Preparation of transparent AlON from powders synthesized by novel CRN method

Aluminum oxynitride (AlON) powders were synthesized by a novel carbothermal reduction and nitridation (CRN) method. Homogenous and fluffy AlOOH/C core-shell nanoparticle precursor was hydrothermally synthesized with aluminum nitrate hydrate, sucrose and urea as starting materials. Then single-phase AlON powders were synthesized by CRN method at 1700 °C for 2 h. The phase transition and growth of Al₂O₃ particles was effectively retarded by the amorphous carbon nano-layers on the surface of precursor, resulting in significantly lower reaction temperature and further smaller particle size. Based on above fine AlON raw material, transparent AlON ceramic was prepared by pressureless sintering at 1880 ℃ with the in-line transmittance above 80 %.     

球磨对碳热还原氮化法制备氮化铝粉末的作用

研究了高能球磨对氧化铝碳热还原反应制备氮化铝的作用。结果表明：氧化铝及没料经高能球磨后,碳热还原反应开始温度降低,完全反应的温度降至1250℃。与未经球磨的反应相比,反应进行程度提高。球磨产生的机械化学作用,如细化晶粒、晶格畸变和大量缺陷及表面断健作用是球磨促进碳热还原反应的主要原因。     

Fabrication of highly transparent AlON ceramics by hot isostatic pressing post-treatment

Highly transparent aluminum oxynitride (AlON) ceramics were fabricated by pressureless sintering with hot isostatic pressing (HIP) post-treatment. The experimental results showed that the optical transparency of AlON ceramics was improved markedly over the visible and near-infrared range by HIP at 1825 °C for 3 h in 200 MPa argon gas, which derived from the elimination of residual pores in the prepared ceramics. For AlON ceramics pre-sintered at 1800 °C, the transmittances of the sample increased from 63.6% to 84.8% at 600 nm and from 75.4% to 86.1% at 2000 nm, respectively. The average grain size of the HIPed sample was about 47.9 μm.     

Transparent AlON ceramics by nitriding combustion synthesis precursors and pressureless sintering method

Fine AION powders (D₅₀ = 607 nm) with high sintering activities were synthesized by a high-efficiency solution combustion-based method, and a high-transmittance AlON ceramics can be achieved by a subsequent sintering process. Moreover, the influences of various aluminum resources (Al(NO₃)₃, AlCl₃ and Al₂(SO₄)₃) on the morphology of precursors and nitride-functionalized products have been studied in detail. Using Al(NO₃)₃ as aluminum source resulted in more active precursors and was beneficial for the preparation of ultrafine AlON powders. Then, single-phase AlON powders were obtained by calcining precursors at 1700 °C for 10 min, while residual Al₂O₃ was observed in the calcined products synthesized by other two aluminum sources. As a result, transparent AlON ceramics with high in-line transmittance of 85.9% at 2 μm were obtained. This research provides valuable reference for rapid preparation of AlON powders and transparent ceramics.     

Planetary ball-milling of AlON powder for highly transparent ceramics

A detailed investigation of planetary ball-milling for coarsened AlON powder was carried out. Our results showed that the weight ratios of milling ball-to-powder, the revolution rate and the planetary ball-milling time have significant impacts on the microscopic morphology, particle size distribution and average particle size of powder. The process and mechanism were analyzed, and the outcome of our study can be used to optimize the complicated planetary ball-milling method by controlling the planetary ball-milling time or adjusting the revolution rate at the final stage of planetary ball-milling. Sequentially, using fine and uniform AlON powder by optimized planetary ball-milling with an average particle size below 300 nm and excellent sintering properties, highly transparent AlON ceramic with an in-line transmittance of 84% at 2000 nm was successfully prepared through pressureless sintering at 1880°C for 6 hours using the elaborative treated powder synthesized from carbothermal nitridation method.     

Preparation of AlN under vacuum by the alumina carbothermal reduction nitridation method

With good electrical, thermal and mechanical properties, aluminium nitride is widely used in microelectronics and other fields. However, high temperatures are still needed for the method of alumina carbothermal reduction nitridation (CRN) for AlN preparation. Herein, thermodynamics calculation and experimental research were adopted to investigate the preparation of AlN under vacuum by the alumina CRN reaction in order to lower reaction temperature. The results demonstrate that lower pressure benefits the alumina CRN reaction. A single phase of AlN was successfully obtained at 1823 K for 2.5 h with a N₂ gas pressure of 300 Pa. Moreover, an underlying growth mechanism of AlN during the alumina CRN process under vacuum was proposed, which is different from that under atmosphere or elevated pressure. The rate limiting step of alumina CRN process was determined. The results also indicate that Al₄C₃ and Al₂OC play important roles in the process of alumina CRN under vacuum.     

