Improvement of AlN Thermal Conductivity Based on Reductive Compound Additives

AlN ceramics were prepared by plasma activation sintering(PAS)with compound additives yttrium acetylacetonate(Y(acac)_(3))and melamine(C_(3)H_(6)N_(6)).The effects of compound additives on the microstructure,density,and thermal properties of Al N ceramic were studied.Y(acac)3and C_(3)H_(6)N_(6)can form Y_(2)O_(3),residual organic carbon and reducing gas during the heating process,which improves the Al N sintering performance at a temperature of 1700℃and the bulk thermal conductivity.When the content of Y(acac)_(3)is 10 wt%and C_(3)H_(6)N_(6)is 3 wt%,the thermal conductivity of Al N ceramics is 105.6 W/(m·K),which is much higher than that of Al N ceramics with Y_(2)O_(3)under the same sintering conditions.This work provides theoretical reference for the preparation of high-performance Al N ceramic.

Temperature-mediated structural evolution of vapor–phase deposited cyclosiloxane polymer thin films for enhanced mechanical properties and thermal conductivity

Polymer-derived ceramic(PDC) thin films are promising wear-resistant coatings for protecting metals and carbon-carbon composites from corrosion and oxidation.However,the high pyrolysis temperature hinders the applications on substrate materials with low melting points.We report a new synthesis route for PDC coatings using initiated chemical vapor deposited poly(1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane)(pV_3D_3) as the precurs or.We investigated the changes in siloxane moieties and the network topology,and proposed a three-stage mechanism for the thermal annealing process.The rise of the connectivity number for the structures obtained at increased annealing temperatures was found with strong correlation to the enhanced mechanical properties and thermal conductivity.Our PDC films obtained via annealing at 850℃ exhibit at least 14.6% higher hardness than prior reports for PDCs synthesized below 1100℃.Furthermore,thermal conductivity up to 1.02 W(mK)^(-1) was achieved at the annealing temperature as low as 700℃,which is on the same order of magnitude as PDCs obtained above 1100℃.Using minimum thermal conductivity models,we found that the thermal transport is dominated by diffusons in the films below the percolation of rigidity,while ultra-short mean-free path phonons contribute to the thermal conductivity of the films above the percolation threshold.The findings of this work provide new insights for the development of wear-resistant and thermally conductive PDC thin films for durable protection coatings.

Low thermal conductivity and high porosity ZrC and HfC ceramics prepared by in-situ reduction reaction/partial sintering method for ultrahigh temperature applications

Porous ultra-high temperature ceramics(UHTCs) are potential candidates as high-temperature thermal insulation materials. However, high thermal conductivity is the main obstacle to the application of porous UHTCs. In order to address this problem, herein, a new method combining in-situ reaction and partial sintering has been developed for preparing porous Zr C and Hf C with low conductivity. In this process, porous Zr C and Hf C are directly obtained from ZrO2/C and HfO2/C green bodies without adding any pore-forming agents. The release of reaction gas can not only increase the porosity but also block the shrinkage. The asprepared porous Zr C and Hf C exhibit homogeneous porous microstructure with grain sizes in the range of 300–600 nm and 200–500 nm, high porosity of 68.74% and 77.82%, low room temperature thermal conductivity of 1.12 and 1.01 W·m-1 K-1, and compressive strength of 8.28 and 5.51 MPa, respectively.These features render porous Zr C and Hf C promising as light-weight thermal insulation materials for ultrahigh temperature applications. Furthermore, the feasibility of this method has been demonstrated and porous Nb C, Ta C as well as Ti C have been prepared by this method.

