B4C对Si3N4/Si2N2O结合SiC材料抗热震性能的影响

首先以Si粉、SiO₂微粉为原料，先在700℃空气气氛处理，然后在1 400℃氮气气氛下合成Si₂N₂O,研究了B₄C添加量(外加质量分数分别为0、1.0%、2.0%、3.0%、4.0%)对Si₂N₂O合成效果的影响。然后根据B₄C最优加入量，先在700℃空气气氛保温5 h,然后在1 400℃氮气气氛保温5 h制备了Si₃N₄/Si₂N₂O结合SiC试样。采用1 300℃风冷5次后试样的抗折强度保持率评价其抗热震性，分析了热震前后试样的物相组成和显微结构。结果表明：1)合成Si₂N₂O的B₄C最优添加量为3%(w);在700℃空气处理时，B₄C优先和气氛中O₂反应生成液相B₂O₃,为1 400℃氮...     

SiC惰性氧化对Al2O3-SiO2浇注料线变化率的影响

以三级矾土（5～0.088、≤0.088 mm）、特级矾土（1～0.088、≤0.088 mm）、碳化硅（≤0.074 mm）为主要原料,以铝酸钙水泥为结合剂,加入适量的α-Al₂O₃微粉、SiO₂微粉和减水剂制备了Al₂O₃-SiO₂浇注料。对经1 400℃空气气氛保温24 h烧后试样的线变化率、显气孔率、化学组成、物相组成和微观结构进行分析,探讨了SiC加入量（w）分别为3%、6%、8%、10%时SiC的惰性氧化对试样线变化率的影响。结果表明：1)SiC惰性氧化产物SiO₂可以填充气孔,降低显气孔率,但SiO₂生成后并不会立即与Al₂O₃发生反应形成莫来石;2)二次莫来石化是试样线变化率提高的主要因素,过量SiC抑制了自身的惰性氧化,氧化产物SiO₂相对减少,不利于试样的烧结以及二次莫来石化的持续进行;3)全组分试样中SiC加入量（w）为6%时...     

K2CO3和CaCO3对低温制备SiC多孔陶瓷性能的影响

采用固相反应法制备SiC多孔陶瓷,通过添加二元复合烧结助剂K₂CO₃和CaCO₃与SiC在空气气氛下氧化生成的SiO₂在较低温度下生成低共融相,促进SiC多孔陶瓷在1100℃-1200℃温度下烧成。研究烧成温度、K₂CO₃和CaCO₃添加量对SiC多孔陶瓷的抗弯强度、显气孔率、气体渗透率、相组成和显微结构的影响。结果表明：当K₂CO₃和CaCO₃添加量分别为1wt%和1wt%,在空气气氛下1100℃保温3h制备的样品综合性能较佳,抗弯强度为33.6MPa,显气孔率为36.3%,气体渗透率为728m³/(m²·h·KPa)。     

ZrC改性石墨对低碳Al2O3-C耐火材料结构与性能的影响

为解决目前低碳Al₂O₃-C耐火材料性能下降、寿命缩短等问题，以Zr粉和鳞片石墨为原料，以NaCl和NaF为熔盐介质，在氩气气氛中于1 000℃保温3 h合成了ZrC改性石墨。然后以电熔白刚玉、α-Al₂O₃粉、Al粉、Si粉、鳞片石墨和ZrC改性石墨为原料，以酚醛树脂为结合剂制备了低碳Al₂O₃-C耐火材料试样。研究了ZrC改性石墨添加量(加入质量分数分别为0、1%、3%、5%)对低碳Al₂O₃-C耐火材料的物相组成、显微形貌、物理性能的影响。结果表明：与仅添加鳞片石墨的试样相比，引入1%～3%(w)的ZrC改性石墨可显著提高低碳Al₂O₃-C耐火材料试样的力学性能，但是当引入5%(w)的ZrC改性石墨时，降低了其性能。添加3%(w)ZrC改性石墨时，试样的力学性能最优，其常温抗折强度和常温耐压强度分别为22.3和97.5 MPa。     

