A1N陶瓷低温烧结过程中氧化控制与性能研究

采用纳米级的A1N粉并以Y2O3-CaF2作烧结助剂于1600℃下制备A1N陶瓷,对AlN陶瓷物相组成、相对密度、微观结构和热性能进行了表征,针对A1N陶瓷烧结过程中易氧化的问题,分析了氮化铝陶瓷在烧结过程中氧化的机理,提出了防止A1N陶瓷制备过程中氧化的措施。研究表明：将A1N坯体置于含有一定量碳粉的A1N埋粉中于N2气氛下烧结,生成还原性气体CO,有效避免了A1N烧结过程中的氧化问题。其中添加3wt％Y2O3-2wt％CaF2作烧结助剂,1600℃常压条件下制备了高热导率的致密A1N陶瓷。     

关于AlN陶瓷导热性的讨论

从声子导热机理出发,探讨了影响A1N陶瓷导热率的主要因素。提出了提高导热率的努力方向,即结构上努力减少晶格缺陷(空位、错界及相畴界,杂质固溶等)和晶界缺陷(第二相拆出、气孔、晶界玻璃相等)。指出了提高A1N陶瓷导热率的途径,即严格控制A1N粉末质量,选择合理的烧结助剂,采用还原气氛烧结工艺。     

Al/NiO复合粒子的制备与表征

以N i(NO3)2.6H2O和CO(NH2)2为主要原料,通过均匀沉淀法,90℃恒温12 h,异相成核,在片状金属铝粉表面包覆一层N i2CO3(OH)2,制备出包覆式复合粒子A l/N i2CO3(OH)2。将复合粒子在马弗炉中400℃恒温灼烧2 h,制备出了A l/N iO复合粒子。通过SEM、XRD及粒度测试等分析方法,对复合粒子的形貌、晶体结构及粒径进行了表征。     

Si3N4-MgO-CeO2陶瓷烧结中的致密化与自动析晶

用等离子放电烧结的方法制备了Si3N4-MgO-CeO2陶瓷，用排水法测定了密度，用X射线衍射的方法测定了物相变化。发现在烧结过程中，高于l450℃时,MgO-CeO2就会与氮化硅粉末表面的SiO2反应形成硅酸盐液相，促进烧结致密化，冷却后形成玻璃相留在晶界，氮化硅的致密化在l500℃接近完成。但高于l550℃烧结，MgO反而会析晶，提高氮化硅陶瓷的高温性能。     

半透明氮化硅陶瓷的放电等离子烧结制备及性能

用放电等离子烧结(spark plasma sintering,SPS)技术,以质量分数(下同)为9%氮化铝(A1N),3%氧化镁(MgO)为烧结助剂,在1850℃烧结5min,成功制备了半透明氮化硅(Si3N4)陶瓷.半透明Si3N4陶瓷在中红外波段表现出良好的透过率,最大透过率为66.4%.SPS的快速致密化过程保证了烧结体具有良好的晶体结构,有利于提高透过率.SPS快速的烧结过程和A1N和MgO的加入能够有效抑制烧结过程中Si3N4陶瓷由α相向β相的转变,是制备光学性能良好的Si3N4陶瓷的关键.报道了半透明Si3N4陶瓷的其他性能.光学性能与其他性能的结合,势必大大拓宽Si3N4陶瓷的应用领域.     

逆反应烧结制备碳化硅/氮化硅复合材料的工艺

制备Si3N4／SiC复合材料的常规反应烧结是以Si和SiC为原料进行氮化烧结,而逆反应烧结是以Si3N4和SiC为原料,首先使Si3N4反向反应为活性氧化物后再进行烧结。建立逆反应烧结工艺制备Si3N4／SiC复合材料的热力学基础。确定了Si3N4先于SiC氧化;氧化产物可以是SiO2,也可以是Si2N2O;形成的SiO2氧化膜不会与基体材料反应;在膜与基体之间可能生成Si2N2O。论证了逆反应烧结的热力学可行性。通过6个烧结实验,证实了其热力学分析的正确性,并从工艺参数与密度变化、残氮率和比强度等关系筛选出最佳的烧结工艺参数。     

