A method for producing aluminum nitride based on the conversion of aluminum compounds

Abstract

The application discloses a method for preparing aluminum nitride based on aluminum compound conversion, and belongs to the field of inorganic non-metallic powder material preparation. The method uses pseudo-boehmite and / or gamma-alumina as an aluminum source, builds a gradient distribution precursor with lower carbon content in the inner layer than in the outer layer through twice coating and controlling the amount of carbon source, and prepares aluminum nitride powder through carbothermal reduction nitridation. The outer layer rich in carbon reacts to generate a porous aluminum nitride shell layer, providing a mass transfer channel for the internal diffusion of nitrogen, and solving the bottleneck that the densification of the outer layer hinders the nitridation of the inner core in the uniform carbon coating scheme. In combination with the phased atmosphere regulation, the reaction process is optimized in the time and space dimensions. The obtained aluminum nitride powder has low oxygen content, narrow particle size distribution and uniform particle morphology, and the process conditions are mild, so the method has a good industrial amplification prospect.

Description

Technical Field

[0001] This invention relates to the field of inorganic non-metallic powder material preparation technology, and more specifically, to a method for preparing aluminum nitride powder by using aluminum oxide or hydroxide as an aluminum source and through a carbothermic reduction nitridation reaction. Background Technology

[0002] Aluminum nitride (AlN) holds an irreplaceable position in applications such as high-end electronic ceramic substrates, IGBT power module packaging, and high-power LED heat dissipation due to its high thermal conductivity, thermal expansion coefficient matching that of silicon, and excellent electrical insulation properties. With the continuous evolution of 5G communication, new energy vehicles, and third-generation semiconductor devices towards higher power densities, the market demand for high-quality aluminum nitride powder is not only reflected in production capacity but also in the increasingly stringent requirements for key indicators such as powder purity, particle size distribution, and microstructure.

[0003] Carbothermic reduction nitriding is one of the mainstream industrial routes for preparing aluminum nitride powder. This method typically uses alumina and a carbon source as starting materials, and involves a high-temperature solid-state reaction under a nitrogen atmosphere. The alumina is reduced by carbon and simultaneously nitrided to produce aluminum nitride. Compared to direct nitriding of metallic aluminum powder, carbothermic reduction nitriding has significant advantages in terms of lower raw material costs, a relatively wider process window, and ease of scalability, making it more popular in industry.

[0004] However, the carbothermic reduction nitriding method faces a long-standing technical bottleneck when applied to high-end applications: the high residual oxygen content in the product. The presence of oxygen impurities significantly degrades the thermal conductivity of aluminum nitride, making it difficult to meet the stringent requirements of high-end thermal management scenarios. The industry has conducted extensive research to address this issue.

[0005] One widely accepted improvement approach, disclosed in numerous patent documents, is to achieve molecular-level homogeneous mixing of carbon sources and alumina using a wet chemical method. The basic idea is to mix and dry water-soluble organic carbon sources (such as glucose, sucrose, polyvinyl alcohol, etc.) with alumina powder in an aqueous solution, allowing the carbon source to uniformly coat the surface of the alumina particles, forming a "carbon-coated alumina" composite precursor. Compared to traditional solid-phase grinding and mixing, this wet mixing method achieves substantial progress in the uniformity of carbon-aluminum contact, enabling the carbothermic reduction reaction to achieve a higher nitriding conversion rate at a lower temperature and in a shorter time.

[0006] However, the applicant noticed a phenomenon generally overlooked in the field during long-term R&D practice. Even with the wet chemical method achieving uniform coating of carbon source on the alumina surface, the oxygen content of the product still exhibits significant batch-to-batch fluctuations and inhomogeneities. Microscopic cross-sectional analysis of the nitridation products revealed that the carbothermic reduction nitridation reaction does not proceed synchronously and uniformly throughout the entire precursor particle, but rather exhibits an "outside-in" progression: the alumina in the outer region preferentially reacts with carbon to form aluminum nitride. As the reaction front advances inward, the outer aluminum nitride shell itself constitutes a physical barrier to the inward diffusion of nitrogen. Nitrogen molecules need to penetrate this continuously thickening aluminum nitride shell to reach the internal reaction interface. When the outer aluminum nitride shell becomes dense to a certain extent, the inward mass transfer of nitrogen is severely hindered, resulting in insufficient nitridation of alumina in the particle core region. The remaining alumina or the incompletely nitrided aluminum-oxygen-nitrogen intermediate phase becomes the main source of oxygen impurities in the product.

[0007] It is evident that the problem does not lie in the widely discussed aspect of "insufficient uniformity in the mixing of carbon source and alumina," but rather in the less-addressed technical detail of "establishing and maintaining nitrogen mass transfer channels during the reaction after sufficient uniformity of mixing." In the precursor design with uniform carbon coating, the carbon on the entire particle surface reacts and is consumed simultaneously with alumina in the early stages of the reaction. The surface area almost synchronously transforms into aluminum nitride and densifies, which actually accelerates the closure of the internal nitrogen mass transfer channels, thus exacerbating the problem of insufficient core nitriding.

[0008] Furthermore, during carbothermic reduction nitridation, the oxidative removal of carbon results in the escape of CO gas. The intensity of this gas escape directly affects the microstructure and particle size distribution of the product: excessively concentrated gas escape can lead to particle breakage and increased fine powder, while poor gas escape may result in closed pores within the particles. Current technologies lack effective means to control this gas escape behavior, often sacrificing powder morphology uniformity in exchange for lower oxygen content.

