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W-Cu复合材料兼具钨的耐高温、高强度、高密度、耐磨损、低膨胀和铜的优良的导热、导电等性能，广泛应用在电子封装、电触头以及金属发汗材料等领域。致密性高、组织结构均匀且晶粒生长可控是获得高性能W-Cu复合材料的关键。目前，熔渗法和复合粉烧结法是制备W-Cu复合材料的常用方法。针对熔渗法制备W-Cu复合材料，关键是制备得到具有一定孔隙率且孔隙均匀贯通的多孔钨骨架。当下，多孔钨骨架的制备多以形貌不规则的钨粉为原料。然而不规则粉体堆积密度低，烧结可控性差，在烧结过程中存在非均匀团聚收缩，导致得到的多孔骨架中存在闭孔或半通孔、且孔隙分布均匀性差，严重影响了熔渗得到复合材料的致密性和组织结构均匀性。针对复合粉烧结法制备W-Cu复合材料，目前多集中在粉体的纳米化及烧结行为研究。但是常规方法得到的纳米粉体分散性差、粒度分布宽且形貌不规则，影响了粉体的烧结活性且导致了晶粒的异常生长，不利于高致密细晶W-Cu复合材料的获得。本文从改善W(Mo)粉体颗粒的特性出发，采用热等离子球化制备得到的球形致密W(Mo)粉和高分散、粒度分布均匀的准球形纳米W粉为原料，以粉体烧结性能研究为基础，分别采用熔渗法和复合粉烧结法，以导热性能及硬度为主要的考察指标，进行高致密性、组织结构均匀且晶粒大小可控的W(Mo)-Cu复合材料的制备及性能研究。主要工作为：（1）以球形致密钨颗粒为原料构建具备均匀贯通孔隙的多孔钨基体。通过研究不同助剂对球形钨烧结和多孔钨结构的影响，最终确定以硬脂酸为成型助剂、以微量Ni为烧结活化剂。为使多孔钨堆积孔隙得到完好的保留且保证一定的烧结颈强度，优化得到了Ni的添加比例（0.03 wt.%）及烧结工艺参数，并分析了球形致密钨粉的活化烧结机理。以同粒度不规则钨粉为原料进行多孔基体的制备，结合烧结动力学分析，证实了球形粉体具有更高的烧结稳定性，有助于孔隙均匀贯通多孔基体的获得。最后，对制备的多孔基体进行了气体通量及渗透性能测试，并且渗铜实验表明，球形粉体有助于制取高导热、高致密的W-Cu复合材料。（2）以第一部分采用球形致密钨粉制备均匀贯通多孔骨架的研究为基础，将其推广至高性能多孔Mo骨架的制备上，并进行Mo-Cu复合材料的熔渗研究。首先以球形致密Mo粉为原料，在1500°C下烧结1 h制备得到多孔Mo骨架。通过成型压力调控多孔骨架的结构，得到了具有不同孔隙率的材料。进而采用熔渗法进行Mo-Cu复合材料的制备，着重考察了熔渗工艺对材料性能的影响，得到了优化的熔渗参数（1300°C熔渗1 h）。最后采用理论模型分析材料的热导率，并通过烧结工艺的优化提升了材料的性能。（3）以热等离子制备得到的高分散、粒度分布均匀的准球形纳米钨粉为原料，进行细晶钨烧结体的制备及强化研究。研究了烧结条件对烧结体结构及性能的影响，分析了烧结体致密化及晶粒演化行为的规律，坯体在1500°C烧结2 h得到了相对密度91.3%、平均晶粒尺寸不足2 μm的烧结体。研究发现，粒度分布均匀的准球形纳米钨粉在烧结前期可有效地抑制晶粒的生长。最后，分析了Al2O3粉体和Ni对钨基体的协同强化作用，研究了二者对钨基体的致密化及晶粒生长行为的影响规律，并确定了最优的添加剂比例。（4）基于第三部分对高分散且粒度分布均匀的准球形纳米钨粉的烧结性能研究，进行细晶W-Cu复合材料的制备。研究了烧结温度和保温时间对材料结构及性能的影响，并分析了材料的晶粒生长行为。计算得到液相烧结阶段的晶粒生长活化能高达338±46 kJ/mol，并且晶粒生长速率低，证实了晶粒的不易生长。研究了坯体的烧结致密化行为，一方面，纳米钨颗粒提升了颗粒重排驱动力（毛细力），并且烧结前期晶粒生长缓慢，从而保证了较高的重排致密化效率；另一方面，钨粉的纳米化在一定程度上改善了其在Cu中的溶解性，从而使物质可通过液相进行传质扩散，加快了体系的致密化进程。最终得到平均晶粒尺寸323 nm，相对密度96.5%的W-Cu复合材料。 ;W-Cu composites are widely used in electronic packaging, electrical contact and metal sweat materials owe to combining the high temperature resistance, good mechanical strength, high density, wear resistance, low thermal expansion coefficient of tungsten and brilliant thermal and electrical properties of copper. Generally, W-Cu composites with high densification, homogeneous microstructure and controllable grain growth are significant to obtain products with good performance. At present, composite powders sintering and infiltration of porous tungsten matrix by liquid copper are two mainly adopted methods to fabricate W-Cu composites. To acquire porous skeleton with proper porosity and uniform and interlinked pores is the main point while W-Cu composites are prepared by infiltration method. Traditionally, irregular tungsten particles are usually used to fabricate porous skeleton. However, irregular particles have low stacking density and poor sintering controllability, and exist uneven contraction during sintering process, which would show adverse effect on the pore structure keeping. As a result, closed or half-connected pores form and pore distribution of porous skeleton is inhomogeneous, which would do harm to the densification and microstructure homogeneity of W-Cu composites obtained by infiltration method. Composite nanopowders are also used as starting materials which fabricated W-Cu composites by composite powders sintering. However, nanopowders prepared by conventional methods usually exhibit poor dispersity, broad particle size distribution and irregular shape, and this would worsen the sintering activity of nanopowders and result in the abnormal grain growth, which would show adverse influence on obtaining high dense and fine-grained W-Cu composites.The research starts from improving the characteristics of W(Mo) particles. Spherical and dense W(Mo) particles and well dispersed quasi-spherical W nanopowders with uniform particle size distribution prepared by thermal plasma process are used as starting materials. W(Mo)-Cu composites with high dense, homogeneous microstructure and controllable grain growth are fabricated by infiltration method and composite powders sintering, while the thermal conductivity (TC) and hardness are used as the key performance index. The results are listed as follows:(1) Spherical and dense tungsten particles are employed to fabricate porous skeleton with homogeneous pore distribution and interlinked pore structure. The influence of additives on the sintering of spherical W particles and the microstructure of porous skeleton is studied, and stearic acid and nickel are used as forming and sintering activator additives, respectively. Optimized Ni contents (0.03 wt.