碳热还原氮化锆英石合成ZrN-Si3N4复合材料

以锆英石(粒度≤44μm)和炭黑(粒度≤30μm)为原料,按m(锆英石):m(炭黑)=100:40的比例配料,于球磨罐中以无水乙醇为介质湿混24 h,然后将料浆放入干燥箱中于60℃下充分干燥,再将干燥后的粉料置于球磨罐中干混10 h。最后,将混匀的物料以60 MPa的压力压制成尺寸为20 mm×5 mm的柱状试样,在120℃下充分干燥后置于N2流量为1.0 L.m in-1的气氛炉内,分别在1 400、1 450、1 480和1 500℃的温度下煅烧,保温时间分别为6、9和12 h。自然冷却至室温后,采用XRD分析试样的相组成,采用SEM观察试样的显微结构,并对碳热还原氮化反应过程进行热力学分析。结果表明:以锆英石和炭黑为原料,利用碳热还原氮化反应,在N2气氛下可以合成出ZrN-Si3N4复合材料;通过控制煅烧温度或炉内CO气体分压,可以获得不同组成的复合材料。在本试验条件下,合成ZrN-Si3N4复合材料的适宜工艺参数为1 500℃保温12 h。     

Preparation of AlN powder of low oxygen content via carbothermal reduction and nitridation by active gas exchange technique

By generating a periodic impulse-like pressure (2.0–4.4 kPa, 84 s) to actively exchange the gas in synthesis furnace, pure AlN powder of low oxygen content was synthesized via additive free carbothermal reduction and nitridation (CRN) of Al₂O₃ powder. Compared with the conventional CRN method, the proposed extra gas exhaust process can more effectively remove the side-produced CO from the reaction sites to accelerate nitridation process and decrease the residual oxygen content in the obtained AlN powder. For example, with 39 wt% activated carbon loaded in the raw material at 1650 °C for 4 h, the prepared AlN powder by the proposed synthesis scheme has only 0.68 wt% residual oxygen. The effects of carbon content, synthesis temperature and holding time on the residual oxygen content in AlN powder by the proposed synthesis scheme were also studied. The ball-milled as-prepared AlN powder was pressureless sintered at 1880 °C for 2.5 h to obtain a translucent AlN ceramics (37.6% at ~5700 nm), which demonstrates the excellent sinterability of the as-prepared AlN powder.     

叶蜡石在碳热还原氮化过程中的相变

研究了叶蜡石在不同温度下碳热还原氮化合成SiAlON过程中的相变。SEM、XRD以及EDS分析结果表明 :130 0℃开始氮化形成O’ SiAlON ,14 0 0℃时O’ SiAlON的XRD峰已经很明显 ,14 5 0℃时O’ SiAlON大量生成并在 15 0 0℃时达到最大值 ;14 5 0℃时开始形成β SiAlON(z=2 )和少量SiC ;15 5 0℃时 ,β SiAlON成为主要的氮化产物 ,与少量的O’ SiAlON并存 ;莫来石和方石英直到 15 0 0℃仍然存在 ,15 5 0℃消失。     

高岭土在碳热还原氮化过程中的相变

系统研究了苏州高岭土碳热还原氮化合成SiAlON过程在不同温度下的相变。对试样的XRD、SEM以及EDXA分析结果表明 ,1 30 0℃之前 ,试样中没有氮化物生成 ,物相为莫来石、石英和方石英 ;1 30 0℃时 ,高岭土开始发生氮化反应 ,生成过渡型SiAlON和β SiAlON。此时 ,石英和方石英相基本消失 ;1 4 0 0℃时 ,过渡型SiAlON、β SiAlON和X SiAlON三相共存 ,β SiAlON有所增多并有少量刚玉相生成 ;从 1 4 50℃到 1 550℃ ,Z值为 3的β SiAlON成为惟一的氮化产物 ,与少量SiC和刚玉相并存。莫来石在 1 50 0℃时完全消失     

Direct coarse powder aqueous slip casting and pressureless sintering of highly transparent AlON ceramics