莫来石晶须原位增强堇青石-莫来石多孔陶瓷制备

为实现煤矸石资源化利用,以高岭土粉、煤矸石粉、滑石粉、氢氧化铝粉为主要原料,制备了莫来石晶须原位增强堇青石-莫来石多孔陶瓷。研究了煤矸石掺量(质量分数分别为0、9%、17%、26%、33%)和AlF₃添加量(质量分数分别为0、1.5%、2.0%、2.5%、3.0%)对多孔陶瓷物相组成、显微结构和性能的影响。结果表明：1)当煤矸石掺量(w)≤17%、AlF₃添加量(w)≤2.5%时,试样主要矿物组成为堇青石和莫来石。2)增加煤矸石掺量可明显提高试样的体积密度和耐压强度,但对保持显气孔率不利;增加AlF₃添加量导致试样体积密度和耐压强度减小,但可提高显气孔率。3)随着煤矸石掺量和AlF₃添加量的增加,莫来石晶须直径增加,长径比增加,呈四方柱状或针状。4)当煤矸石掺量(w)为17%,AlF₃添加量(w)为2.5%时,经1 350℃保温2 h,可获得体积密度1.86 g·cm^-3,显气孔率31.0%,耐压强度27.8 MPa的多孔陶瓷;其针状莫来石晶须形成互锁结构...     

焙烧温度对甲醇水蒸气重整制氢CuO/CeO2-ZrO2/SiC整体催化剂的影响

采用溶胶凝胶法制备了CuO/CeO₂-ZrO₂/SiC整体催化剂，并将其应用到甲醇水蒸气重整制氢反应中。通过XRD、BET、H₂-TPR、N₂O滴定等表征手段，探究焙烧温度对CuO/CeO₂-ZrO₂/SiC整体催化剂的影响。结果表明，焙烧温度对CuO/CeO₂-ZrO₂/SiC整体催化剂催化活性影响较大。结合表征分析，其中500℃焙烧的整体催化剂具有较好的Cu分散度，较大的Cu比表面积。在反应温度为360℃,水醇物质的量之比为1.2,甲醇水蒸气气体空速为4 840 h^-1的条件下甲醇的转化率为77.3%。为整体催化剂的开发提供基础数据。     

SiO2粉末对立体光固化用SiC陶瓷浆料的影响研究

为解决SiC陶瓷浆料立体光固化难题，本文采用添加不同粒径和不同含量的球形氧化硅的方法改性SiC陶瓷浆料。研究发现，SiC陶瓷浆料48 h的稳定性随SiO₂添加量(SiC粉末总质量的5%、10%、15%)的变化基本保持不变，但是随着SiO₂粒径(1μm、5μm、10μm)的增大有降低的趋势。另外，浆料的黏度随着SiO₂添加量的增加明显，随SiO₂粒径的增大有所降低。而固化深度随SiO₂添加量的增加呈现先增大后减小的趋势，随SiO₂粒径的增大变化并不明显。整体而言，粒径为1μm，添加量为10 wt%的SiO₂是比较恰当的选择，适合配制黏度较低、稳定性好以及固化深度满足固化要求的SiC陶瓷浆料，最终通过此配方成功实现了SiC浆料的立体光固化。     

B4C添加量对Al2O3-C材料性能的影响

以板状刚玉(粒度≤1、≤0.075 mm)、Al-Si合金粉(粒度为50μm)、α-Al₂O₃微粉(粒度为5μm)、鳞片石墨(粒度≤0.074 mm)和B₄C粉(粒度为20μm)为原料，硝酸镍为催化剂，酚醛树脂为结合剂制备了Al₂O₃-C材料，研究了B₄C添加量(加入质量分数分别为0、3%、6%和9%)对Al₂O₃-C材料性能的影响。结果表明：1)随着B₄C添加量的增加，试样的线变化率明显减小，常温抗折强度和耐压强度明显增大；当B₄C添加量为3%(w)时，试样经1 450℃处理后的线变化率降至0.65%,常温抗折强度和耐压强度最高，分别为28.7和57.3 MPa。2)当B₄C添加量为6%(w)时，试样经1 400℃空气气氛氧化后的氧化指数降至3.9%,抗氧化能力明显增强。     