%) and sintering parameters are obtained in order to ensure pores open and a certain sintered neck strength together. Besides, the activated sintering mechanism of spherical tungsten particles with dense internal structure is also investigated. In addition, a comparison study used irregular powders with the same particles size as spherical powders is carried out, and the sintering kinetic mechanism is also analyzed, which indicates the more sintering stability of spherical particles. As a result, spherical powders exhibit significant advantages in fabrication porous tungsten skeleton with uniform pore distribution and open pore channel. Finally, gas flux and penetrating quality of porous skeleton are also analyzed, and copper infiltration experiments indicate spherical particles are more beneficial to obtain high dense W-Cu composites with brilliant thermal conductivity.(2) Porous tungsten skeleton with uniform pore distribution and interlinked pores is obtained using spherical particles with dense internal structure as starting materials in the first part, and porous Mo skeleton is also manufactured using the same method. The copper infiltration on the porous skeleton to fabricate Mo-Cu composites is the main research content in this part. Firstly, porous Mo skeleton is obtained at 1500°C for 1 h with no sintering additive using spherical particles with dense internal structure, and the porosity is controlled by adjusting the compacting pressures. And then, Mo-Cu composites are fabricated using infiltration method. The influence of infiltration parameters on the performance of obtained materials is stressly investigated, and optimized parameters (1300°C for 1 h) are obtained. Finally, the thermal conductivity of composites is analyzed using the theoretical models, and the performance of composites is promoted by controlling the sintering process.(3) The fabrication of tungsten compacts with fine grain size and their intensification are researched using quasi-spherical tungsten nanopowders with good dispersity and uniform particle size distribution prepared by thermal plasma as starting materials. The influence of sintering conditions on the microstructure and performance of obtained compacts is investigated. The densification behavior and grain growth of sintered compacts are analyzed, and sintered compacts with the relative density of 91.3% and average grain size under 2 μm are obtained when sintered at 1500°C for 2 h. The results indicate quasi-spherical nanoparticles with uniform particle size distribution could be well suppressed the grain growth at the early stage of sintering process (< 1100°C ). Finally, the coupled effects of alumina reinforced nanosized tungsten matrix during nickel activated sintering process are also studied. The influence of additives on the densification and grain growth is also investigated, and the optimized proportion is obtained.(4) Based on the research on the sintering behavior of quasi-spherical W nanopowders with good dispersity and uniform particle size distribution in the third part, the fabrication of fine-grained W-Cu composites is carried out in this part. The influences of sintering temperatures and holding time on the microstructure and performance of obtained composites are investigated, and the grain growth behavior is also studied. The activation energy of grain growth at liquid phase sintering stage is calculated to be as high as 338±46 kJ/mol, and the rate of grain growth is low, which confirms the suppressed grain growth of sintering process. The densification behavior is also studied. Nanoparticles would improve the driving force of grain rearrangement (capillary force), and the small grain size because of the suppressed grain growth during the early sintering stage would enhance the grain rearrangement. In addition, tungsten particles in nanoscale would improve the solubility of W in liquid copper, and this would enhance the mass diffusion through liquid phase and improve the densification process. Finally, W-Cu composites with the relative density of 96.5% and the average grain size of 323 nm are obtained.
|李保强. 热等离子制备的球形钨粉在W-Cu复合材料中的应用研究[D]. 中国科学院研究生院,2018.|
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