 1-3 minutes; the green body after dry pressing is cold pressed in a cold isostatic pressing mold. The cold isostatic pressing pressure is 30-​50 MPa, and the pressure holding time is 30-40 min. Finally, the surface of the sample has no steps and cracks to be qualified, and a ceramic green body with uniform density is made. If only dry pressing is used, due to the unidirectional compressive stress, there are cases of high and low density inclusions in the green body after pressing. Usually, the upper surface part of the green body has a high density, and the lower surface part has a low density; non-uniform density will cause non-uniform shrinkage of the upper and lower parts of the green body during the sintering process of AlN ceramic, resulting in the phenomena of "edge warping" and "bending", which affect the quality of AlN ceramic. The whole cold isostatic pressing mold is filled with hydraulic oil. The green body after dry pressing is placed in the cold isostatic pressing mold, and at this time the green body is completely immersed in the hydraulic oil. By applying pressure, the pressure is transmitted to the hydraulic oil. The hydraulic oil has fluidity and can wrap the green body 360°. After increasing the pressure, the pressure is transmitted to the whole green body, and the whole green body can be further compacted. And under the condition of all-round stress, the density of the whole green body remains the same, and the overall shrinkage of the sample after sintering is also the same. Therefore, the overall deformation of the sample obtained by sintering after cold isostatic pressing is extremely small, and the relative density is also relatively high, which is easy to further process.

[0022] In the preparation method of the above high thermal conductivity aluminum nitride ceramic, it is characterized in that in step four, the debinding is carried out in an air atmosphere, the heating rate is 1-3 °C / min, and it is kept warm for 1-2 h at 230 °C, 320 °C and 600 °C respectively, and then furnace cooled to room temperature to ensure the full decomposition of PVB and castor oil;

[0023] The secondary sintering includes:

[0024] a) First stage sintering: The temperature is raised from room temperature to 1500℃ and held for 1-2 hours, preferably 2 hours. Li2O reacts with Al2O3 to form lithium aluminate, and MgO reacts with Al2O3 to form MgAl2O4. The active MgO and volatile Li2O are fixed in a stable compound to prevent premature volatilization before reaching higher temperatures. The heating rate from room temperature to 1200℃ is 5-10℃ / min, preferably 10℃ / min, and the heating rate from 1200℃ to 1500℃ is 1-3℃ / min, preferably 2℃ / min. Before 1200℃, the Al2O3 on the AlN surface and the sintering aid hardly react, so the heating rate can be appropriately increased to improve efficiency. After 1200℃, the Al2O3 on the AlN surface and the sintering aid gradually begin to react, so reducing the heating rate is beneficial for the full occurrence of the reaction.

[0025] b) Second stage sintering: The temperature is raised from 1500℃ to 1750℃ and held for 2-6 hours at a rate of 1-3℃ / min, preferably 2℃ / min. Then, the temperature is lowered to 800℃ at a rate of 4-6℃ / min, preferably 5℃ / min. After reaching 800℃, the temperature is cooled to room temperature with the furnace. When raising the temperature from 1500℃ to 1750℃, the holding time is preferably 4 hours. If the holding time is too short, the element diffusion and replenishment are insufficient, the liquid phase wetting is not adequate, the grain growth is incomplete, and the short sintering time is prone to producing pores. The presence of pores will cause phonon scattering and deteriorate the thermal conductivity. If the holding time is too long, the elements have fully diffused and the grains have fully grown and will not continue to grow indefinitely, resulting in no significant improvement in performance and energy waste.

[0026] The cooling rate of 5 °C / min is used to prevent the formation of an amorphous layer in the liquid phase during rapid cooling, which would worsen the thermal conductivity. With this cooling rate, the oxygen dissolved in the AlN lattice has enough time to diffuse from the interior of the lattice to the grain boundaries and react with the sintering aid to form a more stable second-phase compound (crystalline phase), resulting in higher thermal conductivity and better high-temperature stability.

