W-Cu体系功能梯度复合材料的水基流延法制备及其性能研究
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摘要
W-Cu体系复合材料是目前研究多、互补性好的金属基复合材料之一,铜具有良好的导电、导热和抗腐蚀性能,但其强度低；钨具有较高的硬度、强度和良好的耐磨性、热稳定性,W-Cu体系复合材料可结合二者优点,弥补两者缺点,在电子封装、集成电路、国防军工、航空航天等高尖端技术上具有很好的应用前景。随着使用条件要求越来越苛刻,W-Cu体系均质复合材料的综合性能已无法满足极端条件下的使用要求。为了解决均质W-Cu体系复合材料的热学、电学、力学等性能难以满足日益苛刻的使用环境的问题,可同时具备多种优异性能的W-Cu体系功能梯度复合材料被提出了。目前针对不同服役条件,关于W-Cu体系功能梯度复合材料设计、制备和性能的研究已有报道。但从以往的文献报道结果可以看出,W-Cu体系功能梯度复合材料依然存在着成型工艺复杂,烧结难度大,设计组分难以控制等设计和制备难题,无法制备出结构满足设计要求、综合性能高的W-Cu体系功能梯度复合材料。
为此,本文以W-Cu体系功能梯度复合材料为研究对象,针对文献报道的W-Cu体系复合材料所存在的问题和难点,采用化学镀工艺在钨颗粒表面镀铜,改变表面性质,改善钨铜界面润湿性,提高界面结合强度；采用等离子体活化烧结技术(Plasma Activated Sintering,简称PAS)活化烧结W@Cu粉体、降低烧结温度、提高烧结体致密度；采用绿色环保的水基流延成型工艺,解决W-Cu体系功能梯度复合材料组分控制与成型的难题。论文从理论和实验两个方面出发,详细研究了W-Cu体系功能梯度复合材料的关键制备技术(原料处理、成型、烧结等),并详细表征了W-Cu体系均质复合材料和功能梯度复合材料的结构、性能。论文的主要研究工作和结论如下：
首先,本文以环保的乙醛酸取代毒性大的甲醛为还原剂,首次采用无敏化、活化的滴加法直接化学镀技术在钨粉表面化学镀铜制备出定量、高纯的W@Cu复合粉体。通过使用中心实验设计的方法对化学镀铜实验进行设计和工艺条件优化,获得滴加法直接化学镀铜的最优工艺条件：乙醛酸溶液流速5.01mL/min、pH值为12.25、水浴温度为45.35℃。在此工艺条件下,制备的W@Cu复合粉体的铜镀层增重误差仅为0.014%,说明此工艺条件可以制备出定量的W@Cu复合粉体。论文详细考察了pH值、反应温度、还原剂和铜盐加入速度对W@Cu复合粉体结构影响。多种测试结果表明,在本文优化的工艺条件下,铜镀层呈面心立方多晶结构单质Cu,未发现含有1价态和2价态铜离子,包覆层结构紧密,W@Cu复合粉体分散性良好,这说明本实验采用的滴加法直接化学镀技术在乙醛酸-水的绿色体系中成功制备了高纯W@Cu复合粉体,为制备W-Cu体系功能复合材料提供了高纯、定量的包覆粉体原料。
在此基础上,论文还详细分析了滴加法直接化学镀铜制备W@Cu复合粉体的热力学过程和动力学过程,研究表明,滴加法直接化学镀铜过程为异相形核。采用滴加法可很好地控制镀液中铜离子处于低浓度状态,进而,使还原的铜原子同样处于较低的浓度状态,形成了获得良好包覆效果的前提,即避免均相形核的产生-降低了包覆过程的主要推动力-过饱和度。通过对化学镀铜的热力学分析、W@Cu粉体不同时间段的形貌观察和催化机理的研究,建立了钨表面滴加法直接化学镀铜的铜镀层形成模型,提出了其反应机理。分析认为,铜离子首先在钨表面大量聚集,形成带正电的实体,此时铜离子实体表面起到了与金属铜表面一样的效果—自催化,成为催化活性中心,引发乙醛酸与铜离子发生最初的氧化还原反应形成铜镀层,还原出来的金属铜沉积在其表面形成致密的壳层结构：随后,沉积的铜层继续作为活化中心诱导化学镀氧化还原反应继续进行。
其次,本文利用制备的W@Cu复合粉体进行水基流延成型,制备出厚度均匀、固相含量高的W-Cu流延素片,为W-Cu体系复合材料和功能梯度复合材料的烧结提供良好素片。本文基于计算流体力学(CFD),对W-Cu水基流延成型工艺进行数值模拟仿真,详细考察了W-Cu浆料性质和制备工艺制度对流延湿带的影响,获得了稳定流延成型的关键工艺参数和浆料性质：浆料粘度>3P,基带速度>2mm/s,刮刀间隙对流延湿带厚度起主要作用。通过对W-Cu水基流延浆料的实验研究,获得了W-Cu水基流延浆料的最佳配方：粘结剂(PVA-2488)用量与(纯铜)/(W@Cu)质量比满足线性比例关系Y=0.0141+3.39E-4·X,R=0.988；增塑剂(丙三醇)和粘结剂(PVA-2488)的质量比为0.6。通过对W-Cu体系流延浆料固相含量、粘结剂、增塑剂的配比优化,成功制备出了固相含量高、结构均匀、性能良好的不同含量W-Cu水基流延素片。