[0009] Therefore, how to pre-construct a channel that can guide nitrogen gas to continuously transfer mass into the particle during the reaction process at the precursor structure design level, while keeping the gas escape behavior stable and controllable throughout the entire reaction process, so as to synergistically achieve aluminum nitride powder with low oxygen content, narrow particle size distribution and uniform particle morphology, is a key technical problem that urgently needs to be solved in the current carbothermal reduction nitridation method for preparing high-quality aluminum nitride powder. Summary of the Invention

[0010] In view of this, the present invention proposes a method for preparing aluminum nitride based on the conversion of aluminum compounds. The technical concept of the present invention is not to further pursue the "uniform" distribution of carbon source in the precursor, but to do the opposite, to introduce a "controllable non-uniformity" in the precursor—to make the carbon content increase in a gradient distribution from the inside to the outside along the radial direction of the alumina particles, and to take advantage of the preferential occurrence of carbothermic reduction nitridation reaction in the outer carbon-rich region, thus "self-creating" porous channels in situ for nitrogen to diffuse inward, creating mass transfer conditions for the subsequent nitridation reaction in the core region.

[0011] Specifically, the higher carbon content in the outer layer ensures sufficient carbon source to react with alumina in the early stages of the reaction, rapidly generating aluminum nitride on the particle surface and near-surface region, while carbon is consumed and removed, forming a porous aluminum nitride shell with numerous interconnected channels. This porous shell differs from the dense aluminum nitride shell in a uniform carbon coating scheme; it is structurally "permeable," providing continuous mass transfer channels for nitrogen molecules to diffuse inward. The lower carbon content in the inner layer serves two purposes: first, it reduces the dependence of the inner layer reaction on carbon, avoiding the problem of excessive porosity and decreased particle strength left by excessive carbon consumption in the inner layer; second, it allows the reaction front to advance at a relatively uniform rate from the outside in, rather than the core completely stagnating after the surface is "blocked." As the reaction progresses into the middle and later stages, the outer carbon has been largely consumed and has completed its "pore-forming mission," allowing nitrogen to be continuously transported inward through these formed channels, completing the nitriding reaction of the core layer by layer.

[0012] It should be noted that the above description of the porous shell formation mechanism is a technical model proposed by the applicant based on extensive experimental observations and summaries. Its correctness can be objectively verified through the comparison of product performance data between the embodiments and comparative examples of this invention. For example, in the reaction intermediates after only completing the first stage of activation treatment, the BET specific surface area of ​​Example 1 is about 35% higher than that of Comparative Example 1, and the BJH mesoporous pore volume is about 50% higher, which macroscopically confirms the existence of the outer porous shell and its direct correlation with the carbon content gradient. No matter how the mechanism model itself is further modified and improved, it will not affect those skilled in the art from reproducing the technical effects claimed by this invention using the technical means disclosed in this specification.

[0013] The technical solution of this invention is implemented as follows: This invention provides a method for preparing aluminum nitride based on the conversion of aluminum compounds, comprising: Provide aluminum source particles, wherein the aluminum source is selected from boehmite and / or γ-alumina; The aluminum source particles are contacted with an aqueous solution of a first carbon source and then dried to form a carbon-containing inner coating layer on the surface of the aluminum source particles. The particles with the inner coating layer are contacted with an aqueous solution of a second carbon source and dried to form a carbon-containing outer coating layer, thus obtaining a precursor. In forming the inner coating layer and the outer coating layer, the amount of the first carbon source and the second carbon source is controlled so that the mass ratio of carbon elements to aluminum source particles in the inner coating layer is less than the mass ratio of carbon elements to aluminum source particles in the outer coating layer. The precursor was subjected to carbothermal reduction nitriding treatment in a nitrogen-containing atmosphere to obtain aluminum nitride powder.

[0014] The core feature of this invention lies in the gradient design of the carbon content in the precursor. The term "gradient distribution increasing from the inside out" refers to the fact that, radially along a precursor particle, the carbon content is relatively low on the side closer to the aluminum source particle surface (inner layer) and relatively high on the side farther from the aluminum source particle surface (outer layer). It should be noted that the "inner layer" and "outer layer" are identifiable structural layers formed by two coating layers, and do not require a clear interface between them. In actual fabrication, there may be a certain degree of carbon diffusion and a gradual transition zone of carbon content between the two coating layers, which also falls within the scope of the "gradient distribution" defined in this invention.

[0015] Regarding the selection of the aluminum source, this invention preferably uses boehmite and / or γ-alumina. Boehmite (AlOOH·nH2O) has a large specific surface area and abundant surface hydroxyl groups, exhibiting good adsorption affinity for water-soluble carbon sources. During the two coating processes, it can more firmly bind carbon source molecules, making it less prone to large-scale migration and redistribution of the carbon source after drying, thus helping to maintain the designed carbon content gradient. γ-alumina, on the other hand, has high reactivity, which is beneficial for lowering the onset temperature of carbothermic reduction nitridation.

[0016] In some embodiments, the mass ratio of carbon to aluminum source particles in the inner coating layer is 3% to 8%, and the mass ratio of carbon to aluminum source particles in the outer coating layer is 10% to 20%.

