A method for low-temperature sintering of aluminum nitride ceramics

Abstract

This invention discloses a low-temperature sintering method for aluminum nitride ceramics, belonging to the field of ceramic sintering technology. The method includes: Step S1: Preparation of composite powder – Aluminum nitride powder and bifunctional eutectic additive powder are uniformly mixed to form a mixed powder. The bifunctional eutectic additive powder includes metal fluorides, alkaline earth metal compounds, and rare earth oxides, with a total addition amount of 3-10 wt% of the aluminum nitride powder mass; Step S2: Molding – The above mixed powder is dry-pressed or cold isostatically pressed to obtain a green body; Step S3: Sintering – The green body is placed in an atmosphere sintering furnace and sintered under a nitrogen atmosphere; Step S4: Cooling – After sintering, the cooling rate is controlled within the grain boundary phase precipitation temperature range to optimize the crystallization morphology of the grain boundary phase, and then cooled with the furnace. This application utilizes a designed low-eutectic additive system to generate a large amount of effective liquid phase at 1450-1550℃, reducing the main sintering temperature to 1550-1650℃, which is more than 200℃ lower than the sintering temperature of traditional Y2O3 additives, resulting in significant energy savings and reduced consumption.

Description

Technical Field

[0001] This invention relates to the field of ceramic sintering technology, and in particular to a low-temperature sintering method for aluminum nitride ceramics. Background Technology

[0002] Aluminum nitride ceramics are ideal materials for electronic packaging and heat dissipation, but their strong covalent bond characteristics make sintering extremely difficult, usually requiring temperatures above 1850°C. Adding sintering aids and using liquid-phase sintering can lower the temperature to 1750-1850°C, but it still faces problems such as high energy consumption, easy grain coarsening, and demanding equipment requirements.

[0003] Achieving low-temperature sintering of aluminum nitride has been a long-standing pursuit in the industry, but it faces fundamental challenges:

[0004] Insufficient driving force for densification: AlN lattice diffusion is extremely slow at low temperatures, making it difficult to densify through solid-state sintering.

[0005] The contradiction between thermal conductivity and low-temperature sintering: Low-temperature sintering often requires more or more complex additives to promote densification, which usually leads to a large amount of low thermal conductivity grain boundary second phase residue, severely degrading thermal conductivity. How to achieve densification at low temperatures while effectively "purifying" grain boundaries and reducing harmful phases is the biggest technical bottleneck;

[0006] Existing low-temperature systems have poor performance: Existing low-temperature sintering systems (such as those using strong fluxes like Li2O and B2O3) can significantly reduce sintering temperature, but they often introduce a glassy phase with high resistance and low thermal conductivity, or react violently with AlN to damage the matrix, causing the material to lose its application value.

[0007] Therefore, there is an urgent need to develop a new low-temperature sintering strategy that can significantly reduce the sintering temperature while ensuring the material's core performance, especially thermal conductivity, to meet the needs of high-end applications. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a low-temperature sintering method for aluminum nitride ceramics, which can prepare aluminum nitride ceramics with high density and high thermal conductivity in the low-temperature range.

[0009] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0010] A low-temperature sintering method for aluminum nitride ceramics, the low-temperature sintering method comprising the following steps:

[0011] Step S1: Preparation of composite powder

[0012] Aluminum nitride powder is uniformly mixed with bifunctional eutectic additive powder to form a mixed powder. The bifunctional eutectic additive powder includes metal fluorides, alkaline earth metal compounds and rare earth oxides, and its total addition amount is 3-10 wt% of the mass of aluminum nitride powder.

[0013] Among them, the metal fluoride is YF3 or / and CaF2, which functions to partially volatilize or decompose at a lower temperature to form gaseous fluoride, thus performing fluorination cleaning pretreatment on the alumina impurities on the surface of aluminum nitride powder particles in advance.

[0014] Alkaline earth metal compounds, including at least one of CaO, SrO, BaO or their precursors, are used as one of the main fluxing agents.

[0015] The rare earth oxide is at least one of Y2O3, La2O3, and CeO2, and is used to form a favorable grain boundary phase with the residue after cleaning.