[0027] During sintering, when the temperature rises to 1600-1700 °C, additives such as Y₂O₃, CaO (derived from the decomposition of CaCO₃), MgO, and Li₂O (derived from the decomposition of Li₂CO₃) react with Al₂O₃ on the surface of AlN particles to form a multi-component aluminate liquid phase containing Y, Ca, Mg, and Li. The volatilization of Li₂O helps regulate the properties of the liquid phase and promotes its removal in the later stages. Subsequently, at the optimal sintering temperature of 1750 °C, this liquid phase causes AlN particles to rearrange through capillary forces and achieves high densification of the ceramic body through a dissolution-precipitation mechanism, ultimately forming a structure with AlN as the main crystalline phase and yttrium aluminate (Al₂O₃ reacts with Y₂O₃ to form Y₃Al₅O) as the grain boundary phase. 12The microstructure includes minor phases such as Y₂O₃ and calcium aluminate (Al₂O₃ reacts with CaO to form CaAl₃O₇). Because sintering is carried out in a reducing atmosphere, trace amounts of Y₂O₃ and Al₂O₃ remain in the phases. Furthermore, when binary additives Y₂O₃ and CaCO₃ are added, the sintering aids dissolve and precipitate, and during cooling, Y₃Al₅O₇ first crystallizes out. 12 This process fixes free oxygen atoms in the yttrium aluminate lattice, reducing the oxygen content at AlN grain boundaries. This application uses a quaternary sintering aid of Y, Ca, Mg, and Li. The process involves a full reaction at 1200–1750 °C, effectively eliminating residual liquid phase in the later stages of sintering.

[0028] In the above-mentioned method for preparing high thermal conductivity aluminum nitride ceramics, in step four, the ceramic blank is placed in an alumina boat for debinding, and the alumina boat is lined with BN powder; the maximum temperature during debinding is 600 ℃, and the alumina ceramic boat (crucible) used during debinding serves only as a sample container, but other refractory materials can also be used instead.

[0029] The ceramic green body was placed in a graphite boat and sintered in a carbon tube furnace. BN ceramic sheets were placed both above and below the green body inside the graphite boat, with BN powder filling the gaps between the ceramic sheets and the graphite boat. Before sintering, the carbon tube furnace was purged, and then sintering was performed using a flowing N2 atmosphere, maintained at atmospheric pressure throughout, with a gas flow rate of 0.5–1 L / min. A high-temperature resistant graphite boat with good thermal conductivity was used to ensure that the actual sample temperature and the furnace temperature were consistent. The BN ceramic sheets and BN powder primarily served to isolate (block) the carbon atmosphere. Without BN ceramic sheets and BN powder, the entire ceramic would be black after sintering. At high temperatures, carbon from the graphite crucible and the carbon tube furnace could enter the AlN ceramic.

[0030] The present invention also provides a high thermal conductivity aluminum nitride ceramic, which is prepared by the above-described preparation method. The ceramic has a thermal conductivity of not less than 199 W / (m·K), a flexural strength of not less than 269 MPa, and a relative density of not less than 98%.

[0031] This invention also provides an application of high thermal conductivity aluminum nitride ceramic, which, when used as an electronic substrate material, enables heat dissipation in power electronic devices.

[0032] The technical solution of the present invention achieves the following beneficial technical effects:

[0033] 1. This invention retains the reliability of the AlN ceramic matrix by introducing Y2O3, CaCO3, MgO, and Li2CO3 as quaternary sintering aids. Y2O3 and CaCO3 powders are used to prepare the liquid phase, while MgO and Li2CO3 are used as regulating components to adjust the liquid phase distribution. The addition of MgO refines the grains and reduces the fluidity of the liquid phase at high temperatures, preventing the liquid phase from completely encapsulating the grains. The introduction of Li2CO3, with its low saturated vapor pressure, volatilizes at high temperatures, carrying away oxygen impurities and appropriately reducing the liquid phase content. Therefore, under the combined action of these two (Mg and Li) components, the fluidity is changed, and some of the liquid phase and oxygen impurities are volatilized. The heat insulation layer is reduced, the amorphous layer is reduced, and the remaining liquid phase after cooling will shrink at the three-way grain boundaries, resulting in more direct contact between AlN grains, thereby improving thermal conductivity.