再次,本文成功利用PAS烧结技术制备出结构理想、高致密度和性能优良的W-Cu均质复合材料,详细研究了W-Cu均质复合材料的烧结致密化与各种性能。考察了钨体积分数对W-Cu复合材料的致密度、硬度、热膨胀系数、热导率和电导率的影响。结果表明：采用W@Cu复合粉体可降低W-Cu均质复合材料烧结温度,在900℃-30MPa-5min的低温烧结工艺条件下,不同钨含量,特别是高钨含量的W-Cu体系均质复合材料均实现致密化。结果表明,采用W@Cu复合粉体和等离子体活化烧结技术制备出的W-Cu体系均质复合材料具有良好的力学、热学和电学性能。其硬度和杨氏模量力学性能随钨颗粒体积含量的增大而升高。其热膨胀系数、热导率、电导率随着钨体积含量的增大而降低；当Cu含量为30vo1.%时,W-Cu均质复合材料的硬度值为234HV,杨氏模量290GPa,电导率为0.309×108S/m,热导率为235.82W/(m·K),热膨胀系数为8.0μrn/(m·K)。并确定了W-Cu复合材料各项性能参数与体积含量的预测模型公式,为W-Cu体系功能梯度复合材料提供了设计预测模型和烧结工艺制度。
最后,本文针对W-Cu复合材料的应用领域,完成了W-Cu体系功能梯度复合材料的功能性设计；针对电子封装领域对高热导率和低热膨胀系数的要求,电触头或电极行业对高硬度和高电导率的要求,以及面向等离子体壁材料对耐高温和高导热率的要求,论文设计了铜含量范围为35-87vo1.%的三层W-Cu体系梯度功能材料、铜含量范围为35～94.1vol.%的六层W-Cu体系梯度功能材料和铜含量为35-962vo1.%九层W-Cu体系梯度功能材料。采用以上优化的化学镀、流延成型技术和PAS烧结工艺,制备出整体致密、中间过渡层过渡均匀结合紧密、钨体积分数梯度变化的两种结构W-Cu体系功能梯度复合材料,并对W-Cu体系功能梯度复合材料的组织结构进行了表征。结果表明,在同一的烧结制度下制备出的W-Cu体系功能梯度复合材料结构良好,整体致密。三层W-Cu体系梯度功能材料沿着厚度方向的等效热导率为301.8W/(m·K),等效电导率为0.366×108S/m,六层W-Cu体系梯度功能材料沿着厚度方向的等效热导率为309.2W/(m·K),等效电导率为0.378×108S/m,九层W-Cu体系梯度功能材料沿着厚度方向的等效热导率为310.1W/(m·K),等效电导率为0.381×108S/m,高钨含量面的维氏硬度达到225HV,随着层数增多,W-Cu梯度功能材料的综合性能得到了提高,实现了论文的功能性设计要求。本文的研究成果成功为W-Cu体系功能梯度复合材料在电子封装、集成电路、国防军工、航空航天等高尖端技术领域开拓了应用前景。
W-Cu composite is one of the most extensively studied metallic matrix composite materials with the excellent property of complementary. Cu has good electrical conductivity, thermal conductivity and corrosion resistance, but it has low strength; W has high hardness, strength, good wear resistance and thermal stability. The combination of the advantages of Cu and W has made the W-Cu composite become a potential candidate material for a variety of high-tech application such as electron sealing, integrated circuit, national defense military and aerospace. However, with the use condition becoming harsher, such as in the high-temperature environment, the homogeneous composite material would not be able to meet the need as with its thermal and mechanical property. To solve the problem, people proposed a kind of W-Cu composite material with a variety of excellent performances named W-Cu functionally graded material (FGM). Many studies have been focused on the design, preparation and performance of the W-Cu functionally graded material for different service conditions. According to the previous reported results, it's hard to prepare the material with the design requirement structure and the high comprehensive performance, as many problems exist, such as complex molding process, sintering difficulty and hard control of the preparation process.