[0016] The molar ratio of metal fluoride, alkaline earth metal compound, and rare earth oxide satisfies the following: during the sintering process, alkaline earth metal compound and rare earth oxide can preferentially form a binary or ternary eutectic liquid phase with a low eutectic temperature, and the addition of metal fluoride can further reduce the viscosity and surface tension of the eutectic liquid phase.

[0017] Step S2: Molding

[0018] The above-mentioned mixed powder was dry-pressed and cold isostatically pressed to obtain a raw blank;

[0019] Step S3: Sintering

[0020] The green blank was placed in an atmosphere sintering furnace and sintered under a nitrogen atmosphere according to the following procedure:

[0021] Step 1: Low-temperature eutectic infiltration and pre-densification

[0022] Heat to the first holding temperature T1 at a heating rate of 3-8℃ / min. The first holding temperature T1 is 1450-1500℃. Hold at the first holding temperature T1 for 30-120 minutes.

[0023] Step 2: Intermediate-temperature reaction and grain boundary phase regulation

[0024] After completing the first step, raise the temperature to the second holding temperature T2 at a heating rate of 2-5℃ / min. The second holding temperature T2 is 1550-1650℃. Hold at the second holding temperature T2 for 60-240 minutes.

[0025] Step S4: Cooling

[0026] After sintering, the cooling rate is controlled within the grain boundary phase precipitation temperature range to optimize the crystallization morphology of the grain boundary phase, and then the furnace is cooled.

[0027] Preferably, in step S1, the average particle size D50 of the aluminum nitride powder is 0.5 to 1.5 μm, and the oxygen content is less than or equal to 1.2 wt%.

[0028] Preferably, in step S1, the precursor is a CaO precursor, a SrO precursor, or a BaO precursor, where the CaO precursor is its corresponding carbonate, the SrO precursor is its corresponding carbonate, and the BaO precursor is its corresponding carbonate.

[0029] Preferably, in step S4, the cooling rate is 3 to 10 °C / min.

[0030] The above technical solution has the following beneficial effects:

[0031] Lowering the sintering temperature: By utilizing the designed eutectic additive system, a large amount of effective liquid phase is generated at 1450-1550℃, which reduces the main sintering temperature to 1550-1650℃, which is more than 200℃ lower than the sintering temperature of traditional Y2O3 additives, resulting in significant energy saving and consumption reduction.

[0032] Stable process and strong applicability: This method is based on a conventional atmosphere sintering furnace, does not require ultra-high temperature equipment or special pressure, has a wide process window, and is easy to achieve low-cost preparation of large-size and complex-shaped products. Detailed Implementation

[0033] The specific embodiments of the present invention will be further described below. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0034] A low-temperature sintering method for aluminum nitride ceramics specifically includes the following steps:

[0035] Preparation of composite powders:

[0036] Aluminum nitride powder with an average particle size D50 of 0.5–1.5 μm is ball-milled and uniformly mixed with bifunctional eutectic additive powder. The bifunctional eutectic additive powder is composed of metal fluorides, alkaline earth metal compounds, and rare earth oxides in a specific ratio, and its total addition amount is 3–10 wt% of the mass of aluminum nitride powder. Specifically, the average particle size D50 of the aluminum nitride powder is 0.5 μm, 1.5 μm, or 1.0 μm, and the oxygen content of the aluminum nitride powder is less than or equal to 1.2 wt%, specifically 1.2 wt%, 1.0 wt%, or 0.8 wt%. That is, the total mass of oxygen impurities in the form of Al2O3 and other forms in the aluminum nitride powder shall not exceed 1.2 wt%.

[0037] Among them, the metal fluoride is at least one of YF3 and CaF2. Its function is to partially volatilize or decompose at a lower temperature to form gaseous fluoride, and to pre-treat the Al2O3 impurities on the surface of AlN particles by "fluorination cleaning". Specifically, the metal fluoride is YF3 or CaF2, or a mixture of the two.

[0038] The alkaline earth metal compound is at least one of CaO, SrO, BaO or its precursor, and serves as one of the main fluxing agents. Specifically, the alkaline earth metal compound is any one or any two or more of CaO, SrO, BaO or its precursor, wherein its precursor is a CaO precursor, a SrO precursor, or a BaO precursor, which can be the corresponding carbonate, such as CaCO3 as the CaO precursor, SrCO3 as the SrO precursor, and BaCO3 as the BaO precursor.