Enhanced thermal conductivity and mechanical properties of 2D C_(f)/SiC composites modified by in-situ grown carbon nanotubes

In this work,the effects of carbon nanotubes(CNTs)on the microstructure evolution,thermal conductivity,and mechanical properties of C_(f)/SiC composites during chemical vapor infiltration(CVI)densification were investigated in detail.Compared with composites without CNTs,the thermal conductivity,flexural strength,flexural modulus,fracture toughness,interfacial shear strength,and proportional limit stress of specimens with CNTs of 4.94 wt%were improved by 117%,21.8%,67.4%,10.3%,36.4%,and 71.1%,respectively.This improvement was attributed to the role of CNTs in the division of inter-layer pores,which provided abundant vapor growth sites for the ceramic matrix and promoted densification of the whole composite.In addition,the high thermal conductivity network formed by the overlap of CNTs and the rivet strengthening effect of CNTs were beneficial for synergistic improvement of thermal conductivity and mechanical properties of the composites.Therefore,this study has practical significance for the development of thermal protection composite components with enhanced thermal conductivity and mechanical characteristics.

Thermal Conductivity of Superconducting MgB₂

Employing the Geilikman-Kresin (GK) theory, we address the experimental data obtained by Bauer et al., and by Schneider et al., on the thermal conductivity (κ) of superconducting MgB2. The two gaps of this compound have qualitatively been understood via the well-known Suhl, Matthias, and Walker’s (SMW) approach to multigap superconductivity. Since this approach is based on one-phonon exchange mechanism for the formation of Cooper pairs, it cannot give a quantitative account of the values of Tc and the multiple gaps that characterize MgB2 and other high-Tc superconductors (SCs). Despite this fact and some rather ambiguous features, it has been pointed out in a recent critical review by Malik and Llano (ML) that the SMW approach provides an important clue to deal with an SC the two gaps of which close at the same Tc: consider the possibility of the interaction parameters in the theory to be temperature-dependent. Guided by this clue, ML gave a complete summary of parameters that quantitatively account for the Tc and the gaps of MgB2 via the generalized BCS equations (GBCSEs). GBCSEs which we recall, invoke multi-phonon exchange mechanism for the formation of Cooper pairs and multiple Debye temperatures to deal with composite SCs. The parameter-values given in ML are used here to calculate the temperature-dependent gaps, which are an essential input for the GK theory. Notable features of this work are: 1)?kMgB2 is calculated for both—the scenario in which the two gaps of MgB2 close/do not close at the same temperature whence it is found that 2) the latter scenario yields results in better agreement with experiment.

无压烧结制备高致密度AlN-BN复合陶瓷

以低温燃烧合成前驱物制备的比表面积为17.4m2／g的AIN粉末和市售BN粉末为原料,利用无压烧结工艺制备AlN-15BN复合陶瓷,研究了复合陶瓷的烧结行为以及制备材料的性能,结果表明:由于AlN粉末的烧结活性好,复合材料的烧结致密化温度主要集中在1500-1650℃之间,在1650℃烧结后,AlN-15BN复合陶瓷的相对密度可达95.6％。继续升高烧结温度,材料的致密度变化不大,热导率继续增加.在1850℃烧结3h后,可以制备出相对密度为96.1％,热导率为132.6W·m-1·K-1,硬度为HRA64.2的AIN-15BN复合陶瓷.提出了高比表面积的AlN粉末促进复合陶瓷烧结的机理,利用XRD,SEM等手段对烧结体进行了表征.

AlN基板材料研究进展

氮化铝基板因具有高热导率、低介电常数、与硅相匹配的热膨胀系数等优良的物理性能,被誉为新一代理想基板材料。详细综述了AlN板的国内外研究现状及其导热机理;介绍并分析了基片制备的工艺流程和影响因素;概括总结了AlN基板的金属化和烧结工艺方面的研究进展;展望了AlN基板的发展趋势和前景。

复合助剂对低温常压烧结AlN性能的影响

研究了Y2O3,LiO2,CaO烧结助剂对AlN陶瓷常压烧结致密度和性能的影响。结果发现,同时添加Y2O3,LiO2,CaO作为助剂,在1600℃低温烧结就能获得具有高致密度、较小的晶粒尺寸(1μm^4μm)、较高的抗弯强度(331MPa)、断裂韧性(3.8MPa·m1/2)及导热率(118W·m-1·K-1)的AlN陶瓷。