So this work concentrate on the W-Cu functionally graded material. To solve the problems mentioned above, this work use the electroless plating process to plate a uniform copper layer on the surface of the tungsten particals without limiting by the shape or size of the tungsten particles. By changing the state of the tungsten particles surface, the wetting performance between the tungsten and copper was enhanced. Use Plasma Activated Sintering technique to activate the sintering powder, reduce the sintering temperature, increase the density of the sintering body. Use the aqueous tape-casting technique to solve the component controling and molding problem. The paper studied the key preparation process, including the raw material processing, molding and sintering of the W-Cu functionally graded material theoretically and experimentally in detail. And then characterize the structures and the performances of W-Cu homogeneous composites and W-Cu functionally graded composite. The main conclusions are as follow:
Firstly, glyoxic acid was used to replace the formaldehyde as the reducing agent. A novel electroless plating method—copper salt dripping method has been proposed for the first time. Designing of experiment (DOE) technique is used for optimize the data processing. Data analysis show that the optimal conditions are as the followings: flow velocity-5.01ml/min, pH-12.25, Temperature-45.35℃
The effects of the pH value, reaction temperature, flow rate of copper salt and reducing agent were investigated in this work. The deposits obtained in this work was found to be polycrystalline fcc copper, in which no monovalent or divalent Cu ions were detected. In addition, these powders displayed good dispersion property. It means that we've already successfully prepared the Cu@W core-shell structure composite powders through copper salt dripping method in glyoxylic acid-water system, which will provide the high purity, quantitative coated powder raw materials for the preparation of the W-Cu functionally graded material.
On this basis, the thermodynamic and the dynamics processes of the dripping method for direct electroless copper plating on W particle were analysis. The study showed that the heterogeneous nucleation play a decisive role in the dripping method for direct electroless copper plating on W particle process. Dripping method can help keeping the concentration of the copper ions to a low level, a main factor of obtaining a good coating effect, under such condition the pictogram nuclear would be avoid and the main driving force of the coating process-over-saturation would be reduced. According to analysis the reaction mechanism of dripping method for direct electroless copper plating on W particle, the model of the formation of the Cu plating on W particles was established. The model showed that firstly a large amount of Cu2+ions gathered on the tungsten particles surfaces, forming a positively charged entity. In this case the surface of the copper ions entity has the same effect with the surface of the metallic copper-autocatalator, and become the catalytically active centers, which trigger the initial oxidation-reduction reaction between the glyoxylate and copper ions to form a copper plating layer. The metallic copper is deposit on the surface of it to form a dense shell structure. Then the deposited Cu becomes the catalytic centers for the further electroless Cu plating. Subsequently, the deposited copper layer continue to serve as the activation center to induced the oxidation-reduction electroless plating proceed.