[0034] 2. This invention utilizes the strong fluxing effect of Li and Mg to effectively reduce the liquid phase formation temperature. This allows a small amount of highly active liquid phase to form at low temperatures in the early stages of sintering, thus initiating sintering earlier. Furthermore, by utilizing the high entropy effect brought about by the four cations, the presence of multiple cations in the liquid phase allows the liquid phase to remain stable over a wider temperature range, rather than rapidly solidifying or decomposing. The stability of the liquid phase allows for better spreading on the surface of AlN particles in the early stages, wetting the grains. This invention adds fewer additives at the initial stage of the formulation, fundamentally reducing the total amount of residual liquid phase in the final liquid phase, enabling the densification process to be achieved with a smaller amount of liquid phase.

[0035] 3. This invention employs a two-step sintering method. In the first stage sintering temperature, Li2O and MgO, which have low co-sintering temperatures, form an initial liquid phase to enhance atomic diffusion capabilities, providing kinetic preparation for the densification process. In the second stage sintering temperature, the particle rearrangement, dissolution, precipitation, and volatilization processes are rapidly driven, quickly achieving a high green body density. This results in the prepared high thermal conductivity AlN ceramic having good crystallinity, uniform grains, and high stability, with a density of 99% and a linear shrinkage rate of 16.7%. Compared to the one-step sintering method, the two-step sintering method is more stable.

[0036] 4. Furthermore, with the trend towards high integration of components, higher requirements are placed on the strength of AlN heat dissipation substrates. Substrates with insufficient strength are prone to breakage during service, causing permanent device failure. Currently, in pursuit of higher heat dissipation efficiency and more compact designs, AlN components not only need high thermal conductivity but also need to withstand certain mechanical loads. High-strength substrates are a prerequisite for realizing these complex and intricate structural designs. Therefore, this invention starts with materials to prepare high thermal conductivity AlN ceramics with high thermal conductivity and comparable flexural strength. Moreover, this invention retains the AlN matrix and introduces trace amounts of MgO and Li2CO3 to achieve sintering aid behavior and improve stability.

[0037] In summary, this application uses AlN as the ceramic matrix, adds an appropriate amount of a quaternary sintering aid composed of Y2O3, CaCO3, MgO, and Li2CO3 in a specific ratio, mixes it with a specific molding aid, and then selects a reasonable ball milling process for ball milling. Furthermore, it selects a reasonable pressing process and an appropriate sintering temperature in a two-step sintering method, strictly controlling the heating rate and holding time during sintering. This alters the behavior of the sintering aid, effectively reducing the sintering temperature of the AlN ceramic, reducing the liquid phase content, and causing the liquid phase to shrink from a continuous distribution to a contraction at the triple grain boundaries. Simultaneously, it reduces the amorphous layer, which is beneficial for improving thermal conductivity and effectively inhibits crack propagation. The resulting aluminum nitride ceramic exhibits a maximum thermal conductivity of 251 W / (m·K) and a maximum flexural strength of 339 MPa, making it a "dual-effect" AlN ceramic. Compared with other processes, this process has the characteristics of low cost and superior overall performance, providing a valuable reference for actual production and processing. Attached Figure Description

[0038] Figure 1 (a), (b), (c), and (d) are dihedral scanning electron microscope images of the liquid phase distribution of high thermal conductivity aluminum nitride ceramics prepared with sintering aids Y, Y-Ca, Y-Ca-Mg, and Y-Ca-Mg-Li, respectively.

[0039] Figure 2 (a) The sintering aid and sintering conditions were: adding 6 wt.% Y and holding at 1750 ℃ ​​for 4 h. Figure 2 (b) Add 2 wt.% of Y-Ca and incubate at 1750 ℃ ​​for 4 h. Figure 2 (c) After adding 2 wt.% γ-Ca, an additional 0.2 wt.% Mg was added and the mixture was kept at 1750 ℃ ​​for 4 h. Figure 2 (d) Scanning electron microscope image at low magnification of aluminum nitride ceramics prepared by adding 2 wt.% Mg and 0.2 wt.% Li to Y-Ca and holding at 500 °C for 2 h + 1750 °C for 4 h; Figure 2 In the middle, (a1), (b1), (c1), and (d1) are magnified views of parts of figures (a), (b), (c), and (d), respectively, and the corresponding element distribution diagrams.