烧结助剂对锆酸钙材料性能的影响

锆酸钙材料是一种具有潜在应用前景的高温合金熔炼用坩埚材料，以电熔锆酸钙(CaZrO₃)为主原料，添加少量(加入质量分数为0.5%～2.0%)的TiO₂、Al₂O₃、La₂O₃、CeO₂、Y₂O₃粉体作为烧结助剂，在150 MPa下压制成?30 mm×7 mm的坯体，经1 650℃保温3 h烧成。研究了烧结助剂种类及其加入量对锆酸钙材料烧结程度、物相组成及显微结构的影响。结果表明：1)添加2%(w)TiO₂时试样烧结性能最优，这是由于发生固溶反应促进了材料的烧结；2)Al₂O₃的加入量为1%(w)较适宜，由于产生液相促进了材料的烧结，烧后试样中平均晶粒尺寸为10～30μm; 3)Y₂O₃的加入量为0.5%(w)较适宜，由于发生固溶反应促进了材料的烧结，烧后试样中平均晶粒尺寸为10～50...     

TiO2加入量对光固化打印成型Al2O3陶瓷性能的影响

以光敏丙烯酸树脂与分散剂SP-710为液相，以d₅₀=2.38μm的Al₂O₃粉和添加剂TiO₂粉为固相，制备了固含量为78%（w）的Al₂O₃陶瓷浆料。采用数字光固化成型技术（DLP）打印Al₂O₃陶瓷素坯，经1 450、1 500、1 550、1 600℃保温4 h烧后，分析TiO₂加入量（质量分数分别为0、1%、2%、3%、5%）对Al₂O₃陶瓷试样性能的影响。结果表明：TiO₂的加入可促进Al₂O₃陶瓷的烧结，显著提高致密性，降低烧结温度，并确定TiO₂最优掺量为3%（w）,烧结温度最优为1 600℃,此时试样在x轴、y轴、z轴的收缩率分别为15.7%、15.8%与23.8%,显气孔率为2.41%,体积密度为3.74 g·cm^-3,...     

Effect of Annealing Environment on the Crack Healing and Mechanical Behavior of Silicon Carbide-Reinforced Alumina Nanocomposites

The crack healing and strength behavior of an alumina-silicon carbide (Al₂O₃-SiC) nanocomposite (Al₂O₃+ 5 vol% 0.2 μm SiC particles) has been studied, as a function of the crack size and the annealing environment. Results show that annealing treatments can significantly increase the indentation strength. The annealing atmosphere has a profound influence on the extent of crack healing and the degree of strength recovery. Annealing in argon results in a strength increase of 50%, whereas annealing in air yields a three-fold improvement in the indentation strength. Scanning electron microscopic observation has shown that healing of indentation cracks occurs in both environments, with the greater degree of healing occurring during annealing in air. Implications of the findings to the strengthening mechanism in Al₂O₃ (SiC) nanocomposites will be discussed.     

In Situ Synthesis of Ultrafine ZrB2–SiC Composite Powders and the Pressureless Sintering Behaviors

Ultrafine ZrB₂–SiC composite powders have been synthesized in situ using carbothermal reduction reactions via the sol–gel method at 1500°C for 1 h. The powders synthesized had a relatively smaller average crystallite size (<200 nm), a larger specific surface area (∼20 m²/g), and a lower oxygen content (∼1.0 wt %). Composites of ZrB₂+20 wt% SiC were pressureless sintered to ∼96.6% theoretical density at 2250°C for 2 h under an argon atmosphere using B₄C and Mo as sintering aids. Vickers hardness and flexural strength of the sintered ceramic composites were 13.9±0.3 GPa and 294±14 MPa, respectively. The microstructure of the composites revealed that elongated SiC grain dispersed uniformly in the ZrB₂ matrix. Oxidation from 1100° to 1600°C for 30 min showed no decrease in strength below 1400°C but considerable decrease in strength with a rapid weight increment was observed above 1500°C. The formation of a protective borosilicate glassy coating appeared at 1400°C and was gradually destroyed in the form of bubble at higher temperatures.     