Secondly, W-Cu composite cast sheet with the uniform thickness, high solid content were prepared by the powder synthetized above and the aqueous tape-casting technique, which provide the raw material for the sintering of the W-Cu functionally graded material. In this paper, the Computational Fluid Dynamics (CFD) and the Bingham model were chose to help the numerical simulation of the whole tape casting process. The influences of the blade gap, the casting speed and the slurry's rheological property to the W-Cu cast sheet were studied. The critical process parameters and slurry properties for the stable cast molding were achieved: Speed>2mm/s, Viscosity>3P By experiment the composition of the tape casting slurry and the technological condition of the tape casting, optimal recipe for the aqueous slurry was got: Y=0.0141+3.39E-4·X, R=0.988Where X is the volume of the solid powder, Y is the mass ratio of the binder (PVA-2488) with the solid phase powder. The mass ratio of the plasticizer (glycerin) and a binder is0.6. Through optimizing the solid content of the casting slurry, the binder and the plasticizer, the tungsten copper aqueous tape-casting film with high solid content, uniform structure, great performance were successfully produced.
Thirdly, this work successfully prepared the homogeneous W-Cu composite with the ideal structure, high density and excellent performance by the PAS sintering technology. The densification and various performance of the W-Cu homogeneous composite were studied in details. The influence of the W volume fraction to the density, coefficient of thermal expansion, thermal conductivity and electrical conductivity of the composite were researched. The result showed that using the powder with the coating structure greatly reduced the difficulty of the densification. Under the low temperature sintering condition of950degrees-100MPa-lh, different component W-Cu composites all achieved the high density of more than98%. The density, thermal expansion coefficient, thermal conductivity, electrical conductivity decreased and the hardness increased with the increasing of the W. We also got the prediction model formula about the performance and the component of the W-Cu composition, which can provide the predictive design models and the sintering condition for W-Cu functionally graded material.
Finally, this work completes the functional design of the W-Cu functionally graded materials directing at the W-Cu composite applications. For The requirements of the high thermal conductivity and low thermal expansion coefficient for the electronic packaging field, high hardness and high electrical conductivity for the electrical contacts or electrodes industry, as well as the high temperature resistance and high thermal conductivity for the plasma-oriented wall material. This work prepared two kinds of W-Cu functionally graded material. Three layers with the copper content in the range of35～87vol.%. Six layers with the copper content in the range of35～94.1vol.%and nine layers with the copper content in the range of35～96.2vol.%. By using optimized tape casting and plasma activated sintering (PAS) processing above, The whole sample is dense. The intermediate layer combines tightly. And the volume fraction of the W varied gradiently. The properties of the W-Cu functionally gradient material were then studied in detail. The results show that W-Cu functionally graded material achieved good structure and distributed compactly. The thermal conductivity along the thickness direction of the three layers W-Cu functionally gradient material with the distribution index of1was301.8W/(m·K), and the electrical conductivity was0.366×108S/m. The thermal conductivity along the thickness direction of the six layers W-Cu functionally gradient material with the distribution index of1was309.2W/(m·K), and the electrical conductivity was0.378×108S/m. The thermal conductivity along the thickness direction of the nine layers W-Cu functionally gradient material with the distribution index of1was310.1W/(m·K), and the electrical conductivity was0.381×108S/m. With the layers increase, the performance of the W-Cu FGM was improved. This work successfully developed the application aspect for the W-Cu functionally gradient material in the high-tech areas of electronic packaging, integrated circuit, defense industry and aerospace.