[0040] Figure 3 Transmission electron microscopy (TEM) image of the high thermal conductivity aluminum nitride ceramic prepared in Example 1 of this invention;

[0041] Figure 4(a) and (b) are transmission electron microscopy (TEM) images of different regions of high thermal conductivity aluminum nitride ceramics prepared by adding 2 wt.% Mg and 0.2 wt.% Li to Y-Ca and holding at 1750℃ for 4 h, respectively. (mainly Y3Al5O) 12 (phase) and the corresponding element distribution diagram;

[0042] Figure 5 XRD diffraction pattern of the standard diffraction pattern of the high thermal conductivity aluminum nitride ceramic prepared in the embodiment of the present invention;

[0043] Figure 6 These are statistical images of the bulk density and compaction density of the high thermal conductivity aluminum nitride ceramics prepared in the embodiments of the present invention;

[0044] Figure 7 (a), (b), and (c) are statistical images of the thermal conductivity, thermal diffusivity, and specific thermal conductivity of the high thermal conductivity aluminum nitride ceramics prepared in Examples 1, 3, and 2 of this invention, respectively.

[0045] Figure 8 (a), (b), and (c) are statistical images of the flexural strength of the high thermal conductivity aluminum nitride ceramics prepared in Examples 1, 3, and 2 of this invention, respectively. Detailed Implementation

[0046] Example 1:

[0047] The method for preparing high thermal conductivity AlN ceramics in this embodiment is as follows:

[0048] Step 1: Mixing AlN, Y2O3, CaCO3, MgO and Li2CO3 powders

[0049] The raw materials were weighed according to the following percentages: AlN powder accounted for 97.513% of the total mass of the mixture, Y2O3 powder accounted for 1.632% of the total mass of the mixture, CaCO3 powder accounted for 0.358% of the total mass of the mixture, MgO powder accounted for 0.199% of the total mass of the mixture, and Li2CO3 powder accounted for 0.298% of the total mass of the mixture. Using PVB, castor oil, and anhydrous ethanol as molding aids, the mixture was ball-milled in a planetary ball mill at a speed of 300 r / min for 4 h to ensure uniform mixing of the five powders. The ceramic slurry was then placed in a vacuum drying oven at 60 ℃ and dried for 8 min. After drying, the ceramic slurry was ground again through an agate mortar and finally passed through a 60-mesh sieve. After aging for 12 h, the multiphase powder was obtained.

[0050] Step 2: Sintering of high dielectric constant and low loss aluminum nitride-based multiphase ceramics

[0051] Weigh the above-ground multiphase powder, pour it into a stainless steel mold, press it under axial pressure of 10 MPa for 2 min, and then perform cold isostatic pressing under pressure of 40 MPa for 40 min to obtain a ceramic green body with high thermal conductivity and high strength. Then, heat at a rate of 1 ℃ / min, holding at 230 ℃ for 1 h, 320 ℃ for 1 h, and 600 ℃ for 2 h, followed by furnace cooling to room temperature to ensure complete decomposition of PVB and castor oil. Next, place a BN ceramic sheet on a graphite boat, fill the gaps between the sheet and the graphite boat with an appropriate amount of BN powder, place the ceramic green body on top, then place another BN ceramic sheet on top, and finally fill all gaps with BN powder. Place the graphite boat in a carbon tube furnace for sintering. The specific sintering process is as follows: room temperature ~ 1200 ℃, heating rate of 10 ℃ / min; 1200 ~ 1500 ℃, heating rate of 2... The furnace was heated at 1500 ℃ for 2 h at a rate of ℃ / min, and the sintering temperature was 1750 ℃ ​​for 4 h. The temperature was then lowered to 800 ℃ at a rate of 5 ℃ / min, and then cooled to room temperature in the furnace. The total N2 gas flow rate was 0.5 L / min. This yielded aluminum nitride ceramics with high thermal conductivity and comparable strength.

