摘要
TiB_2-BN复相导电陶瓷具有优良的机加工性和方便的电性能可设计性等，其应用前景十分广阔。但还存在许多有待解决的问题：烧结比较困难、烧结体性能不稳定、不均匀等等。本文采用通电热压快速烧结技术进行了制备TiB_2-BN复相陶瓷的研究，从烧结过程的传热问题入手，运用计算、数值及实验相结合的方法，研究烧结过程中温度分布的均匀性问题；采用渗流理论及正交试验方法，优化烧结过程中的主要影响因素；分析并揭示TiB_2-BN复相导电陶瓷发生潮解的原因；在模拟工况条件下考察试样的热疲劳问题。
根据烧结初期升温速率突变的现象，从实验上确认烧结过程中电——热转换存在二种不同的机制，从而证实了烧结初期样品中产生了放电现象。烧结初期，满足一定的烧结条件下，温度场测量结果揭示：烧结初期的升温速率发生了突变，高效的能量转换证实了放电现象的存在，烧结初期的真空度、样品收缩测量结果也提供了佐证。烧结绝缘样品时，升温速率无突变，故样品烧结过程中不发生放电现象。
烧结中期，样品中的热能主要来自压头。数值计算结果表明，不论样品是导电的还是绝缘的，烧结过程中的热流方向与压力方向一致，热量由压头流向样品。不同样品烧结过程中温度分布不同，绝缘样品中形成封闭的环状等温线，导电样品中形成梯度等温线。径向的温度分布为：导电样品中呈单调分布，绝缘样品中呈波浪分布。
揭示了TiB_2-BN复相导电陶瓷的通电热压快速烧结末期存在大的温度梯度，并且温度梯度的大小与材料的电导率、热导率直接相关。烧结末期，本文推导了通电热压快速烧结中样品及模具内二维稳态温度分布，计算结果是：在样品的径向和轴向，温度都呈抛物线分布。重点分析了径向温度分布问题，样品内对称面上任一点与样品中心的温差为：ΔT=-q_1r~2／4k_1。只要烧结样品为导电材料，样品和模具内温差的存在是不可避免的，且中心温度最高。不同的导电样品，由于导热系数差异及样品发热率不同，样品中存在的温差各异，但均不为零。只有当烧结样品为绝缘材料，样品发热率为零时，样品中才可能实现无温差。
烧结末期温度场的测量结果是：样品为TiB_2-BN复相导电陶瓷时，由于所需烧结温度较高，导热能力较低，导致样品中存在大的温度梯度，样品中心与样品边界的温差最高可达450℃(烧结温度1700℃，无保温措施)；当样品为金属Cu时，样品中温差较小，小于50℃(烧结温度900℃)；当样品为绝缘相BN时，样品中的温差随升温速率高低变化而变化，升温速率高(8℃／s)时，样品中温差大，可达220℃(烧结温度1300℃)，升温速率较低(1.8℃／s)时，样品中温差小，小于50℃(烧结温度1100℃)，样品中的温度分
布完全取决于模具及压头的传热。实验结果与计算结果相吻合。
采用渗流理论解释了TIBZ碉N复相陶瓷的导电性能，确定了通电热压快
速烧结型TIBZ－BN复相陶瓷符合渗流模型。采用通电热压快速烧结方法制备
fiBZ－BN复相导电陶瓷，其电阻率与导电相fiBZ百分含量的关系符合渗流理
论，其渗流模型为：PIP广4．785X10’Xcyc2．9）”’·的。说明快速烧结型iBz
与BN复相陶瓷，hBZ颗粒无规则地分布在BN绝缘相中，且在TffiZ颗粒边
界上不存在BN相的绝缘包裹层。
运用正交试验方法确定了影响烧结材料电性能的几个主要影响因素，其
优化结果为：TIBZ配比大于29．2％（体积百分比入烧结助剂的添加量为 0．5％
（重量百分比），烧结温度约为1650’C。添加烧结助剂，既有效地促进了样
品中导电网络的构成，又对llBZ－BN复相导电陶瓷的电阻率产生很大影响，
而且TIBZ配比越接近渗流阀值，烧结助剂的影响越大。
通过分析TIBZ－BN复相导电陶瓷表面析出物及fiBZ粉和BN粉的氧化产
物，揭示TIBZ－BN复相导电陶瓷中残留的氧化产物一一氧化硼是Tl32－BN复
相导电陶瓷引起潮解的主要原因。复相陶瓷表面析出物是硼酸，作者认为
TIB＊BN复相导电陶瓷在烧结过程中发生了氧化反应，并且在烧结体内残留
了部分氧化硼，这是潮解的主要原因。TIBZ－BN复相导电陶瓷中存在开口气
孔，是TIBZBN复相导电陶瓷潮解的诱因。样品放置于空气中，开口气孔中
吸附水汽后，氧化硼溶解于水并生成硼酸，发生体积膨胀，导致样品破坏。
根据TIBZ粉和BN粉高温氧化产物的不同。与fiBZ－BN复相陶瓷中生成的氧
化物进行对比，可以确定TIBZEN复相陶瓷烧结过程中的氧化，主要发生在
BN相。
热循环对TIBZ－BN复相导电陶瓷不断产生损伤。多次循环后，两相界面
及导电网络均受影响，情况严重时试样破坏。样品电阻率与热震损伤量N中。
之间的关系为：
优化烧结条件后获得的样品，在工况条件（120℃）下使用1200分钟“0
个循环）仍旧完好，只是样品电阻率在每一次循环后略有增加。但在工况条
件门500C）下经受11次循环u20分钟）样品就已无法使用。工作温度也
是影响 TIBZ·BN复相导电陶瓷寿命的一个重要因素，需得到严格的控制。
TiB2-BN composite has a great application potential owing to its machining property and designability of resistivity. But many problems such as sintering difficulty, instability and inhomogeneous etc still exist. This paper starts with the heat transfer problem of newest sintering technology, using the method of calculation, simulation and experiment to study the temperature distribution in sintering process, using the percolation theory and the orthogonal experimental design method to optimize the major factors influencing the sintering process, reasons for the deliquescence of the composite were studied. The thermal fatigue problems of sample under the imitation operating mode condition were analysed.