[0017] The aforementioned carbon content range was determined through systematic orthogonal experimental optimization, with the following technical considerations: If the inner layer carbon content is below 3%, it cannot provide sufficient reduction driving force for the nitridation of the inner alumina, resulting in a significant decrease in the core nitridation completion rate. If the inner layer carbon content is above 8%, the amount of CO gas consumed during the reaction increases, easily forming excessive porosity in the inner layer, affecting the overall strength of the particles. It also makes the carbon content gradient between the inner and outer layers insufficiently significant, weakening the control effect of the carbon gradient design on the reaction front advancement. If the outer layer carbon content is below 10%, the porosity of the aluminum nitride shell formed in the early stages of the reaction is insufficient, limiting its promoting effect on nitrogen mass transfer inward. If the outer layer carbon content is above 20%, the outer layer reaction is too vigorous, and the concentrated escape of CO gas easily leads to particle breakage, which is detrimental to particle size distribution control. Within the above range, adjustments can be made according to the specific surface area and particle size of the aluminum source.

[0018] It should be noted that when the carbon content of the inner layer approaches the upper limit of the range (e.g., close to 8%) and the carbon content of the outer layer approaches the lower limit of the range (e.g., close to 10%), the carbon content gradient between the inner and outer layers becomes relatively gentle. Under this gentle gradient condition, although the effects of pore formation in the outer layer and nitriding in the inner layer are still better than those of the uniform carbon coating scheme, the improvement will tend to narrow. This is not a denial of the invention, but a natural manifestation of the gradient effect: the more significant the gradient, the more prominent the effect. The preferred ranges of 3%~8% and 10%~20% provided by the applicant are determined under the premise of ensuring the significance of the gradient (the difference in carbon content between the inner and outer layers is not less than about 2 percentage points), while taking into account factors such as reaction driving force and morphology control. After reading this specification, those skilled in the art can independently select a suitable ratio of inner and outer layer carbon content within the above range according to the specific requirements of the target product performance.

[0019] In some embodiments, the first carbon source and the second carbon source are each independently selected from at least one of glucose, sucrose, and polyvinyl alcohol.

[0020] Glucose and sucrose are preferred water-soluble organic carbon sources. Their high solubility in water allows them to be uniformly adsorbed onto the surface of alumina particles in molecular form, forming a uniform carbon source film with controllable thickness after drying. Polyvinyl alcohol exhibits good film-forming properties after drying, which is beneficial for forming a continuous coating layer, making it particularly suitable as a carbon source for the outer coating layer. In the two coating processes, the first and second carbon sources can be the same or different substances to meet different requirements for coating layer thickness, carbon content, and film quality. It should be noted that different carbon sources have inherent differences in thermal decomposition behavior, residual carbon rate, and microstructure of the coating, but this does not preclude the application of the core design principle of carbon content gradient. When using different types of carbon sources, those skilled in the art should understand and implement this invention based on the actual mass ratio of carbon elements, rather than simply considering the carbon source raw materials as equal in mass.

[0021] In some embodiments, after obtaining the precursor and before performing carbothermic reduction nitriding, the process further includes: pre-carbonizing the precursor at 300-600°C under an inert atmosphere to carbonize the carbon-containing coating layer. Further, the inert atmosphere for the pre-carbonization treatment is nitrogen or argon, and the pre-carbonization temperature is preferably 400-600°C.

[0022] The role of pre-carbonization in this invention is not simply that of conventional pretreatment. For carbon content gradient precursors, the special significance of pre-carbonization lies in the fact that when organic carbon sources (such as glucose and sucrose) directly enter the high-temperature nitriding section, they undergo violent thermal decomposition within a narrow temperature window, releasing a large amount of volatile gases in a short time. If these gases escape in a concentrated manner, they can disrupt the coating structure of the precursor particles, even causing localized blistering and detachment of the coating layer, thus destroying the established carbon content gradient. This is precisely the unique vulnerability of carbon content gradient precursors compared to uniformly carbon-coated precursors—uniform carbon coating only needs to ensure the carbon content meets the standard, while carbon content gradient precursors also need to additionally protect the spatial distribution relationship of the inner and outer carbon layers from being disrupted by the thermal decomposition process. Pre-carbonization advances these thermal decomposition processes to a lower temperature and a controlled atmosphere, thus protecting the integrity of the carbon gradient structure. Furthermore, the amorphous carbon formed after pre-carbonization has higher reactivity, which is beneficial for lowering the starting temperature of the subsequent carbothermic reduction nitriding reaction.

[0023] In some embodiments, the carbothermic reduction nitriding treatment is performed in the following stages: The first stage involves treatment at 800~1100℃ in a nitrogen atmosphere containing 0.5%~3% hydrogen by volume. The second stage involves treatment at 1100~1400℃ in a pure nitrogen atmosphere. The third stage involves treatment at 1400~1550℃ in a nitrogen atmosphere containing 0.5%~2% ammonia by volume.

[0024] The aforementioned phased atmosphere control strategy is a further refinement of the carbon content gradient precursor design in this invention, and the two have a synergistic effect. The design considerations and synergistic principles of each stage are explained below.

[0025] The first stage (800~1100℃, nitrogen + hydrogen): This stage is defined as the "activation and pore-forming stage." Hydrogen at high temperatures has a certain activating effect on the alumina surface, which helps to lower the energy barrier of the carbon-alumina interface reaction. More importantly, hydrogen molecules are small in size and have a high diffusion coefficient, allowing them to quickly penetrate the microporous structure of the precursor particles and preferentially enter the carbon-alumina reaction interface in the shallow region of the particles. In this stage, the carbothermic reduction reaction starts at a relatively mild rate. The hydrogen-assisted reducing microenvironment promotes the initial solid-solid reaction between carbon and alumina. The carbon source begins to be consumed at a controllable rate, generating CO. Microporous structures gradually form on the particle surface, "pre-opening" channels for the subsequent deep diffusion of nitrogen. This is highly compatible with the structural design of the carbon-content gradient precursor: the outer carbon-rich region provides sufficient carbon source supply for this stage, enabling surface pore formation with a relatively low carbon consumption ratio.