[0052] Upon testing, the high thermal conductivity aluminum nitride ceramic prepared in this embodiment contains AlN phase and Y3Al5O phase. 12 Xiang, like Figure 3 As shown, the overall phase content is low, and Figure 5 The XRD test data matches the standard PDF card data.

[0053] The ceramics prepared in Example 1 of this invention, as well as ceramics formed using mono-, binary, and ternary sintering aids, were characterized. Figure 1 Scanning electron microscopy (SEM) images of the dihedral angles of AlN ceramics show that the dihedral angles φ for the Y, Y-Ca, Y-Ca-Mg, and Y-Ca-Mg-Li additive systems are 60°, 45°, 90°, and 135°, respectively. The Y-Ca system has a smaller dihedral angle than the Y system, indicating that the liquid phase can penetrate deeper into the grain boundaries. The Y-Ca-Mg system has a moderate dihedral angle, but exhibits more overall porosity. The Y-Ca-Mg-Li additive system has a dihedral angle of 135°, indicating that the addition of MgO and Li₂O increases the solid-liquid interfacial energy. After cooling, the liquid phase cannot spread at the grain boundaries and tends to be distributed in an isolated, discontinuous form at the triple grain boundaries. In this case, the direct contact area between grains is large and the structure is more stable. Li and Mg volatilize in the later stages of sintering, carrying away oxygen impurities and reducing the content of the grain boundary phase before cooling, causing it to exist as isolated particles at the triple grain boundaries.

[0054] like Figure 2As shown in the figure, the white area represents the cooled additive phase (liquid phase), the black polygons represent AlN grains, and the circular black pores are gas pores. From... Figure 2 As shown in (a) and (b), in the sintered ceramic samples with the Y and Y-Ca additive systems, the sintering aids adhere to the surface of the AlN particles, resulting in a relatively large number of pores. Furthermore, the binary additive samples exhibit large-area bonding regions in the sintering aids. Figure 2 As shown in (c) and (d), the pores in the Y-Ca-Mg and Y-Ca-Mg-Li samples are located at the triple grain boundaries of the AlN grains. Figure 2 (d) The inset clearly shows that the liquid phase in the Y-Ca-Mg-Li sample gradually decreases, remaining at the three grain boundaries in an island-like distribution. This island-like distribution of the grain boundary phase (liquid phase) disrupts the continuous brittle grain boundary network. Figure 1 The results are consistent; and the elemental distributions in Figures (a1), (b1), (c1), and (d1) show that with the addition of Mg and Li elements, Y elements gradually decrease, and the segregation and enrichment of Ca and Mg elements are almost not observed in the later stage of sintering. This also indicates that the YCML sintering aid wets the grains in the early stage and volatilizes and carries away the impurities at the grain boundaries in the later stage.

[0055] The bulk density of the prepared AlN ceramic was 3.26846 g / cm³. 3 It has a relative density of 99.35%, a thermal conductivity of 251 W / (m·K), and a thermal diffusivity of 59.9 mm. 2 Its specific heat capacity is 1.23 J / (g·K), and its maximum flexural strength is 339 MPa.

[0056] Example 2:

[0057] The method for preparing high thermal conductivity AlN ceramics in this embodiment is as follows:

[0058] Step 1: Mixing AlN, Y2O3, CaCO3, MgO and Li2CO3 powders

[0059] The raw materials were weighed according to the following percentages: AlN powder accounted for 96.838% of the total mass of the mixture, Y2O3 powder accounted for 1.621% of the total mass of the mixture, CaCO3 powder accounted for 0.356% of the total mass of the mixture, MgO powder accounted for 0.197% of the total mass of the mixture, and Li2CO3 powder accounted for 0.988% of the total mass of the mixture. PVB, castor oil, and anhydrous ethanol were used as molding aids. The mixture was ball-milled in a planetary ball mill at a speed of 400 r / min for 6 h to ensure uniform mixing of the five powders. The resulting ceramic slurry was then dried in a vacuum drying oven at 50 ℃ for 5 min. After drying, the ceramic slurry was ground again in an agate mortar and finally passed through a 60-mesh sieve. After aging for 12 h, the multiphase powder was obtained.