Decomposition Suppression of Silicon Nitride by Hexagonal Boron Nitride Nanocoatings

We report a stabilized Si₃N₄ simply with nanocoatings of h-BN. Very thin BN coatings are enough for suppressing the decomposition of Si₃N₄ particles. This approach should open up a new potential way to prepare stabilized Si₃N₄. Reduced nitridation of H₃BO₃-coated Si₃N₄ powder at 1050°C in a flowing mixed 40% N₂+60% H₂ atmosphere, and then following heat-treatment at 1500°C in a flowing N₂ atmosphere can realize the nanocoating of BN on Si₃N₄ particles. Compared with the Si₃N₄ powder without nanocoatings of h-BN, TG and XRD analysis showed that the obtained h-BN nanocoated Si₃N₄ powder demonstrated obviously improved stability in argon atmosphere.     

Porous 2H-Silicon Carbide Ceramics Fabricated by Carbothermal Reaction between Silicon Nitride and Carbon

Porous SiC ceramics were synthesized by sintering pressed and pressed/CIPed powder compacts of α-Si₃N₄, carbon (Si₃N₄:C = 1:3 mol as ratio), and sintering aids, at 1600°C for few hours to achieve a reaction, and subsequently sintering at a temperature range of 1750°–1900°C, in an argon atmosphere. High porosities from 45%–65% were achieved by low shrinkage with large weight loss. Formation of pure 2H-SiC phase via a reaction between Si₃N₄ and carbon can be demonstrated by X-ray diffractometry. The resultant porous SiC samples were characterized by SiC grain microstructures, pore-size distribution, and flexural strength. This method has the advantage of fabricating high-porous SiC ceramics with fine microstructure and good properties at a relatively low temperature.     

Synthesis and characterization of non-oxide ceramic coatings on graphite substrates by solid–vapor reaction process

Silicon carbide (SiC) and silicon carbide/silicon nitride (SiC/Si₃N₄) were successfully synthesized on graphite substrates by the use of solid–vapor reaction (SVR) process. Layers of SiC and SiC/Si₃N₄ were synthesized on graphite substrate through the reaction between SiO and substrate the (SiO_(vapor) + 2C_{(from graphite)}) and N₂ and substrate the (3SiO_(vapor) + 2N_{2 (vapor)} + 3C_{(from graphite)}), respectively. With the increase of dwell time and synthesis temperature the thickness of SiC layers increases up to 1500 °C temperature. Also, with the increase of synthesis temperature hardness value of SiC coatings is increased, which is 10–15 times higher than the substrate. The critical load of SiC coatings for wear resistance is about 22 N, which was observed by scratch tests. The synthesized SiC coatings appear to consist of a β-SiC phase mixed with a minor amount of an -SiC phase, and its thickness is mainly affected by porosity of the substrate. The thickness of SiC/Si₃N₄ coatings is much thinner than that of SiC coatings, but gives higher value of surface hardness.     

Oxidation and Strength Retention of Monolithic Si3N4 and Nanocomposite Si3N4-SiC with Yb2O3 as a Sintering Aid

The oxidation behaviors of monolithic Si₃N₄ and nanocomposite Si₃N₄-SiC with Yb₂O₃ as a sintering aid were investigated. The specimens were exposed to air at temperatures between 1200° and 1500°C for up to 200 h. Parabolic weight gains with respect to exposure time were observed for both specimens. The oxidation products formed on the surface also were similar, i.e., a mixture of crystalline Yb₂Si₂O₇ and SiO₂ (cristobalite). However, strength retention after oxidation was much higher for the nanocomposite Si₃N₄-SiC compared to the monolithic Si₃N₄. The SiC particles of the nanocomposite at the grain boundary were effective in suppressing the migration of Yb³⁺ ions from the bulk grain-boundary region to the surface during the oxidation process. As a result, depletion of yttribium ions, which led to the formation of a damaged zone beneath the oxide layer, was prevented.     