[0026] The second stage (1100~1400℃, pure nitrogen): This stage is the main reaction section of carbothermic reduction nitridation. The reaction between alumina and carbon and the nitridation reaction occur simultaneously, which is the stage for the formation of the main crystalline phase of aluminum nitride in the product. At this time, the porous surface structure formed in the first stage begins to play its role: nitrogen gas diffuses continuously inward through these pre-established channels, contacting the reaction front. The distribution characteristic of carbon decreasing from the outside to the inside in the carbon content gradient precursor guides the reaction front to advance radially layer by layer inward, rather than concentrating and stagnating at a certain point on the surface. A pure nitrogen atmosphere is used in this stage because: at this time, the reaction system is in the temperature range with the highest carbothermic reduction activity, and any excess auxiliary atmosphere may interfere with the chemical equilibrium of the carbothermic reduction nitridation reaction. Pure nitrogen can ensure that the main reaction proceeds according to the expected stoichiometric relationship.

[0027] In this stage, the heating rate is controlled at 2–5 °C / min. The technical significance lies in maintaining a dynamic balance between the carbon oxidation rate and the CO gas escape rate. Too rapid a heating rate leads to excessive carbon oxidation per unit time, causing the CO gas generation rate to exceed its diffusion rate through the pores, creating a high-pressure zone inside the particles, which can severely lead to particle breakage. Too slow a heating rate, on the other hand, reduces the economic efficiency of the entire process cycle. A heating rate window of 2–5 °C / min represents the empirically optimal range for achieving a reasonable balance between stable reaction progress and production efficiency.

[0028] The third stage (1400~1550℃, nitrogen + ammonia): In the middle and late stages of carbothermic reduction nitridation, the outer carbon source has been largely consumed, and the carbon-depleted conditions in the core region make the nitridation of the core alumina more thermodynamically and kinetically difficult. Introducing 0.5%~2% volume fraction of ammonia at this stage utilizes the high-temperature decomposition of ammonia to generate more reactive nitrogen-containing species (such as NH2·, NH·, and N· radicals). These reactive nitrogen-containing species have stronger penetrating power and reactivity than nitrogen molecules, enabling them to enter micro-regions inside the particles that are difficult for nitrogen molecules to reach, thus promoting the final nitridation of the residual alumina in the core. This forms a precise functional coupling with the carbon-depleted inner layer structure of the carbon content gradient precursor: the low carbon content in the inner layer makes it more dependent on ammonia-assisted nitridation, and the introduction of ammonia in the third stage precisely meets this requirement.

[0029] It is particularly important to note that the use of hydrogen and ammonia in the aforementioned three-stage atmosphere control strategy is a known technique in the field. The fundamental difference between this invention and other methods lies in the timing and synergistic logic employed: instead of using hydrogen and ammonia as the atmosphere throughout the entire reaction process, this invention, based on a refined understanding of the kinetic characteristics of each stage of the carbothermic reduction nitridation reaction, precisely positions a specific auxiliary atmosphere at the most valuable and irreplaceable reaction stage. This "temporal programming" design of the atmosphere, in conjunction with the "spatial programming" design of the carbon content gradient precursor, constitutes a precise time- and space-dimensional reaction process control system. The carbon content gradient, spatially, predetermines the advancement of the reaction front, while the staged atmospheres, temporally, provide a matching chemical driving force for the phased advancement of the reaction front; the absence of either significantly diminishes the effectiveness.

[0030] Regarding the contribution weights between staged atmosphere control and carbon content gradient, Table 1 provides a clear answer: In the context of carbothermic reduction nitridation for AlN preparation, the carbon content gradient design plays a fundamental and prerequisite role—it fundamentally solves the inherent defect of closed nitrogen mass transfer channels in uniform carbon coating schemes; staged atmosphere control, on the other hand, further optimizes the kinetic conditions of each reaction stage based on the already opened mass transfer channels by the carbon gradient, playing a deepening and auxiliary role. The technical effects of the two are not simply a linear sum, but rather exhibit a positive synergistic characteristic.

[0031] In some embodiments, the carbothermic reduction nitriding treatment further includes: heat treatment at 500-700°C in an oxygen-containing atmosphere to remove residual carbon.

[0032] This decarbonization step is used to remove unreacted carbon remaining in the product after the carbothermic reduction nitridation reaction. It should be noted that the design of the lower carbon content in the inner layer of the carbon content gradient precursor ensures that the residual carbon is mainly concentrated in the outer layer after the reaction, which facilitates oxygen contact and oxidation removal. This is an additional advantage of the carbon content gradient design in subsequent processes.

[0033] In some embodiments, the aluminum nitride powder prepared by the method of the present invention exhibits a slight increase in carbon residue radially from the inside to the outside in terms of elemental distribution across its particle cross-section. This structural imprint is a natural remnant of the carbon content gradient precursor after carbothermic reduction nitridation and can serve as a potential distinguishing feature from products prepared by the uniform carbon coating method. Of course, the significance of this feature depends on the specific parameters of the carbothermic reduction nitridation process and the degree of decarbonization treatment.