[0060] Step 2: Sintering of high thermal conductivity AlN ceramics

[0061] Weigh the above-ground multiphase powder, pour it into a stainless steel mold, press it under an axial pressure of 8 MPa for 2 min, and then perform cold isostatic pressing under a pressure of 40 MPa for 30 min to obtain a ceramic green body of high thermal conductivity aluminum nitride ceramic. Then, heat at a rate of 1 ℃ / min, holding at 230 ℃ for 1 h, 320 ℃ for 1 h, and 600 ℃ for 2 h, followed by furnace cooling to room temperature to ensure complete decomposition of PVB and castor oil. Next, place a BN ceramic sheet on a graphite boat, fill the gaps between the sheet and the graphite boat with an appropriate amount of BN powder, place the ceramic green body on top, then place another BN ceramic sheet on top, and finally fill all gaps with BN powder. Place the graphite boat in a carbon tube furnace for sintering. The specific sintering process is as follows: room temperature ~ 1200 ℃, heating rate 10 ℃ / min; 1200 ~ 1500 ℃, heating rate 2 ℃ / min. The furnace was heated at 1500 ℃ for 2 h at a rate of 2 ℃ / min from 1500 to 1750 ℃, and held at 1750 ℃ ​​for 4 h. The temperature was then lowered to 800 ℃ at a rate of 5 ℃ / min, and then cooled to room temperature with the furnace. The total N2 gas flow rate was 0.5 L / min. This yielded aluminum nitride ceramics with high thermal conductivity and comparable strength, as well as aluminum nitride ceramics with high thermal conductivity and comparable mechanical properties.

[0062] Upon testing, the high dielectric constant and low loss aluminum nitride-based multiphase ceramic prepared in this embodiment contains AlN phase and Y3Al5O phase. 12 The phase contains a very small amount of Al2O3 phase, and Figure 5 The XRD test data matches the standard PDF card data; Figure 4 The transmission electron microscope (TEM) images of a local area show that the liquid phase, after cooling, is Y3Al5O. 12 The AlN grain boundaries are straight, and elemental distribution reveals no elemental enrichment at the grain boundaries, indicating that the Y-Ca-Mg-Li additive system effectively reduces the amorphous layer at the grain boundaries. Furthermore, the bulk density of the prepared AlN ceramic is 3.29035 g / cm³. 3 It has a relative density of 99.07%, a thermal conductivity of 251 W / (m·K), and a thermal diffusivity of 59.5 mm. 2 Its specific heat capacity is 1.229 J / (g·K), and its maximum flexural strength is 290MPa.

[0063] Example 3:

[0064] The method for preparing high thermal conductivity AlN ceramics in this embodiment is as follows:

[0065] Step 1: Mixing AlN, Y2O3, CaCO3, MgO and Li2CO3 powders

[0066] The raw materials were weighed according to the following percentages: AlN powder accounted for 97.513% of the total mass of the mixture, Y2O3 powder accounted for 1.632% of the total mass of the mixture, CaCO3 powder accounted for 0.358% of the total mass of the mixture, MgO powder accounted for 0.199% of the total mass of the mixture, and Li2CO3 powder accounted for 0.298% of the total mass of the mixture. PVB, castor oil, and anhydrous ethanol were used as molding aids. The mixture was ball-milled in a planetary ball mill at a speed of 250 r / min for 4 h to ensure uniform mixing of the five powders. The ceramic slurry was then placed in a vacuum drying oven at 50 ℃ and dried for 5 min. After drying, the ceramic slurry was ground again through an agate mortar and finally passed through a 60-mesh sieve. After aging for 12 h, the multiphase powder was obtained.