Direct coarse powder aqueous slip casting was proposed to prepare AlON green body for pressureless sintering of highly transparent AlON ceramics. It was reported that anti-hydrolysis treatment to AlON powder before aqueous slip casting was essential due to hydrolysis reaction between AlON and water. However, both XRD of hydrolyzed powder and pH value of aqueous slurry indicate that the hydrolyzate amount is positively correlating with hydrolysis reaction area, i.e., surface area of powder. Therefore, replacing fine particle with coarse one to reduce surface area of powder is an efficient hydrolyzates reduction scheme by lessening hydrolysis reaction sites. As a result, the hydrolyzate in green body of direct slip casting coarse AlON powder (91 vol% of particles have a size of >1.0 μm) was successfully reduced to an insignificant amount so that the sintered ceramics had a high transmittance (84.1% at ~3750 nm), which also verified the effectiveness of the proposed hydrolyzate-reduction scheme.     

Direct ink writing of aluminium oxynitride (AlON) transparent ceramics from water-based slurries

In this study, transparent AlON ceramics were fabricated via the direct ink writing (DIW) method from the water-based ceramic slurry. The solids loading of the ceramic slurry was optimised by changing the dispersant content, and the printability and water content were then adjusted by adding hydroxyethyl cellulose (HEC). The structure of the green body was complete and no impurity phases were detected. The effects of sintering temperature and dwell time on the bulk density, phase evolution, microstructure, in-line transmittance, and mechanical properties of the ceramics were studied systematically. High optical and mechanical properties of 10 × 10 × 0.9 mm³ single-phase AlON ceramic tiles were obtained by sintering at 1960 °C for 10 h in a nitrogen atmosphere: in-line transmittance of 81.90% at a wavelength of 780 nm, fracture toughness of 1.74 MPa·m^1/2 (2.94 N), and Vickers hardness of 18.56 GPa (2.94 N). This study provides a novel method for synthesising AlON transparent ceramics from water-based ceramic slurries.     

碳热还原氮化法制备碳氮化钛粉末

以物质的量比为1∶2.5的TiO_2粉和活性炭粉为原料,于N2气氛下采用碳热还原氮化法在不同的合成温度(分别为1500℃、1600℃、1650℃、1700℃、1750℃,N2压力固定为0.1MPa)和N2压力(分别为0.05MPa、0.1MPa、0.15MPa、0.2MPa,温度1700℃)下保温3h合成了碳氮化钛粉末。研究结果表明提高合成温度和降低N2压力有利于合成碳含量高的碳氮化钛粉末;在N2压力为0.1MPa的条件下,于1700℃保温3h热处理后,可以获得平均粒径为2μm的碳氮化钛粉末。     

Fast one-step carbothermal reduction and nitridation method using nano-sized Al2O3 and carbon black to prepare aluminium oxynitride powder for highly transparent ceramics

By fast heating the nano-sized Al₂O₃ and carbon black mixtures at 50°C/min to 1750°C for 30–120 min, single-phase AlON powders were successfully obtained by a fast one-step carbothermal reduction and nitridation (CRN) method. The AlON ceramics pressureless sintered at 1880°C for 150 min by these powders show high transmittances up to 83%–84%, which indicates that the proposed fast one-step CRN method is an effective and efficient way with strong robustness to synthesize single-phase AlON powder for highly transparent AlON ceramics. It was found that α-Al₂O₃ particles do not have enough time to aggregate and coalesce during heating due to the tremendously shortened heating span, which significantly inhibited particle coarsening until the formation of AlON starts. The fast-formed AlON further inhibits the coarsening of α-Al₂O₃ during dwelling. Consequently, single-phase AlON powder of small primary particles can be obtained after 30 min dwelling at 1750°C.     

碳热还原氮化制备Ti(C,N)技术的现状与发展

简单概述了Ti(C,N)的结构、性质、应用及其制备方法,并从热力学和动力学方面对碳热还原氮化制备Ti(C,N)技术进行了论述,分析了其制备过程中的工艺因素(如C/Ti摩尔比,原料种类及粒度大小,原料混合方式,气体流速,燃烧温度,添加剂等)的影响,同时还对碳热还原氮化制备Ti(C,N)技术的发展方向进行了展望。     