AlN低温烧结助剂的研究现状

介绍了在 Al N烧结中 ,烧结助剂的种类和加入量对 Al N陶瓷材料的烧结致密度和热传导系数的影响。通过分析不同烧结助剂对 Al N低温烧结 (<16 0 0℃ )的影响 ,认为有效的烧结助剂应该满足以下 4个原则 :1和 Al N颗粒表面 Al2 O3反应生成的化合物的液化温度应该低于 16 0 0℃ ;2在烧结过程中不要诱发 Al N的分解和氧化 ;3应该能以晶界析出 Al2 O3化合物的形式降低 Al N晶格中的氧 ;4形成的 Al2 O3化合物应该有不亚于 Al N的导热性 ,或者该化合物在晶界的含量要足够低。由于单一的烧结助剂无法同时满足以上 4个条件 ,Y2 O3- Ca O- L i2 O复合烧结助剂 ,极有希望成为高性能的 Al

CaF_2烧结助剂对热压烧结AlN-BN陶瓷复合材料的影响

用热压工艺制备了AlN-BN复合陶瓷材料,研究了不同含量CaF_2烧结助剂对致密化、介电和热导性能的影响.研究表明:CaF_2添加剂可促进材料致密化,净化材料晶界,优化材料的综合性能.热压1850℃保温3h可获得高致密度的烧结体,添加3wt%～4wt%的CaF_2,可获得98.53%～98.54%的相对密度.制备的AlN-BN复合材料其介电常数在7.29～7.60之间,介电损耗值最小为6.28×10^(-4),添加3wt%的CaF_2获得的热导率为110W·m^(-1)·K^(-1).

无压烧结高导热AlN陶瓷的研究进展

AlN以其优异的高热导率、与Si相匹配的热膨胀系数及其它优良的物理化学性能受到了国内外学术界的广泛关注,被誉为新一代高密度封装的首选基板材料。本文详细综述了AlN陶瓷的导热机理和无压烧结工艺等方面的研究进展,并介绍了烧结助剂的选取原则和AlN陶瓷热导率与温度的关系,以及展望了AlN基板的发展趋势和前景。

可加工AlN-BN复合陶瓷的制备

以碳热还原法合成的 AlN 粉末和市售 BN 粉末为原料,添加 5%Y2O3 为烧结助剂,利用无压烧结制备 AlN-15BN复合陶瓷,研究了烧结温度对 AlN-15BN 复合陶瓷相变、致密度、微观结构以及性能的影响,结果表明:Y2O3 可与 AlN粉末表面的 Al2O3 发生反应生成液相促进烧结,随着烧结温度的升高,复合陶瓷的致密度、热导率和硬度逐渐增加,片状的 BN 形成的卡片房式结构会阻碍复合陶瓷的收缩和致密。在 1 850℃烧结 3 h,可以制备出相对密度为 86.4%,热导率为104.6 W?m-1?K-1,硬度为 HRA56.2的 AlN-15BN复合陶瓷。研究表明,通过添加加工性能良好的 BN制备可加 AIN-BN复合陶瓷,是解决 AIB 陶瓷复杂形状成形问题的一个重要途径。

AlN陶瓷基板材料热导率与烧结助剂的研究进展

AlN陶瓷因具有高热导率、低介电常数、与硅相匹配的热膨胀系数等优异性能,被认为是替代Al2O3和BeO陶瓷的理想基板材料。主要讨论了AlN陶瓷的导热机理及影响热导率的因素;介绍了AlN陶瓷烧结助剂的选取原则、几种烧结助剂的作用机理及优缺点,并从高温烧结助剂体系和低温烧结助剂体系2个方面介绍了AlN陶瓷研究的最新进展。

Al_2O_3加入量对AlN-Al_2O_3复相陶瓷制备及性能的影响

以AlN和Al2O3为原料,Y2O3为烧结助剂,N2气氛下在1650℃下热压烧结制备出了AlN-Al2O3复相陶瓷;采用X射线衍射、扫描电子显微镜和透射电子显微镜等对复相材料的相组成、显微结构进行表征。研究了Al2O3加入量对AlN-Al2O3复相材料的强度、热导率和介电性能的影响。结果表明,Al2O3加入量的增加可促进材料的烧结致密化和抑制AlN基体晶粒的生长。Al2O3含量为20 wt%和30 wt%时可制得致密的AlN-Al2O3复相陶瓷。Al2O3利用其高强度和弥散强化作用对AlN基体起到了明显的增强效果。随着Al2O3加入量的增加,试样的抗弯强度显著提高,热导率和介电性能也得到改善。加入30 wt%Al2O3的复相陶瓷的抗弯强度和热导率达到最大值457 MPa和57 W/(m.K),介电常数和介电损耗达到最低值9.32和2.6×10-4。当Al2O3含量达到40 wt%以后,材料中部分AlN与Al2O3反应生成AlON,从而使材料的抗弯强度、热导率和介电性能又明显下降。