[0067] Step 2: Sintering of high thermal conductivity AlN ceramics:

[0068] Weigh the above-ground multiphase powder, pour it into a stainless steel mold, press it under an axial pressure of 8 MPa for 2 min, and then perform cold isostatic pressing under a pressure of 30 MPa for 30 min to obtain a ceramic green body of high thermal conductivity aluminum nitride ceramic. Then, heat at a rate of 1 ℃ / min, holding at 250 ℃ for 1 h, 320 ℃ for 1 h, and 600 ℃ for 4 h, followed by furnace cooling to room temperature to ensure complete decomposition of PVB and castor oil. Next, place a BN ceramic sheet on a graphite boat, fill the gaps between the sheet and the graphite boat with an appropriate amount of BN powder, place the ceramic green body on top, place another BN ceramic sheet on top, and finally fill all gaps with BN powder. Place the graphite boat in a carbon tube furnace for sintering. The specific sintering process is as follows: room temperature ~ 1200 ℃, heating rate 10 ℃ / min; 1200 ~ 1500 ℃, heating rate 2 ℃ / min. The furnace was heated at 1500 ℃ for 2 h at a rate of 2 ℃ / min from 1500 to 1750 ℃, and held at 1750 ℃ ​​for 2 h. The temperature was then lowered to 800 ℃ at a rate of 5 ℃ / min, and then cooled to room temperature in the furnace. The total N2 gas flow rate was 0.5 L / min. This yielded aluminum nitride ceramics with high thermal conductivity and comparable strength, as well as aluminum nitride ceramics with high thermal conductivity and comparable mechanical properties.

[0069] Upon testing, the high thermal conductivity aluminum nitride ceramic prepared in this embodiment contains AlN phase and Y3Al5O phase. 12 It consists of two phases, and Figure 5 The XRD test data matches the standard PDF card data; Figure 4It was observed that the AlN ceramic grains had a large contact area, clean grain boundaries without sintering aid encapsulation, and the liquid phase shrank independently as grain boundary phases after cooling; Figure 4 (a) and (b) confirm that the grain boundary phase is Y3Al5O 12 Furthermore, the illustration shows a clean interface, and the elemental distribution reveals that there is no elemental enrichment at the grain boundaries, which also indicates that Y-Ca-Mg-Li effectively reduces the amorphous layer at the grain boundaries.

[0070] The bulk density of the ceramic prepared in this embodiment is 3.2746 g / cm³. 3 It has a density of 98.97%, a thermal conductivity of 199 W / (m·K), and a thermal diffusivity of 53.06 mm. 2 The parameters of the three embodiments are as follows: 1.094 J / (g·K) for specific heat capacity, 269 MPa for maximum flexural strength, and 1.094 J / (g·K). Figures 6-8 As shown.

[0071] In summary, this invention uses in-situ synthesized AlN powder as the matrix powder for ceramic sintering. By introducing Y2O3, CaCO3, MgO, and Li2CO3 components, where Y2O3 and CaCO3 powders are used to prepare the liquid phase, and MgO and Li2CO3 are used as regulating components to adjust the liquid phase distribution, a high thermal conductivity functional material is finally prepared by two-step pressureless liquid phase sintering using quaternary sintering aids. This material also has high thermal conductivity and suitable bending strength, enabling it to play a unique role in mobile phones, heat dissipation substrates, and chip packaging materials.

[0072] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A method for preparing high thermal conductivity aluminum nitride ceramics, characterized in that, Includes the following steps: Step 1: Provide ceramic matrix powder and quaternary sintering aid respectively; the ceramic matrix powder is AlN powder, and the quaternary sintering aid includes Y2O3, CaCO3, MgO and Li2CO3; Step 2: Mix the ceramic matrix powder and the quaternary sintering aid to obtain a mixture, and then add the molding aid to obtain a ceramic slurry; Step 3: After drying the ceramic slurry, grind and sieve it to obtain multiphase powder. Press the multiphase powder into shape to obtain ceramic blank. Step 4: The ceramic green body is debinded to obtain a ceramic green body; then the ceramic green body is sintered a second time to obtain aluminum nitride ceramic with high thermal conductivity.

2. The method for preparing a high thermal conductivity aluminum nitride ceramic according to claim 1, characterized in that, In step one, the average particle size of AlN powder is 1~3 μm, the mass of AlN powder is 96~98 wt% of the total mass of the mixture, the total amount of quaternary sintering aid is 2~4 wt% of the total mass of the mixture, wherein the molar ratio of Y2O3 to CaCO3 is 2:1, the amount of MgO added is 0.1~0.2 wt% of the total mass of the mixture, and the amount of Li2CO3 added is 0.2~1 wt% of the total mass of the mixture.