为此,本文以W-Cu体系功能梯度复合材料为研究对象,针对文献报道的W-Cu体系复合材料所存在的问题和难点,采用化学镀工艺在钨颗粒表面镀铜,改变表面性质,改善钨铜界面润湿性,提高界面结合强度；采用等离子体活化烧结技术(Plasma Activated Sintering,简称PAS)活化烧结W@Cu粉体、降低烧结温度、提高烧结体致密度；采用绿色环保的水基流延成型工艺,解决W-Cu体系功能梯度复合材料组分控制与成型的难题。论文从理论和实验两个方面出发,详细研究了W-Cu体系功能梯度复合材料的关键制备技术(原料处理、成型、烧结等),并详细表征了W-Cu体系均质复合材料和功能梯度复合材料的结构、性能。论文的主要研究工作和结论如下：
首先,本文以环保的乙醛酸取代毒性大的甲醛为还原剂,首次采用无敏化、活化的滴加法直接化学镀技术在钨粉表面化学镀铜制备出定量、高纯的W@Cu复合粉体。通过使用中心实验设计的方法对化学镀铜实验进行设计和工艺条件优化,获得滴加法直接化学镀铜的最优工艺条件：乙醛酸溶液流速5.01mL/min、pH值为12.25、水浴温度为45.35℃。在此工艺条件下,制备的W@Cu复合粉体的铜镀层增重误差仅为0.014%,说明此工艺条件可以制备出定量的W@Cu复合粉体。论文详细考察了pH值、反应温度、还原剂和铜盐加入速度对W@Cu复合粉体结构影响。多种测试结果表明,在本文优化的工艺条件下,铜镀层呈面心立方多晶结构单质Cu,未发现含有1价态和2价态铜离子,包覆层结构紧密,W@Cu复合粉体分散性良好,这说明本实验采用的滴加法直接化学镀技术在乙醛酸-水的绿色体系中成功制备了高纯W@Cu复合粉体,为制备W-Cu体系功能复合材料提供了高纯、定量的包覆粉体原料。
在此基础上,论文还详细分析了滴加法直接化学镀铜制备W@Cu复合粉体的热力学过程和动力学过程,研究表明,滴加法直接化学镀铜过程为异相形核。采用滴加法可很好地控制镀液中铜离子处于低浓度状态,进而,使还原的铜原子同样处于较低的浓度状态,形成了获得良好包覆效果的前提,即避免均相形核的产生-降低了包覆过程的主要推动力-过饱和度。通过对化学镀铜的热力学分析、W@Cu粉体不同时间段的形貌观察和催化机理的研究,建立了钨表面滴加法直接化学镀铜的铜镀层形成模型,提出了其反应机理。分析认为,铜离子首先在钨表面大量聚集,形成带正电的实体,此时铜离子实体表面起到了与金属铜表面一样的效果—自催化,成为催化活性中心,引发乙醛酸与铜离子发生最初的氧化还原反应形成铜镀层,还原出来的金属铜沉积在其表面形成致密的壳层结构：随后,沉积的铜层继续作为活化中心诱导化学镀氧化还原反应继续进行。
其次,本文利用制备的W@Cu复合粉体进行水基流延成型,制备出厚度均匀、固相含量高的W-Cu流延素片,为W-Cu体系复合材料和功能梯度复合材料的烧结提供良好素片。本文基于计算流体力学(CFD),对W-Cu水基流延成型工艺进行数值模拟仿真,详细考察了W-Cu浆料性质和制备工艺制度对流延湿带的影响,获得了稳定流延成型的关键工艺参数和浆料性质：浆料粘度>3P,基带速度>2mm/s,刮刀间隙对流延湿带厚度起主要作用。通过对W-Cu水基流延浆料的实验研究,获得了W-Cu水基流延浆料的最佳配方：粘结剂(PVA-2488)用量与(纯铜)/(W@Cu)质量比满足线性比例关系Y=0.0141+3.39E-4·X,R=0.988；增塑剂(丙三醇)和粘结剂(PVA-2488)的质量比为0.6。通过对W-Cu体系流延浆料固相含量、粘结剂、增塑剂的配比优化,成功制备出了固相含量高、结构均匀、性能良好的不同含量W-Cu水基流延素片。
再次,本文成功利用PAS烧结技术制备出结构理想、高致密度和性能优良的W-Cu均质复合材料,详细研究了W-Cu均质复合材料的烧结致密化与各种性能。考察了钨体积分数对W-Cu复合材料的致密度、硬度、热膨胀系数、热导率和电导率的影响。结果表明：采用W@Cu复合粉体可降低W-Cu均质复合材料烧结温度,在900℃-30MPa-5min的低温烧结工艺条件下,不同钨含量,特别是高钨含量的W-Cu体系均质复合材料均实现致密化。结果表明,采用W@Cu复合粉体和等离子体活化烧结技术制备出的W-Cu体系均质复合材料具有良好的力学、热学和电学性能。其硬度和杨氏模量力学性能随钨颗粒体积含量的增大而升高。其热膨胀系数、热导率、电导率随着钨体积含量的增大而降低；当Cu含量为30vo1.%时,W-Cu均质复合材料的硬度值为234HV,杨氏模量290GPa,电导率为0.309×108S/m,热导率为235.82W/(m·K),热膨胀系数为8.0μrn/(m·K)。并确定了W-Cu复合材料各项性能参数与体积含量的预测模型公式,为W-Cu体系功能梯度复合材料提供了设计预测模型和烧结工艺制度。