纳米Al_2O_3和Cr_2O_3对镁铬材料烧结与力学性能的影响

利用纳米粉的粒径小及活性高等特性,在镁铬材料中加入不同质量分数(分别为2%、4%、6%)的纳米A l2O3或纳米Cr2O3粉体,研究了这两种纳米粉取代相应的微粉后对不同温度烧成的镁铬耐火材料烧结与力学性能的影响。结果表明:适量地用纳米粉体取代相应微粉可有效改善镁铬材料的烧结,提高其常温与高温力学性能,且烧成温度越低,纳米粉对镁铬材料性能的提高作用越明显;在本试验的镁铬材料的颗粒级配条件下,两种纳米粉的加入质量分数均以4%为最佳;纳米A l2O3的加入能降低镁铬质耐火材料的烧结温度,加入4%质量分数的纳米A l2O3可降低约100℃。     

氮化铝粉末特性对氮化铝-氮化硼复合陶瓷结构和性能的影响

以比表面积为4.26m2/g、氧含量(质量分数,下同)为O.98%和比表面积为17.4m2/g、氧含量为1.69%的2种AlN粉末为原料,用无压烧结工艺制备氮化铝氮化硼(A1N-15BN,BN为15%)复合陶瓷,研究了A1N粉末对复合陶瓷显微结构和性能的影响.结果表明:A1N粉末对复合陶瓷的致密化过程以及陶瓷的性能有重要影响.由于高比表面积A1N粉末的烧结活性好,AlN-15BN复合陶瓷的烧结致密化温度主要集中在1500～1650℃之间.在1650℃烧结3h后,A1N-15BN复合陶瓷的相对密度可达95.6%,热导率为108.4W/(m·K),硬度HRA为72.继续升高烧结温度,A1N-15BN复合陶瓷的致密度变化不大,热导率升高,硬度下降.在1850℃烧结后,A1N-15BN复合陶瓷的热导率为132.6W/(m·K),Rockwell硬度(HRA)为64.2.低比表面积的AIN粉末所制备的A1N-15BN复合陶瓷的致密化过程主要发生在1650～1800℃间.在1850℃烧结3h,制备出A1N-15BN复合陶瓷的相对密度为86.4%,热导率为104.2W/(m·K),HRA为56.2.     

α-Si_3N_4与γ-Si_3N_4、α-Si_3N_4混合粉体超高压烧结的比较研究

以Y2O3-Al2O3-La2O3体系作烧结助剂,在5.4～5.7GPa、1620K～1770K的高温高压条件下进行了α-Si3N2与γ-Si3N4、α-Si3N4粉体的烧结研究.探讨了烧结温度及压力对烧结体性能的影响.实验测试结果表明:α-Si3N4、γ-Si3N4完全相变为β-Si3N4,相同的烧结条件下,α-Si3N4比γ-Si3N4、α-Si3N4混合粉体烧结试样的相对密度、维氏硬度高.α-Si3N4与γ-Si3N4、α-Si3N4混合粉体烧结试样的最高相对密度与维氏硬度分别为98.78%、21.87GPa和98.71%、21.76GPa.烧结体由相互交错的长柱状β-Si3N4晶粒组成,显微结构均匀.     

α-Si3N4与γ-Si3N4、α-Si3N4混合粉体超高压烧结的比较研究

以Y2O3-Al2O3-La2O3体系作烧结助剂,在5．4～5．7GPa、1620-1770K的高温高压条件下进行了α-Si3N4与γ-Si3N4、α-Si3N4粉体的烧结研究,并探讨了烧结温度及压力对烧结体性能的影响。实验结果表明：α-Si3N4、γ-Si3N4完全相变为β-Si3N4;在相同的烧结条件下,α-SigN4比γ-Si3N4、α-Si3N4混合粉体烧结试样的相对密度、维氏硬度高。α-Si3N4与γ-Si3N4、α-Si3N4混合粉体烧结试样的最高相对密度与维氏硬度分别为98．78％、21．87GPa和98．71％、21．76GPa。烧结体由相互交错的长柱状β—Si3N4晶粒组成．显微结构均匀。     