[0039] The rare earth oxide is at least one of Y2O3, La2O3, and CeO2, used to form a favorable grain boundary phase with the residue after cleaning. Specifically, the rare earth oxide is any one or any two or more of Y2O3, La2O3, and CeO2.

[0040] The molar ratio of metal fluoride, alkaline earth metal compound, and rare earth oxide should satisfy the following: during sintering, the alkaline earth metal compound and rare earth oxide can preferentially form a binary or ternary eutectic liquid phase with a low eutectic temperature, and the addition of metal fluoride can further reduce the viscosity and surface tension of the eutectic liquid phase. Specifically, the molar ratio of metal fluoride, alkaline earth metal compound, and rare earth oxide is 1.2-1.8:2.2-2.8:1. The molar ratio of metal fluoride, alkaline earth metal compound, and rare earth oxide can be 1.5:2.5:1, 1.2:2.2:1, or 1.8:2.8:1.

[0041] forming:

[0042] The mixed powder is dry-pressed and cold isostatically pressed to form a raw blank;

[0043] sintering:

[0044] The green blank was placed in an atmosphere sintering furnace and sintered under a nitrogen atmosphere according to the following procedure:

[0045] Step 1: Low-temperature eutectic infiltration and pre-densification

[0046] The temperature is increased to the first holding temperature T1 at a rate of 3-8℃ / min. The first holding temperature T1 is 1450-1500℃, which is higher than the theoretical eutectic temperature of the eutectic system. The temperature is then held at the first holding temperature T1 for 30-120 minutes. Specifically, the temperature can be increased to the first holding temperature T1 at a rate of 3℃ / min or 8℃ / min. The first holding temperature T1 can be 1450℃, 1500℃, or 1480℃. After heating, the temperature is held at the first holding temperature T1 for 30 minutes, 120 minutes, or 75 minutes.

[0047] At this temperature, alkaline earth metal compounds and rare earth oxide components rapidly form a large amount of low-viscosity, high-fluidity eutectic liquid phase. With its excellent wettability, this liquid phase quickly penetrates into the gaps between aluminum nitride particles. This stage mainly completes the initial process of particle rearrangement and dissolution-precipitation, achieving rapid pre-densification of the billet. The relative density after densification can reach 92-96%.

[0048] Step 2: Intermediate-temperature reaction and grain boundary phase regulation

[0049] After completing the first step, the temperature is increased to the second holding temperature T2 at a rate of 2-5℃ / min. The second holding temperature T2 is 1550-1650℃. The temperature is then held at the second holding temperature T2 for 60-240 minutes. Specifically, the temperature is increased to the second holding temperature T2 at a rate of 2℃ / min or 5℃ / min. The second holding temperature T2 is 1550℃ or 1650℃, or it can be 1600℃. The temperature is then held at the second holding temperature T2 for 60 minutes or 240 minutes, or it can be held for 150 minutes.

[0050] Mechanism: After heating to the second holding temperature T2, the amount and fluidity of the liquid phase further increase, and the solubility of aluminum nitride in the liquid phase is significantly improved. During this stage, a full dissolution-precipitation process occurs, achieving final densification. The evolution of the liquid phase composition is as follows: During the second holding temperature T2, fluoride ions (from the residue or decomposition products of metal fluorides), alkaline earth metal ions (from alkaline earth metal compounds), oxygen impurities continuously extracted from the surface and lattice of aluminum nitride, and rare earth ions (from rare earth oxides) in the liquid phase undergo complex interfacial reactions. This reaction tends to generate high-melting-point, discretely distributed fluorinated rare earth aluminate crystalline phases, or promote the transformation of the grain boundary phase from a continuous glassy state to an isolated crystalline state. After cooling, these crystalline phases are distributed in the form of islands at the three-way grain boundaries, rather than encapsulating the grains.

[0051] cool down:

[0052] After sintering, the cooling rate is controlled (e.g., 3-10℃ / min) through the grain boundary phase precipitation temperature range to optimize the crystallization morphology of the grain boundary phase. Then, the furnace is cooled. Specifically, the cooling rate is controlled at 3-10℃ / min to reduce the temperature to 1200℃. Then, the furnace is cooled, specifically at 3℃ / min or 10℃ / min. After furnace cooling, the sintering of aluminum nitride ceramic is completed.