最后,本文针对W-Cu复合材料的应用领域,完成了W-Cu体系功能梯度复合材料的功能性设计；针对电子封装领域对高热导率和低热膨胀系数的要求,电触头或电极行业对高硬度和高电导率的要求,以及面向等离子体壁材料对耐高温和高导热率的要求,论文设计了铜含量范围为35-87vo1.%的三层W-Cu体系梯度功能材料、铜含量范围为35～94.1vol.%的六层W-Cu体系梯度功能材料和铜含量为35-962vo1.%九层W-Cu体系梯度功能材料。采用以上优化的化学镀、流延成型技术和PAS烧结工艺,制备出整体致密、中间过渡层过渡均匀结合紧密、钨体积分数梯度变化的两种结构W-Cu体系功能梯度复合材料,并对W-Cu体系功能梯度复合材料的组织结构进行了表征。结果表明,在同一的烧结制度下制备出的W-Cu体系功能梯度复合材料结构良好,整体致密。三层W-Cu体系梯度功能材料沿着厚度方向的等效热导率为301.8W/(m·K),等效电导率为0.366×108S/m,六层W-Cu体系梯度功能材料沿着厚度方向的等效热导率为309.2W/(m·K),等效电导率为0.378×108S/m,九层W-Cu体系梯度功能材料沿着厚度方向的等效热导率为310.1W/(m·K),等效电导率为0.381×108S/m,高钨含量面的维氏硬度达到225HV,随着层数增多,W-Cu梯度功能材料的综合性能得到了提高,实现了论文的功能性设计要求。本文的研究成果成功为W-Cu体系功能梯度复合材料在电子封装、集成电路、国防军工、航空航天等高尖端技术领域开拓了应用前景。
W-Cu composite is one of the most extensively studied metallic matrix composite materials with the excellent property of complementary. Cu has good electrical conductivity, thermal conductivity and corrosion resistance, but it has low strength; W has high hardness, strength, good wear resistance and thermal stability. The combination of the advantages of Cu and W has made the W-Cu composite become a potential candidate material for a variety of high-tech application such as electron sealing, integrated circuit, national defense military and aerospace. However, with the use condition becoming harsher, such as in the high-temperature environment, the homogeneous composite material would not be able to meet the need as with its thermal and mechanical property. To solve the problem, people proposed a kind of W-Cu composite material with a variety of excellent performances named W-Cu functionally graded material (FGM). Many studies have been focused on the design, preparation and performance of the W-Cu functionally graded material for different service conditions. According to the previous reported results, it's hard to prepare the material with the design requirement structure and the high comprehensive performance, as many problems exist, such as complex molding process, sintering difficulty and hard control of the preparation process.