[0034] The present invention has the following advantages over the prior art: This invention addresses a crucial technical detail in the carbothermic reduction nitridation reaction process: nitrogen mass transfer. This process, which has long been neglected in the field, breaks through the traditional approach of pursuing uniform carbon source distribution on the alumina surface. For the first time, it proposes and implements a precursor structure and its preparation method characterized by a gradient carbon content distribution from the inside out. This gradient carbon content design allows the outer carbon-rich region to preferentially react in the early stages of the reaction, forming a porous aluminum nitride shell layer in situ that allows nitrogen to diffuse inward. This creates a critical mass transfer channel for subsequent nitridation in the core region, fundamentally improving upon the inherent defect in uniform carbon coating schemes where rapid densification of the outer layer hinders nitrogen diffusion. Simultaneously, the lower carbon content in the inner layer ensures that the reaction front can continuously and uniformly advance inward radially, preventing stagnation in core nitridation. Building upon this foundation, the present invention further introduces a staged atmosphere control strategy. Based on the differentiated kinetic characteristics of the carbothermic reduction nitriding reaction in its three stages—activation and pore formation, main nitriding, and deep completion—hydrogen, pure nitrogen, and ammonia are respectively positioned at the specific reaction stages where their respective functions are most demanding. This, combined with the spatial structure design of the carbon content gradient precursor, forms a close spatiotemporal synergy, achieving precise control of the reaction process across all dimensions. This synergistic approach does not rely on extreme reaction temperatures or excessively long reaction times, and can obtain high-quality aluminum nitride powder with low oxygen content, low carbon residue, narrow particle size distribution, and uniform particle morphology under relatively mild and economical process conditions, demonstrating good feasibility for industrial scale-up and promising practical applications. Detailed Implementation

[0035] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0036] In the following examples and comparative examples, all raw materials used were industrial-grade or chemically pure reagents unless otherwise specified, and all water used was deionized water. The phase composition of the aluminum nitride powder was determined by X-ray diffraction analysis, the oxygen and carbon contents were determined by an oxygen-nitrogen-hydrogen analyzer and a carbon-sulfur analyzer, the particle size distribution of the powder was determined by a laser particle size analyzer, and the microstructure was observed by a scanning electron microscope. The BET specific surface area and BJH pore size distribution were determined by a nitrogen adsorption-desorption analyzer.

[0037] Example 1 This embodiment provides a method for preparing a carbon content gradient distribution precursor and the preparation of aluminum nitride powder thereon.

[0038] (I) Precursor Preparation 100 g of boehmite powder (AlOOH·nH2O, specific surface area approximately 280 m² / g, median particle size D50 approximately 15 μm) was weighed and dispersed in 300 mL of deionized water. The mixture was stirred for 30 min to form a homogeneous suspension. 6 g of glucose (equivalent to approximately 3.3 wt% of the carbon equivalent of the boehmite) was added to the solution, and stirring was continued for 60 min to allow the glucose to fully dissolve and adsorb onto the surface of the boehmite particles. The resulting slurry was placed in a forced-air drying oven and dried at 80 °C for 12 h to obtain boehmite powder with an inner carbon coating.

[0039] The powder was redispersed in 300 mL of deionized water, and 25 g of glucose (equivalent to approximately 13.8 wt% of the carbon equivalent of boehmite) was added to the solution. After stirring for 60 min, the mixture was dried at 80 °C for 12 h to obtain a precursor with a gradient distribution of carbon content. In this precursor, the carbon equivalent of the inner coating layer was approximately 3.3 wt%, and the carbon equivalent of the outer coating layer was approximately 13.8 wt%, with the carbon content increasing radially from the inside to the outside of the particle.

[0040] The obtained precursor was placed in a tube furnace and pre-carbonized under a flowing nitrogen atmosphere at a rate of 5°C / min to 500°C and held for 2 hours to convert the organic carbon source into amorphous carbon.

[0041] (ii) Carbothermic reduction nitriding The pre-carbonized precursor is placed in an atmosphere tube furnace, with a flowing atmosphere throughout the process. The total gas flow rate is controlled at 1 L / min. Carbothermic reduction nitriding is then performed according to the following three-stage procedure: First stage: Increase the temperature to 950℃ at 10℃ / min and keep it at that temperature for 1.5 h in a mixed atmosphere of N2 + 2 vol% H2; Second stage: Switch to pure N2 atmosphere, raise the temperature from 950℃ to 1350℃ at 3℃ / min, and keep it in pure N2 atmosphere for 3h; Third stage: Switch to a mixed atmosphere of N2 + 1 vol% NH3, and heat from 1350℃ to 1500℃ at a rate of 5℃ / min, and hold for 2 hours.

[0042] After the reaction was completed, the mixture was naturally cooled to room temperature under a N2 atmosphere to obtain the nitrided product.

[0043] (III) Carbon removal treatment The above nitriding product was placed in a muffle furnace and heated to 650°C at 5°C / min in air atmosphere and held for 3 h to remove residual carbon, thus obtaining aluminum nitride powder.

[0044] Example 2 The difference between this embodiment and Embodiment 1 is that the ratio of carbon content in the inner and outer layers has been adjusted.

[0045] (I) Precursor Preparation The inner coating used 8 g of glucose (approximately 4.4 wt% carbon equivalent), and the outer coating used 30 g of glucose (approximately 16.5 wt% carbon equivalent). The remaining operations were the same as in Example 1.