3. The method for preparing a high thermal conductivity aluminum nitride ceramic according to claim 1, characterized in that, In step two, the molding aid includes a binder, a dispersant, and a solvent. The molding aid is prepared by dissolving the binder in the solvent under water bath heating and shaking, and then adding the dispersant dropwise and stirring to prepare the molding aid.

4. The method for preparing a high thermal conductivity aluminum nitride ceramic according to claim 3, characterized in that, The binder is polyvinyl butyral, the dispersant is castor oil, and the solvent is anhydrous ethanol; The total mass of the polyvinyl butyral is 1-1.5% of the total mass of the mixture, the total mass of castor oil is 0.2-0.5% of the total mass of the mixture, the volume ratio of the mixture to the molding aid is 1:1.2-1.5; the water bath heating temperature is 50-70℃, and the viscosity of the molding aid is 1843.6±300 mPa·s.

5. The method for preparing a high thermal conductivity aluminum nitride ceramic according to claim 1, characterized in that, In step two, the mixture and molding aid are ball-milled to form a ceramic slurry. The ball milling parameters are as follows: ball milling is performed by alternating forward and reverse directions, the ball milling speed is 250~400 r / min, and the ball milling time is 4~6 h; the ball milling media are zirconia balls, wherein the volume ratio of φ3 mm to φ4 mm zirconia balls is 2:1, and the volume ratio of the mixture, zirconia balls and molding aid is always maintained at 1:2:(1.2~1.5).

6. The method for preparing a high thermal conductivity aluminum nitride ceramic according to claim 1, characterized in that, In step three, drying is carried out in a vacuum environment at a temperature of 50-70 ℃ for 5-10 min. After grinding, the powder is finally passed through a 60-mesh sieve. The powder after grinding and sieving is then aged in a vacuum to obtain multiphase powder. The pressing process includes dry pressing and cold isostatic pressing, with the dry pressing pressure being 8~12 MPa and the holding time being 1~3 minutes; the cold isostatic pressing pressure being 30~50 MPa and the holding time being 30~40 minutes.

7. The method for preparing a high thermal conductivity aluminum nitride ceramic according to claim 1, characterized in that, In step four, the glue removal is carried out in an air atmosphere, with a heating rate of 1~3 ℃ / min, and the temperature is held at 230 ℃, 320 ℃ and 600 ℃ for 1~2 hours respectively, followed by furnace cooling to room temperature; Secondary sintering includes: a) First stage sintering: heating from room temperature to 1500℃ and holding at that temperature for 1~2 h; wherein, the heating rate in the range from room temperature to 1200℃ is 5~10 ℃ / min, and the heating rate in the range from 1200 to 1500℃ is 1~3 ℃ / min. b) Second stage sintering: The temperature is raised from 1500 ℃ to 1750 ℃ ​​and held for 2~6 h at a rate of 1~3 ℃ / min. Then the temperature is lowered to 800 ℃ at a rate of 4~6 ℃ / min. After reaching 800 ℃, the temperature is cooled to room temperature with the furnace.

8. A method for preparing a high thermal conductivity aluminum nitride ceramic according to claim 1 or 7, characterized in that, In step four, the ceramic blank is placed in an alumina boat for debinding, and the alumina boat is lined with BN powder. The ceramic blank was placed in a graphite boat and sintered in a carbon tube furnace. BN ceramic sheets were placed both above and below the ceramic blank in the graphite boat, and the gaps between the BN ceramic sheets and the graphite boat were filled with BN powder. Sintering was carried out in a nitrogen atmosphere with a gas flow rate of 0.5~1L / min.

9. A high thermal conductivity aluminum nitride ceramic, characterized in that, The ceramic prepared by any one of claims 1 to 8 has a thermal conductivity of not less than 199 W / (m·K), a flexural strength of not less than 269 MPa, and a relative density of not less than 98%.

10. An application of a high thermal conductivity aluminum nitride ceramic, characterized in that, The high thermal conductivity aluminum nitride ceramic prepared according to any one of claims 1 to 9 can be used as an electronic substrate material to achieve heat dissipation of power electronic devices.

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