The paper proves that sparking has occurred in the samples during the initial stage of sintering based on the sudden temperature rise in the experiment and the two mechanisms for the conversion from electricity to heat. In the initial stage of sintering, when certain sintering conditions are satisfied, the result of the measurement shows that there is a sudden temperature rise. This high energy conversion shows the existence of sparking. This result is further proved by the measurement of the vacuum level and the sample shrinkage. On the other hand, there is no sparking, if the sintering sample is insulant and there is no sudden temperature rise, either.
In the middle stage of sintering, the heat of the sample comes chiefly from the punches. The result of numerical calculation shows that the direction of the thermal current is consistent with that of the pressure. Whether the sample is insulating or conducting, the direction of thermal current is always from the punch to the sample. The temperature distribution forms closed ring isotherm in insulating sample, and forms gradient isotherm in electric conducting sample. The temperature distribution of radial appears monotone function in electric conducting sample, and appears undulation in insulating sample.
The paper further reveals that there is a big temperature gradient in the final stage of Spark Plasma Sintering TiB2-BN and its size is directly related to electric and heat conductivities of the material. In the final stage of the sintering, this paper also works on discovering two-dimension steady-state temperature distribution in sample. The results are: at the radial and the axial directions of
sample, the temperature distributing assumes parabola. Focusing on the radial temperature distribution problem, the temperature difference between a point of symmetry plane and the center of sample is: It points out that as long as the sintering sample is an electric conducting material, the existence of temperature difference inside sample and the mould is unavoidable and the center temperature is the highest. Owing to different thermal conductivity and heat release rate, the temperature differences that exist in different samples are different, but none of them equals to zero. If the sample is insulant, it is possible to reach zero temperature difference sintering with zero heat release rate.
The measuring results of temperature field in final stage of sintering are as the following: With high sintering temperature and low thermal conductivity, there is high temperature gradient in TiB2-BN sample, the biggest temperature difference reaches 450, (at sintering temperature 1700C ) . If the sample is metal Cu, the temperature difference is small, the temperature difference is less than 50C (at sintering temperature 900 , ) . If the sample is BN, the higher of the heat-up rate, the bigger of the temperature difference, the temperature difference can reach 220 C(at sintering temperature 1300C, 8C/s), and less than 50C (at sintering temperature 1100C, 1.8C/s) .Depending on heat transfer from the die and the punch the temperature difference is adjustable. The experiment results match with the calculated results.
The percolation model that is compatible with the SPS TiB2-BN composite can be established by using the theory of percolation to explain the conducti
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