[0046] (ii) Carbothermic reduction nitriding and decarbonization treatment are the same as in Example 1.

[0047] Example 3 The difference between this embodiment and Embodiment 1 lies in the change of the type of carbon source and the adjustment of the pre-carbonization atmosphere.

[0048] (I) Precursor Preparation The inner coating used 5 g of sucrose (approximately 2.1 wt% carbon equivalent), and the outer coating used 24 g of polyvinyl alcohol (PVA-1788, approximately 13.2 wt% carbon equivalent). The pre-carbonization treatment was carried out in an argon atmosphere, and the remaining operations were the same as in Example 1.

[0049] (ii) Carbothermic reduction nitriding and decarbonization treatment are the same as in Example 1.

[0050] It should be noted that in Example 3, polyvinyl alcohol was used as the carbon source for the outer layer, unlike the glucose used in Examples 1 and 2. Polyvinyl alcohol, as a high-molecular-weight carbon source, inherently differs from glucose in its thermal decomposition behavior and film-forming properties. This is a normal effect resulting from the change in carbon source type, and not an effect of the carbon content gradient itself. Those skilled in the art should consider the objective existence of this difference in carbon source type when comparing the data of Example 3 with other examples. The setup in Example 3 illustrates that the carbon content gradient principle defined in this invention is feasible under different combinations of carbon sources, and is not limited to a specific type of carbon source.

[0051] Example 4 The difference between this embodiment and Embodiment 1 is that different process parameters are used in the carbothermic reduction nitriding stage.

[0052] The carbothermic reduction nitriding treatment is carried out in the following three stages: First stage: Increase the temperature to 850℃ at 10℃ / min and keep it at that temperature for 2 h in a mixed atmosphere of N2 + 1 vol% H2; Second stage: Switch to pure N2 atmosphere, raise the temperature from 850℃ to 1300℃ at 2℃ / min, and keep it in pure N2 atmosphere for 4 hours; Third stage: Switch to a mixed atmosphere of N2 + 1.5 vol% NH3, and heat from 1300℃ to 1450℃ at a rate of 5℃ / min, and hold for 3 h.

[0053] The precursor preparation and decarbonization process were the same as in Example 1.

[0054] Comparative Example 1 This comparative example corresponds to a representative scheme of "uniform carbon coating" in the prior art, and is used to compare with the embodiments of the present invention to highlight the technical effect brought about by the gradient distribution of carbon content.

[0055] (I) Precursor Preparation 100 g of boehmite powder was weighed and dispersed in 300 mL of deionized water, and stirred for 30 min to form a uniform suspension. 31 g of glucose (total carbon equivalent equivalent to the sum of the inner and outer carbon equivalents in Example 1, approximately 17.1 wt%) was added at once, and the mixture was stirred for 60 min and then dried at 80 °C for 12 h to obtain a precursor uniformly coated with carbon source. In this precursor, the carbon source is uniformly distributed in a monolayer on the surface of the alumina particles, and there is no radial gradient of carbon content.

[0056] The pre-carbonization conditions were the same as in Example 1: the temperature was increased to 500°C at a rate of 5°C / min and held for 2 hours under a flowing nitrogen atmosphere.

[0057] (ii) Carbothermic reduction nitriding The same three-stage carbothermal reduction nitridation procedure as in Example 1 was used to compare the advantages of the carbon gradient structure over the uniform structure under the same atmosphere conditions.

[0058] (iii) The carbon removal treatment is the same as in Example 1.

[0059] Comparative Example 2 This comparative example is used to examine the performance of the "carbon gradient precursor" under conditions without staged atmosphere control, in order to verify the synergistic effect of staged atmosphere control on the carbon gradient precursor.

[0060] (I) Precursor Preparation The same carbon content gradient distribution precursor and pre-carbonization treatment as in Example 1 were used.

[0061] (ii) Carbothermic reduction nitriding The precursor was placed in a tube furnace and a pure N2 atmosphere was introduced throughout the process. The temperature was increased from room temperature to 1500°C at a rate of 5°C / min, and held at 1500°C for 6 hours (the total high-temperature period was equivalent to the sum of the three holding times in Example 1). After the reaction was completed, the mixture was allowed to cool naturally under an N2 atmosphere.

[0062] (iii) The carbon removal treatment is the same as in Example 1.

[0063] Comparative Example 3 This comparative example is used to examine the performance of "uniform carbon-coated precursor" under non-staged atmosphere conditions, serving as a comprehensive benchmark reference for existing technologies.

[0064] (I) Precursor Preparation The same uniform carbon-coated precursor and pre-carbonization treatment as Comparative Example 1 were used.

[0065] (ii) Carbothermic reduction nitriding The carbothermal reduction nitriding process was carried out in the same pure N2 atmosphere as Comparative Example 2, with a one-step temperature increase to 1500℃.

[0066] (iii) The carbon removal treatment is the same as in Example 1.

[0067] Comparative Example 4 This comparative example is used to examine the technical effect of using only a single coating but with opposite carbon distributions in the inner and outer layers (i.e., carbon-poor outer layer and carbon-rich inner layer).

