【作者】 仝建峰；

【导师】 陈大明；

【作者基本信息】 北京航空材料研究院 ， 材料学， 2002， 博士

【摘要】 本文利用普通的氧化铝和和经过处理的碳黑作为原料,首次采用水基凝胶注模工艺成型出了柔韧性良好、可以冲切加工的氧化铝-碳陶瓷坯片,在流动氮气氛下通过碳热还原氮化反应直接制备出了AlN陶瓷基片。这为氮化铝陶瓷基片的低成本制备开辟了一条新的思路。系统研究了氧化铝-碳陶瓷水基料浆的制备工艺和流变学特性。首次分别在中性和碱性条件下制备出固含量高达56vol%的氧化铝-碳陶瓷水基料浆。在流变学研究中发现:氧化铝-碳陶瓷料浆在低的剪切速率(<150s-1)时,表现为“剪切变稀”,高的剪切速率下表现为“剪切增厚”;在一定的范围内(50vol%～56vol%),固含量对碱性料浆产生“剪切增厚”的临界剪切速率影响不大,然而对中性料浆,随着固含量的降低,“剪切增厚”的临界剪切速率呈明显的升高趋势;分散剂的加入量对氧化铝-碳料浆的粘度有显著的影响,在恒剪切速率下,随着分散剂加入量的升高,其粘度逐渐降低,但当加入量超过一定值时,料浆的粘度又会出现升高现象,而且分散剂加入量对氧化铝-碳料浆的“剪切增厚”临界剪切速率具有显著的影响,当分散剂含量为1.0wt%时,料浆的“剪切增厚”的临界剪切速率较高。这说明当固含量为56vol%时,分散剂合适的量为1.0wt%;随着球磨时间的延长,粉料平均粒径逐渐减小,当球磨时间超过一定值后,粒径变化出现一个“平台”,即球磨对粉料细化是有一定限度的。如果粉料原始粒径较小,那么“平台”出现的早一些,反之,“平台”出现的时间晚一些,而且随着球磨时间的延长,氧化铝-碳料浆的“剪切增厚”临界剪切速率增大,而且逐渐不再有“剪切变稀”阶段。此外,随着球磨时间的延长,料浆逐渐显示出一定的触变性。研究了氧化铝-碳陶瓷水基料浆的高分子凝胶工艺和机理。系统研究了加热、催化剂、氧化-还原等不同的凝胶方式,研究了单体含量、引发剂、催化剂加入量对凝胶速率的影响,结果表明:随单体含量、引发剂、催化剂加入量的增加,凝胶速率增大,但如果加入量太大,最终坯片中高分子含量过高,坯片经排胶后内部孔隙较多,所以它们的添加量应适当。对氧化铝-碳凝胶注模陶瓷坯片的干燥过程进行了系统的研究,并对陶瓷坯片进行了多方位的表征。研究表明:凝胶注模陶瓷坯片的干燥过程与试样外形尺寸、环境温度和湿度有关;料浆的固含量影响到凝胶注模陶瓷坯片的坯片尺寸收缩率和坯片的密度值;随着料浆固含量的提高,坯片的抗弯强度呈下降趋势;随着料浆固含量的增加,坯片内部的孔隙率降低,平均孔隙直径从110nm降为90nm,而且均为单峰分布;凝胶注模成型的陶瓷坯片在干燥后,颗粒与颗粒之间有明显的有机物粘结,可进行适当的冲、切加工。通过XRD物相分析,优化了氧化物、氟化物等不同反应助剂的合适添加量。研究了碳热还原氮化的工艺对反应过程的影响。重点研究了反应助剂含量、工艺温度、保温时间对反应相变过程的影响。随后对碳热还原氮化进行了热力学分析,建立了碳热还原氮化的模型。添加适量的反应助剂对碳热还原氮化反应有很大的帮助。在助剂为:2.0wt% CaF2,反应工艺为1750℃×4h的条件下,氧化铝通过碳热还原氮化反应能够完全转变成为氮化铝,热导率达到了50 W/m?K。助剂为:1.0wt% Y2O3,反应工艺为1750℃×4h的条件下,反应的最终产物为氮化铝和极少量的氧化铝,热导率达到了49W/m?K。当采用2.0wt%CaF2+1.0wt%Y2O3和2.0wt%CaF2+1.0wt%YF3作为复合反应助剂时,较合适的反应工艺为(1650～1750)℃×4h,在此条件下,反应能够完全进行,能够完全转变成为氮化铝,热导率分别达到了54W/m?K和59W/m?K。利用SEM和TEM研究了反应烧结体的断口形貌和材料内部的微观结构,重点研究了晶界相在材料内部的分布情况。研究表明,Y-Al-O和Ca-Al-O相多数出现在三晶粒交界处。更多还原