Sintering characteristics of plasma-sprayed TBCs: Experimental analysis and an overall modelling

In this study, sintering behaviour of plasma-sprayed thermal barrier coatings (PS-TBCs) was investigated experimentally and theoretically. Results show that the sintering kinetics of PS-TBCs is highly stage-sensitive. The sintering proceeds significantly faster at initial short thermal exposure (<20 h), while it slows down dramatically at following long thermal exposure. A detailed examination on microstructural evolution of the PS-TBCs was carried out to understand their sintering behaviour. Results show that, different from the conventional sintering theory, the healing of 2D pores was dominantly responsible for the stage-sensitive sintering kinetics during thermal exposure. In brief, the sintering characteristics of the PS-TBCs are highly structure specific. In addition, a structural model was developed based on the structural characteristics of the PS-TBCs; and the model predicts a well consistent sintering behaviour with experiments. Finally, an outlook towards TBCs with higher performance was put forward.     

Thermal conductivity of spark plasma sintered SiC ceramics with Alumina and Yttria

In this study, dense SiC ceramics were fabricated at 1650?1750 °C for 10?60 min by spark plasma sintering (SPS) using 3?10 wt.% Al₂O₃-Y₂O₃ as sintering additives. Effects of sintering temperature, sintering additive content and holding time on microstructure as well as correlations between microstructure and thermal conductivity were investigated. An increase in the sintering temperature promotes grain growth. Extending holding time has little influence on grain size but results in formation of continuous network of sintering additive, which increases interfacial thermal resistance and thus decreases thermal conductivity. For SiC ceramics composed of continuous SiC matrix and discrete secondary phase (yttrium aluminum garnet, YAG), an increase in the sintering additive content results in smaller grain size and lower thermal conductivity. The lower thermal conductivity of the SiC ceramic with higher sintering additive content is mainly due to the smaller grain size rather than the low intrinsic thermal conductivity of YAG.     

初始碳酸根含量和热处理对碳酸羟基磷灰石制备和性能的影响

以湿化学法制备了不同碳酸根含量的碳酸羟基磷灰石(carbonated hydroxyapatite,CHA).研究了热处理温度和气氛对这些CHA的烧结、热稳定性和碳酸根替代的影响.通过X射线衍射、Fourier变换红外光谱及热分析等多种测试手段表征了CHA粉体的特性.结果表明:CHA坯体的烧结温度和粉体的热稳定性与初始的碳酸根含量有关,含量越高,烧结温度越低,热稳定性越差.热处理过程中,CHA中碳酸根的替代量随热处理温度的升高而减少,并在热处理过程中A位的碳酸根较B位的碳酸根在更低的温度下失去.湿二氧化碳气氛能促进CHA的烧结,减少碳酸根的损失,提高热稳定性,有利于生成以B型替代为主的CHA.     

碳化硅陶瓷固相烧结的烧结机理及研究进展简

碳化硅陶瓷材料具有高硬度、高强度、抗氧化、耐高温、高热导率、低线胀系数等优良性能,同时具有优良的化学稳定性且能够耐大多数种类的酸碱溶液腐蚀,在石油、化工、建材、航空、机械等诸多领域得到了广泛应用。本文主要阐述了碳化硅陶瓷固相烧结的烧结机理,并对目前国内外关于碳化硅陶瓷固相烧结的研究进展进行了阐述。     

烧结熔剂高温特性的实验研究

利用ZDHR-3型微机灰熔融性测定仪和综合热分析仪研究了常用的7种烧结熔剂在烧结过程中的高温反应特性及热分解特性.测定了模拟烧结矿在配加7种熔剂的矿粉的变形温度、软化温度、半球温度和流动温度,以及7种烧结熔剂的分解率与分解温度.结果表明,烧结过程中,7种熔剂的分解温度在1215℃及1250℃左右:相比于其他5种熔剂,白云石A、B的流动性较差;7种熔剂的分解失重率分为"一段式失重"和"两段式失重"两种,前者失重温度区间在690～810℃,而后者在400～460℃及610～710℃,且前者大于后者;两种白云石和石灰石的失重率最大,烧损也最大.     