[0068] (I) Precursor Preparation 100 g of boehmite powder was weighed and dispersed in 300 mL of deionized water, and stirred for 30 min to form a homogeneous suspension. 25 g of glucose (approximately 13.8 wt% carbon equivalent) was added to the solution, and the mixture was stirred for 60 min and then dried at 80 °C for 12 h to obtain boehmite powder with a carbon-rich inner layer. This powder was then dispersed in 300 mL of deionized water, and 6 g of glucose (approximately 3.3 wt% carbon equivalent) was added. After stirring for 60 min, the mixture was dried at 80 °C for 12 h to obtain a reverse gradient precursor with a carbon-depleted outer layer and a carbon-rich inner layer.

[0069] The pre-carbonization treatment was the same as in Example 1.

[0070] (ii) Carbothermic reduction nitriding The same three-stage carbothermal reduction nitridation procedure as in Example 1 was used.

[0071] (iii) The carbon removal treatment is the same as in Example 1.

[0072] Comparative Example 5 This comparative example is used to examine the technical effect when the carbon content of the inner and outer layers is the same (i.e., although a two-coating process is used, a uniform carbon distribution is actually formed), in order to verify that the "gradient" itself, rather than the "two-coating" process, is the fundamental reason for the difference in effect.

[0073] (I) Precursor Preparation 100 g of boehmite powder was weighed and dispersed in 300 mL of deionized water, and stirred for 30 min to form a homogeneous suspension. 15.5 g of glucose (approximately 8.6 wt% carbon equivalent) was added to the solution, and the mixture was stirred for 60 min and then dried at 80 °C for 12 h to obtain boehmite powder with an inner carbon coating. This powder was then dispersed again in 300 mL of deionized water, and 15.5 g of glucose (approximately 8.6 wt% carbon equivalent) was added. After stirring for 60 min, the mixture was dried at 80 °C for 12 h to obtain a precursor with the same carbon content in both the inner and outer layers, exhibiting no gradient. Although this precursor underwent two coating processes, the carbon content showed no significant difference along the radial direction.

[0074] The pre-carbonization treatment was the same as in Example 1.

[0075] (ii) Carbothermic reduction nitriding The same three-stage carbothermal reduction nitridation procedure as in Example 1 was used.

[0076] (iii) The carbon removal treatment is the same as in Example 1.

[0077] The aluminum nitride powders prepared in Examples 1-4 and Comparative Examples 1-5 were characterized in terms of performance, and the results are summarized in Table 1.

[0078] Table 1 Performance indicators of AlN powder obtained in each embodiment and comparative example

[0079] The technical effects of the present invention and the underlying technical logic can be clearly seen from the data in Table 1.

[0080] Examples 1-4 all yielded aluminum nitride powders with an oxygen content of less than 0.6 wt%, a carbon content of less than 400 ppm, a median particle size D50 in the range of 1.5-2.1 μm, a particle size distribution width of no more than 1.5, and a particle morphology mainly of near-spherical or equiaxed shape, with good dispersibility.

[0081] Comparing Example 1 with Comparative Example 1, both employed the exact same three-stage carbothermal reduction nitriding procedure and the same total carbon content. The only difference lay in the carbon distribution of the precursor—Example 1 featured a gradient carbon content distribution, while Comparative Example 1 had a uniform carbon content distribution. The oxygen content in Example 1 was 0.42 wt%, while that in Comparative Example 1 was as high as 0.85 wt%. This difference clearly demonstrates the significant advantage of a gradient carbon content distribution in improving the core nitriding completion rate: in a gradient distribution, the porous aluminum nitride shell formed by the preferential reaction of the outer carbon-rich region provides an effective channel for nitrogen diffusion, whereas in a uniform distribution, the outer carbon reacts simultaneously with alumina to rapidly form a dense aluminum nitride shell, hindering nitrogen mass transfer inward, resulting in insufficient core nitriding and a high oxygen content. The partial sintering and adhesion phenomenon observed in the particle morphology of Comparative Example 1 is also related to the stagnation of core nitriding in the later stages of the reaction and the localized sintering of residual alumina with aluminum nitride at high temperatures.

[0082] Comparing Example 1 and Comparative Example 5, both employed a two-coating process, resulting in comparable total carbon content (17.1 wt% in Example 1, 17.2 wt% in Comparative Example 5). The only difference lay in the carbon content of the inner and outer layers in Example 1 (3.3 wt% vs. 13.8 wt%), while the carbon content of the inner and outer layers in Comparative Example 5 was the same (8.6 wt%). The oxygen content of Comparative Example 5 was 0.79 wt%, slightly better than Comparative Example 1 (0.85 wt%), but significantly worse than Example 1 (0.42 wt%). This comparison clearly demonstrates that the improvement in technical performance does not stem from the "two-coating" process itself, but rather from the substantial characteristic of the carbon content gradient formed by the two coatings. Because Comparative Example 5 had the same carbon content in both the inner and outer layers, it was essentially equivalent to a single uniform coating, failing to create a reaction pattern where the outer layer was carbon-rich, preferentially forming pores, while the inner layer was carbon-poor, uniformly advancing. Therefore, its effect was only slightly better than a single uniform coating, far inferior to Example 1, which had a significant gradient.

[0083] Comparing Example 1 and Comparative Example 2, both used the exact same carbon content gradient precursor, the difference being the carbothermic reduction nitridation procedure—Example 1 employed staged atmosphere control, while Comparative Example 2 used a one-step heating process with pure nitrogen throughout. Example 1 showed better oxygen content (0.42 wt%) and particle size distribution width (1.3) than Comparative Example 2 (0.72 wt% oxygen content, 1.7 wt% particle size distribution width). This indicates a significant synergistic effect between the carbon content gradient precursor and staged atmosphere control: the carbon gradient structure provides the material basis for the progressive advancement of the reaction front in the spatial dimension, but without the staged atmosphere providing matching chemical driving forces for each reaction stage in the temporal dimension, the potential of the carbon gradient structure cannot be fully realized. Furthermore, Comparative Example 2 exhibited broken fine powder in its particle morphology, due to the concentrated escape of CO gas during the one-step heating process, a problem effectively avoided by the staged heating design in Example 1.