Combustion Synthesis of Si3N4–SiC Composite Powders

Fine Si₃N₄-SiC composite powders were synthesized in various SiC compositions to 46 vol% by nitriding combustion of silicon and carbon. The powders were composed of α-Si₃N₄, β-Si₃N₄, and β-SiC. The reaction analysis suggested that the SiC formation is assisted by the high reaction heat of Si nitridation. The sintered bodies consisted of uniformly dispersed grains of β-Si₃N₄, β-SiC, and a few Si₂N₂O.     

Selection of Materials for Use at Temperatures above 1500°C in Oxidizing Atmospheres

The purpose of this work was to select materials for applications in oxidizing atmospheres at temperatures of a minimum of 1500°C. Several requirements had to be fulfilled, e.g. long-term oxidation resistance, low vapour pressure, high creep resistance and low gas permeability. Another important point was to avoid reactions and sintering in material combinations with silica forming materials. The investigation was based on a detailed literature screening, followed by compatibility tests at 1600°C in air with potential materials (Al₂O₃, ZrO₂, CeO₂, La₂O₃, Y₂O₃, HfO₂, MgO·Al₂O₃) and Mosi₂ as an example of a silica-forming material. The result of these experiments was, that only Y₂O₃ and HfO₂ did not show severe reactions in contact with MoSi₂. Since most of the materials available did not fulfill all the requirements, some new materials were investigated: CIPed MoSi₂ with different additives (SiC, ZrB₂) and recrystallized SiC (RSiC) with a polymer (polysiloxane) derived MoSi₂-filled coating. The oxidation behaviour of these materials was evaluated by continuous thermogravimetric measurements at 1500°C over a period of 100 h in air and by detailed postexperimental investigations.     

WO3/TiO2-ZrO2脱硝催化剂制备及其NH3活化机理

采用共沉淀法制备了WO₃/TiO₂-ZrO₂脱硝催化剂,并用固定床反应器进行活性评价,采用BET、XRD、TPD、氨气吸附漫反射FT-IR进行表征。结果显示,ZrO₂掺杂增强了TiO₂的Lewis酸性;负载WO₃之后,位于3.1~1.7 nm之间孔隙的稳定性显著增强;NH₃的吸附与活化分别由TiO₂-ZrO₂载体和WO₃完成;WO₃中W元素强大的电负性,促进了NH₃的N-H键由共价键向离子键过渡,进而导致了NH₃的活化。脱硝活性结果显示:当WO₃含量为9%(质量)时,催化剂脱硝活性最高,并在320~420℃的温度窗口保持94%以上。(9%)WO₃/TiO₂-ZrO₂具有更加稳定的孔隙结构(4.4~1.7 nm),表面Brønsted酸中心数量增加,Lewis酸中心的强度和酸量增加的幅度最大,NH₃-SCR过程中的活性中间产物NH₂的吸收峰更加明显,这些特征可能是其脱硝活性最好的原因。     

Strengthening and Prevention of Oxidation of Aluminum Nitride by Formation of a Silica Layer on the Surface

A silica (SiO₂) layer was deposited on the surface of an AlN ceramic in order to increase the strength and to prevent the high-temperature oxidation of the material. The layer was formed on the surface by exposing coupons to the atmosphere downstream of a bed of SiC powder in a flowing H₂–0.1% H₂O atmosphere at 1450°C. A reaction between the SiC powder and H₂O in the H₂ gas resulted in the generation of SiO₂"smoke" in the product gas stream. Part of the SiO₂ smoke was subsequently deposited on the surface of the AlN specimen to form a dense and uniform SiO₂ layer. The strength of AlN was improved by about 20% apparently because of blunting of surface defects by SiO₂. More importantly, the layer was very effective in protecting the AlN from the oxidation at elevated temperatures, through the inhibition of transport of oxidants to the sample surface.