添加 Y_2O_3 或 CaO 对 AlN 陶瓷的电性能和热导性能的影响

本文采用 Y_2O_3,CaO 为 AlN 陶瓷烧结的添加剂,测量了该材料电性能(包括体电阻率,介电常数,介质损耗因子)和热导性能以及这些性能随温度变化的关系。研究了添加量对该材料第二相组成的影响及对材料电性能和热性能影响,并对所得的结果进行了初步讨论。指出要制得具有优良电性能和热导性能的 AlN 材料,应严格控制添加剂加入量和 AlN 粉末中的含氧量。

流延成型用AlN无苯浆料的制备及其性能研究

氮化铝(AlN)陶瓷具有优良的热、电、力、光学性能,具有广阔的应用前景,已经引起了国内外研究者的广泛关注。以无水乙醇和异丙醇为混合溶剂,研究了分散剂添加量、Y值(表示粘结剂和增塑剂质量之比)、固含量及溶剂种类对AlN浆料性能的影响。在不添加二甲苯的前提下,当分散剂添加量为1.0%(质量分数)、Y值为0.75时,制备了固含量为39.5%(体积分数)的低黏度AlN浆料,并利用流延成型制备了AlN生坯,在氮气气氛中1 825℃,保温4 h烧结后得到相对密度为99.8%、热导率为178 W·m–1·K–1的AlN陶瓷。

衰减剂对AlN陶瓷热导率的影响

在AlN基相中加入衰减剂石墨粉及炭黑,采用热压工艺,制备复相微波衰减材料。通过矢量网络分析仪、SEM等测试手段,研究衰减剂C在基相中的微观存在形状和分布方式,同时研究保温时间对AlN-C复相材料的热导率的影响。结果表明,以球形孤岛状分布的炭黑使材料具有较高的热导率,达到116 W/(m.K);保温4～6 h为较佳的工艺条件。

高导热AlN陶瓷材料制造及其应用研究

对AlN陶瓷材料制备过程中影响其性能尤其是导热性能的诸多因素进行实验 ,摸索出一套较合理的制备工艺 ,制造出性能较稳定、导热率高的AlN陶瓷材料。观察了AlN陶瓷的微结构并得出某些结论。已加工出AlN陶瓷夹持杆、收集极和输出窗 ,在微波管上试用 ,取得一些经验。

AlN-C复相陶瓷热导率影响因素分析

在AlN基相中加入不同的C,采用热压工艺,制备了AlN-C系列微波衰减瓷,通过矢量网络分析仪、SEM、XRD及热导率仪等测试手段系统研究了影响衰减瓷热导率的因素。研究结果表明,复相陶瓷中氧化钛添加物的含量、晶界相分布状态、碳的形貌及分布形式对复相陶瓷的热导率有重要影响。制备出热导率高达116W/m.KAlN-C(炭黑)的材料,符合微波能量衰减要求。

超微粉（AlN＋Al）的添加对AIN的烧结和导热的影响

将混合超微粉（ＡｌＮ＋Ａｌ）添加到ＡｌＮ细粉中，在不同温度下进行了常压烧结试验。测定了烧结体的导热系数，进行了Ｘ射线衍射分析和电镜观察。结果表明：当烧结温度≤２０２３Ｋ时，超微粉的添加使ＡｌＮ的烧结密度和导热率提高；当烧结温度升至２１２３Ｋ时，超微粉的添加反而引起ＡｌＮ的烧结密度和导热率的下降。讨论了超微粉对ＡｌＮ的烧结和导热的作用条件和机理。