[0084] A horizontal comparison of Comparative Examples 1, 2, and 3 reveals a significant trend: Comparative Example 1 (uniform carbon coating + staged atmosphere, oxygen content 0.85 wt%) is superior to Comparative Example 3 (uniform carbon coating + pure nitrogen throughout, oxygen content 1.25 wt%), and Comparative Example 2 (carbon gradient + pure nitrogen throughout, oxygen content 0.72 wt%) is superior to Comparative Example 3. However, Comparative Example 2 (carbon gradient + pure nitrogen throughout) is even superior to Comparative Example 1 (uniform carbon coating + staged atmosphere). This further confirms that the advantage of the carbon gradient structure over the uniform structure is a more fundamental contribution to reducing oxygen content. Comparative Example 3, representing the most traditional scheme in the prior art (uniform carbon coating + one-step heating with pure nitrogen throughout), has an oxygen content as high as 1.25 wt%, a carbon residue as high as 520 ppm, a particle size distribution width of 2.3, and exhibits severe sintering and a large number of agglomerates, clearly illustrating the inherent limitations of traditional schemes in simultaneously achieving low oxygen content and good morphology.

[0085] Comparing Example 1 with Comparative Example 4, which employed a reverse gradient precursor with a carbon-depleted outer layer and a carbon-rich inner layer, revealed a high oxygen content of 1.08 wt% and a large number of broken particles. This is because, in the early stages of the reaction, the carbon-depleted outer region could not form an effective porous shell, and a dense aluminum nitride layer formed early on, blocking the nitrogen gas channels. Meanwhile, the carbon-rich inner layer was consumed in the later stages of the reaction, resulting in a large amount of gas with nowhere to go, creating high pressure inside and ultimately leading to particle breakage. This powerfully demonstrates, from the opposite perspective, the criticality and non-obviousness of the specific gradient direction of "carbon-rich outer layer and carbon-depleted inner layer" in this invention.

[0086] Based on the above comparative analysis, it can be concluded that the core inventive point of this invention is that the precursor with a gradient distribution of carbon content increasing from the inside to the outside has independent and significant technical effects in reducing oxygen content, improving particle size distribution, and particle morphology. Furthermore, the staged atmosphere control strategy has a further synergistic effect on the carbon gradient precursor. The superposition relationship between the two in terms of technical effects is not a simple linear addition, but rather exhibits a clear positive synergistic characteristic.

[0087] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing aluminum nitride based on the conversion of aluminum compounds, characterized in that, include: Provide aluminum source particles, wherein the aluminum source is selected from boehmite and / or γ-alumina; The aluminum source particles are contacted with an aqueous solution of a first carbon source and then dried to form a carbon-containing inner coating layer on the surface of the aluminum source particles. The particles with the inner coating layer are contacted with an aqueous solution of a second carbon source and dried to form a carbon-containing outer coating layer, thus obtaining a precursor. In forming the inner coating layer and the outer coating layer, the amount of the first carbon source and the second carbon source is controlled so that the mass ratio of carbon elements to aluminum source particles in the inner coating layer is less than the mass ratio of carbon elements to aluminum source particles in the outer coating layer. The precursor was subjected to carbothermal reduction nitriding treatment in a nitrogen-containing atmosphere to obtain aluminum nitride powder.

2. The method according to claim 1, characterized in that, The mass ratio of carbon to aluminum source particles in the inner coating layer is 3% to 8%, and the mass ratio of carbon to aluminum source particles in the outer coating layer is 10% to 20%.

3. The method according to claim 1, characterized in that, The first carbon source and the second carbon source are each independently selected from at least one of glucose, sucrose, and polyvinyl alcohol.

4. The method according to claim 1, characterized in that, After obtaining the precursor and before performing carbothermal reduction nitriding treatment, the process further includes: pre-carbonizing the precursor at 300~600°C under an inert atmosphere to carbonize the carbon-containing coating layer.

5. The method according to claim 1, characterized in that, The carbothermic reduction nitriding treatment is carried out in the following stages: The first stage involves treatment at 800~1100℃ in a nitrogen atmosphere containing 0.5%~3% hydrogen by volume. The second stage involves treatment at 1100~1400℃ in a pure nitrogen atmosphere. The third stage involves treatment at 1400~1550℃ in a nitrogen atmosphere containing 0.5%~2% ammonia by volume.

6. The method according to claim 5, characterized in that, The heating rate in the second stage is 2~5℃ / min.

7. The method according to claim 4, characterized in that, The inert atmosphere for the pre-carbonization treatment is nitrogen or argon, and the pre-carbonization temperature is 400~600℃.

8. The method according to claim 1, characterized in that, The carbothermal reduction nitriding treatment further includes: heat treatment at 500~700℃ in an oxygen-containing atmosphere to remove residual carbon.

9. An aluminum nitride powder, prepared by the method according to any one of claims 1 to 8, characterized in that, The aluminum nitride powder has an oxygen content of less than 0.6 wt%, a carbon content of less than 400 ppm, and a median particle size D50 between 0.5 and 3 μm.

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