[0053] Example 1

[0054] A low-temperature sintering method for aluminum nitride ceramics, specifically including:

[0055] Composite powder: Take 100g of aluminum nitride powder (D50=1.0μm) with an oxygen content of 0.9%, add 5wt% of bifunctional eutectic additive based on the total addition amount of aluminum nitride powder. The bifunctional eutectic additive is a mixture of CaF2, CaO, and Y2O3, prepared according to a CaF2:CaO:Y2O3 molar ratio of 1.5:2.5:1. Ball mill the mixture for 12 hours and dry it.

[0056] Molding: The mixed powder is dry-pressed at 200MPa and then cold isostatically pressed at 300MPa to complete the molding process;

[0057] Sintering: Under a flowing N2 mixed with 3% H2 atmosphere;

[0058] The temperature is increased to the first holding temperature T1=1480℃ at a heating rate of 5℃ / min, and held for 60 minutes.

[0059] The temperature was increased to the second holding temperature T2 = 1600℃ at a heating rate of 3℃ / min, and held for 120 minutes.

[0060] The temperature was cooled to 1200℃ at a cooling rate of 5℃ / min, and then cooled in the furnace to complete the sintering.

[0061] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A low-temperature sintering method for aluminum nitride ceramics, characterized in that, The low-temperature sintering method includes the following steps: Step S1: Preparation of composite powder Aluminum nitride powder is uniformly mixed with bifunctional eutectic additive powder to form a mixed powder. The bifunctional eutectic additive powder includes metal fluorides, alkaline earth metal compounds, and rare earth oxides. The total amount of bifunctional eutectic additive powder added is 3-10 wt% of the mass of aluminum nitride powder. Among them, the metal fluoride is YF3 or / and CaF2, which functions to partially volatilize or decompose at a lower temperature to form gaseous fluoride, thus performing fluorination cleaning pretreatment on the alumina impurities on the surface of aluminum nitride powder particles in advance. Alkaline earth metal compounds, including at least one of CaO, SrO, BaO or their precursors, are used as one of the main fluxing agents. The rare earth oxide is at least one of Y2O3, La2O3, and CeO3, and is used to form a favorable grain boundary phase with the residue after cleaning. The molar ratio of metal fluoride, alkaline earth metal compound, and rare earth oxide satisfies the following: during the sintering process, alkaline earth metal compound and rare earth oxide can first form a binary or ternary eutectic liquid phase with a low eutectic temperature, and the addition of metal fluoride can further reduce the viscosity and surface tension of the eutectic liquid phase. Step S2: Molding The above-mentioned mixed powder was dry-pressed and cold isostatically pressed to obtain a raw blank; Step S3: Sintering The green blank was placed in an atmosphere sintering furnace and sintered under a nitrogen atmosphere according to the following procedure: Step 1: Low-temperature eutectic infiltration and pre-densification Heat to the first holding temperature T1 at a heating rate of 3-8℃ / min. The first holding temperature T1 is 1450-1500℃. Hold at the first holding temperature T1 for 30-120 minutes. Step 2: Intermediate-temperature reaction and grain boundary phase regulation After completing the first step, raise the temperature to the second holding temperature T2 at a heating rate of 2-5℃ / min. The second holding temperature T2 is 1550-1650℃. Hold at the second holding temperature T2 for 60-240 minutes. Step S4: Cooling After sintering, the cooling rate is controlled within the grain boundary phase precipitation temperature range to optimize the crystallization morphology of the grain boundary phase, and then the furnace is cooled.

2. The low-temperature sintering method for aluminum nitride ceramics according to claim 1, characterized in that, In step S1, the average particle size D50 of the aluminum nitride powder is 0.5 to 1.5 μm, and the oxygen content is less than or equal to 1.2 wt%.

3. The low-temperature sintering method for aluminum nitride ceramics according to claim 1, characterized in that, In step S1, the precursors are CaO precursor, SrO precursor, and BaO precursor, where CaO precursor is its corresponding carbonate, SrO precursor is its corresponding carbonate, and BaO precursor is its corresponding carbonate.

4. The low-temperature sintering method for aluminum nitride ceramics according to claim 1, characterized in that, In step S4, the cooling rate is 3 to 10 °C / min.