A general method to determine optimal thermal cycles based on solid-state sintering fundamentals

Most technical ceramics require processing up to and including final-stage sintering to obtain a high-density bulk while inhibiting grain growth as dominant sintering process as far as possible. The literature typically highlights the qualitative interdependence of the sintering variables and microstructural parameters, focusing on very simple particulate systems. However, a quantitative method to achieve optimum sintering of actual polycrystalline solids is still lacking.This paper puts forward such a method, which has been satisfactorily tested by the authors. The method consists of a mathematical model, based on the physical phenomena that take place during solid-state sintering. The method leads to two differential equations: a densification rate and a pore-dragged normal grain growth rate equation during final-stage sintering, which mainly depend on sintering temperature and shaping conditions. Simultaneous numerical integration of these two rate equations allows design of an optimal thermal cycle (enhancing densification and controlling grain growth) to obtain the targeted sintered polycrystalline microstructure. Application of this method yields staggered thermal cycles, in addition to the number of steps, as well as the sintering temperature and dwell time in each step.     

A comprehensive sintering mechanism for TBCs‐Part I: An overall evolution with two‐stage kinetics

A comprehensive sintering mechanism for lamellar thermal barrier coatings was reported experimentally and theoretically in this study. To begin with, an overall property evolution with two‐stage kinetics was presented during thermal exposure. The increase in mechanical property at initial thermal exposure duration (stage‐I) was much faster with respect to that in the following longer duration (stage‐II). At the stage‐I, the in situ pore healing behavior revealed that the significant faster sintering kinetics was attributed to the rapid healing induced by multipoint connection at the intersplat pore tips, as well as a small quantity of the narrow intrasplat cracks. At the following stage‐II, the residual wide intersplat pore parts and the wide intrasplat cracks decreased the possibility of multiconnection at their counter‐surfaces, resulting in a much lower sintering kinetic. Moreover, a structural model based on the microstructure of plasma sprayed YSZ coatings was developed to correlate the microstructural evolution with mechanical property. Consequently, the model predicted a two‐stage evolutionary trend of mechanical property, which is well consistent with experiments. In brief, by revealing the pore healing behavior, this comprehensive sintering mechanism shed light to the structure tailoring toward the advanced TBCs with both higher thermal‐insulating effect and longer life time.     

Combined effect of internal and external factors on sintering kinetics of plasma-sprayed thermal barrier coatings

Understanding the sintering kinetics of plasma-sprayed thermal barrier coatings (PS-TBCs) is crucial to retard their performance degradation. However, under real service condition, the sintering kinetics is often affected by multiple factors. This study investigated the sintering kinetics, in a novel scale-progressive view, under the combined-effect of internal and external factors. Results show that the sintering kinetics of PS-TBCs was highly associated with their unique sintering process from nanoscale to microscale. Firstly, sintering leads to nanoscopic roughening of the pore surface. Subsequently, multiple contacts are formed between counter-surfaces. As a result, microscopic healing of pores can be finally observed. In terms of external factors, the temperature further affects the level and rate of nanoscopic roughening. This is responsible for the differences of the microscopic healing ratios, as well as the macroscopic elastic modulus.     

Microstructures and thermal conductivities of reaction sintered SiC ceramics

Abstract

Reaction sintered SiC ceramics were prepared by the silicon melt infiltration method over temperatures of 1450?1550°C. The effects of the carbon and silicon contents of the starting materials as well as the sintering temperature and time on the thermal conductivities and microstructures of the ceramic materials were studied. The thermal conductivities and microstructures of the samples were characterised using thermal conductivity measurements, X-ray diffraction analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy and mercury injection porosimetry. The results showed that sintering temperature and time as well as the carbon and silicon contents of the green specimens are the main factors affecting the microstructure and porosity of reaction bonded SiC ceramics. Increasing the reaction temperature and time decreased the porosity of the ceramics. This was due to the infiltration of the silicon melt into the ceramic specimens. The thermal conductivity and porosity of the sample sintered at 1550°C for 3 h in an argon atmosphere were 102·5 W m K^?1 and 0·3% respectively.