工艺参数对锆英石碳热还原氮化反应速率及物相组成的影响

以锆英石细粉(≤0.045 mm)和活性炭为起始物料,N2为氮源,研究了配碳量(质量分数为10%、20%、22%和30%)、反应温度(1 3501、400、1 450和1 500℃)以及成型压力(25、501、00和150 MPa)等工艺参数对锆英石碳热还原氮化反应速率及产物相组成的影响。研究结果表明:1)配碳量不同,锆英石碳热还原氮化反应产物的相组成不同;此外,配碳量的增加还会降低锆英石碳热还原氮化反应的开始温度。2)反应温度对反应速率和产物相成分的影响显著;配碳量(w)为22%时,随着反应温度的升高,产物相中锆英石相和m-ZrO2含量减少,ZrN含量不断增加,而Zr7N8O4先增加后减少。3)成型压力对产物相组成的影响不大,但成型压力的增加会降低反应的速率。     

Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering

Transparent MgO ceramics were fabricated by spark plasma sintering (SPS) of the commercial MgO powder using LiF as the sintering additive. Effects of the additive amount and the SPS conditions (i.e., sintering temperature and heating rate) on the optical transparency and microstructure of the obtained MgO ceramics were investigated. The results showed that LiF facilitated rapid densification and grain growth. Thus, the MgO ceramics could be easily densified at a moderate temperature and under a low pressure. In addition, the transparency and microstructure of the MgO ceramics were found to be strongly dependent on the temperature and heating rate. For the MgO ceramics sintered at 900 °C for 5 min with the heating rate of 100 °C/min and the pressure of 30 MPa from the powders with 1 wt% LiF, the average in-line transmittance reached 85% in the range of 3 – 5 μm, and the average grain size is ∼0.7 μm.     

Phase evolution and microstructure characteristics during the carbothermal reduction of Hf(C,N,O) ceramics derived from a precursor

In this study, nanosized Hf(C,N,O) ceramics were successfully prepared from a novel precursor synthesised by combining HfCl₄ with ethylenediamine and dimethylformamide. Subsequently, the carbothermal reduction of these Hf(C,N,O) ceramics into hafnium carbide was investigated. The Hf(C,N,O) ceramics comprised Hf₂ON₂ and HfO₂ nanocrystals and amorphous carbon. Upon carbothermal reduction, conversion began at 1300 °C, when HfC first appeared, and continued to completion at 1500 °C, resulting in irregularly shaped crystallites measuring 50–150 nm. Upon increasing the dwelling time, the oxides were completely converted into carbides at 1400 °C. Furthermore, nitrogen was introduced into the reaction to catalyse the conversion of oxides into carbides considering the beneficial gas–solid reaction between CO and Hf₂ON₂. We expect that the ceramics prepared in this study will be suitable for the fabrication of high-performance composite ceramics, with properties superior to those of current materials.     

Characterization of SnO2 nanowires synthesized from SnO by carbothermal reduction process

In this paper, single-crystalline SnO₂ nanowires have been successfully prepared by a carbothermal reduction process employing SnO as the starting material and CuO as the catalyst. Their morphologies, purity and sizes of the products were characterized by transmission electron microscopy (TEM), selected area electron diffraction, X-ray diffraction, field emission scanning electron microscopy (FESEM) and Raman spectroscopy, respectively. The FESEM images reveal wire-like and rod-shaped nanowires of about 100–800 μm in length and 30–200 nm in the transverse dimensions. The three observed Raman peaks at 474, 634 and 774 cm⁻¹ indicate the typical rutile phase of the SnO₂ which is in agreement with the X-ray diffraction results. The influence of some reaction parameters, including the temperature and the reaction duration, on the forming, morphology and particle size of SnO₂ crystallize is discussed.     

Thermodynamic and kinetic analyses of vacuum synthesis of AlN by the alumina carbothermal reduction nitridation method

Ultrafine single-phase aluminium nitride (AlN) particles were successfully synthesized by the vacuum carbothermal reduction nitridation (VCRN) method at shorter times and lower temperatures. The mechanism of the process was investigated. By analyzing the X-ray diffraction (XRD) patterns and thermodynamic calculations, the main reaction path was determined to be Al₂O₃→Al₂CO→Al₄C₃→Al₅C₃N→AlN. By investigating the kinetics mechanism, the process was a gas–solid reaction model, which was controlled by the crystalline chemical reaction from 0.5  to 1.5 h, combination control from 1.5 to 2 h, and gas diffusion control from 2 to 2.5 h. Moreover, the apparent activation energy (E_a) was calculated from the nitridation ratio, the value obtained, 237.71 kJ mol^–1, is 31% of the energy reported for the conventional synthesis method. These results show that ultrafine AlN could be effectively synthesized at 1823–1873 K and 300 Pa N₂ pressure for 2.5 h.