【Abstract】 Aluminum nitride has excellent potential for use as a substrate material for high-density, high thermal conductivity, excellent electrical insulation and low thermal expansion mismatch relative to Si. The thermal conductivity of AlN is several times higher than that of Al2O3 at room temperature and is almost equal to that of BeO at 150℃. But the development of aluminum nitride is restricted by high preparation cost,so low-cost preparation is a key technique to realize industrialization. In this research, a new fabrication technology is developed. Aluminum nitride substrate with thermal conductivity of ~59W/m?K is prepared by gelcasting and carbothermal reduction nitridation methods using alumina and carbon black as raw materials.The alumina-carbon suspensions were prepared and their rheology characteristics were studied systematically. The aqueous alumina-carbon suspensions of 56vol% solid loading were prepared in neutral and alkaline surroundings respectively for the first time. It is found that alumina-carbon suspension shows“shear thinning”behavior in lower shear rate (<150s-1), and“shear thickening”in higher shear rate. In given range, the critical shear rate of“shear thickening”is impervious to the solid loading in alkaline suspension,while it is increased with the decrease of the solid loading clearly in neutral suspension.The dispersant content has as remarkable effects on the viscosity of the alumina-carbon suspension. Under constant shear rate, the viscosity of the suspension first decreases with the increment of the dispersant content , and then begin to rise at a certain value. However, the dispersant content remarkably affects the suspension’s critical shear rate of the“shear thickening”. The critical shear rate is higher when the content is 1.0wt%. So it can be taken as the appropriate contents of dispersant to the suspension with the solid loading of 56vol%.Ball milling can smash the power in suspension, but this effect is limited. As the milling time prolongs, the average diameter of powder first decreases first, then the plateau appears and the average diameter keeps changeless. The finer the particle diameter is, the earlier the plateau appears. Furthermore, the critical shear rate of the“shear thickening”of the alumina-carbon suspension increases with the ball-milling time, also the stage of“shear thinning”disappears in long time ball-milling, and the suspension becomes thixotropic gradually.The polymer gelating procedure and mechanism of the alumina–carbon suspension were investigated. The different gelating ways such as heating, catalysis, oxidation-deoxidization and the effect of monomer content and initiator content on the gelating rate were studied in detail. It reveals that the gelling rate accelerates by the increase of the content of monomer and initiator. However, in view of more cavity in the green body caused by too much polymer, excessive content is not proposed.The drying procedure of alumina-carbon gel flan was studied carefully. Comprehensive analyses show that the drying procedure of the gel-casting ceramic flan is associated with sample size, surrounding temperature and humidity. Moreover, the shrinking rate and volume density of the ceramic flan are affected by solid loading of suspension. As the solid loading increases, the shrinking rate and density increases, and average pore diameter decreases.Aluminum nitride ceramics were successfully fabricated by carbothermal reduction-nitridation in flowing nitrogen gas using different reacting aids. In appropriate condition, the alumina-carbon ceramic flans can be completely converted to aluminum nitride. When CaF2 was chosen as reaction additive, the appropriate content was 2.0wt% and preferable reaction temperature was 1750℃. When Y2O3 was chosen as reaction additive, the appropriate content was 1.0wt% and preferable reaction temperature was 1750℃. But if CaF2 and Y2O3 were chosen as reaction additive, the appropriate composition was 2.0wt%CaF2 and 1.0wt%Y2O3, and in this condition, the preferable reaction temperature was 1650℃, and the reaction temperature was reduced by 100℃.In this paper, the effects of the carbothermal reduction-nitridation parameters on the reaction was studied, especially the additive contents, reaction temperature and soaking time. The appropriate content of different reaction additives such as oxide and fluoride were optimized respectively. The thermodynamics of the carbothermal reduction-nitridation procedures were analyzed and the model of the reaction was established.The aluminum nitride specimen has a thermal conductivity of (49~59) W/m?K, which is about as twice as that of alumina. There will be a widely application as promising substrate material.更多还原