So this work concentrate on the W-Cu functionally graded material. To solve the problems mentioned above, this work use the electroless plating process to plate a uniform copper layer on the surface of the tungsten particals without limiting by the shape or size of the tungsten particles. By changing the state of the tungsten particles surface, the wetting performance between the tungsten and copper was enhanced. Use Plasma Activated Sintering technique to activate the sintering powder, reduce the sintering temperature, increase the density of the sintering body. Use the aqueous tape-casting technique to solve the component controling and molding problem. The paper studied the key preparation process, including the raw material processing, molding and sintering of the W-Cu functionally graded material theoretically and experimentally in detail. And then characterize the structures and the performances of W-Cu homogeneous composites and W-Cu functionally graded composite. The main conclusions are as follow:
Firstly, glyoxic acid was used to replace the formaldehyde as the reducing agent. A novel electroless plating method—copper salt dripping method has been proposed for the first time. Designing of experiment (DOE) technique is used for optimize the data processing. Data analysis show that the optimal conditions are as the followings: flow velocity-5.01ml/min, pH-12.25, Temperature-45.35℃
The effects of the pH value, reaction temperature, flow rate of copper salt and reducing agent were investigated in this work. The deposits obtained in this work was found to be polycrystalline fcc copper, in which no monovalent or divalent Cu ions were detected. In addition, these powders displayed good dispersion property. It means that we've already successfully prepared the Cu@W core-shell structure composite powders through copper salt dripping method in glyoxylic acid-water system, which will provide the high purity, quantitative coated powder raw materials for the preparation of the W-Cu functionally graded material.
On this basis, the thermodynamic and the dynamics processes of the dripping method for direct electroless copper plating on W particle were analysis. The study showed that the heterogeneous nucleation play a decisive role in the dripping method for direct electroless copper plating on W particle process. Dripping method can help keeping the concentration of the copper ions to a low level, a main factor of obtaining a good coating effect, under such condition the pictogram nuclear would be avoid and the main driving force of the coating process-over-saturation would be reduced. According to analysis the reaction mechanism of dripping method for direct electroless copper plating on W particle, the model of the formation of the Cu plating on W particles was established. The model showed that firstly a large amount of Cu2+ions gathered on the tungsten particles surfaces, forming a positively charged entity. In this case the surface of the copper ions entity has the same effect with the surface of the metallic copper-autocatalator, and become the catalytically active centers, which trigger the initial oxidation-reduction reaction between the glyoxylate and copper ions to form a copper plating layer. The metallic copper is deposit on the surface of it to form a dense shell structure. Then the deposited Cu becomes the catalytic centers for the further electroless Cu plating. Subsequently, the deposited copper layer continue to serve as the activation center to induced the oxidation-reduction electroless plating proceed.
Secondly, W-Cu composite cast sheet with the uniform thickness, high solid content were prepared by the powder synthetized above and the aqueous tape-casting technique, which provide the raw material for the sintering of the W-Cu functionally graded material. In this paper, the Computational Fluid Dynamics (CFD) and the Bingham model were chose to help the numerical simulation of the whole tape casting process. The influences of the blade gap, the casting speed and the slurry's rheological property to the W-Cu cast sheet were studied. The critical process parameters and slurry properties for the stable cast molding were achieved: Speed>2mm/s, Viscosity>3P By experiment the composition of the tape casting slurry and the technological condition of the tape casting, optimal recipe for the aqueous slurry was got: Y=0.0141+3.39E-4·X, R=0.988Where X is the volume of the solid powder, Y is the mass ratio of the binder (PVA-2488) with the solid phase powder. The mass ratio of the plasticizer (glycerin) and a binder is0.6. Through optimizing the solid content of the casting slurry, the binder and the plasticizer, the tungsten copper aqueous tape-casting film with high solid content, uniform structure, great performance were successfully produced.
Thirdly, this work successfully prepared the homogeneous W-Cu composite with the ideal structure, high density and excellent performance by the PAS sintering technology. The densification and various performance of the W-Cu homogeneous composite were studied in details. The influence of the W volume fraction to the density, coefficient of thermal expansion, thermal conductivity and electrical conductivity of the composite were researched. The result showed that using the powder with the coating structure greatly reduced the difficulty of the densification. Under the low temperature sintering condition of950degrees-100MPa-lh, different component W-Cu composites all achieved the high density of more than98%. The density, thermal expansion coefficient, thermal conductivity, electrical conductivity decreased and the hardness increased with the increasing of the W. We also got the prediction model formula about the performance and the component of the W-Cu composition, which can provide the predictive design models and the sintering condition for W-Cu functionally graded material.
Finally, this work completes the functional design of the W-Cu functionally graded materials directing at the W-Cu composite applications. For The requirements of the high thermal conductivity and low thermal expansion coefficient for the electronic packaging field, high hardness and high electrical conductivity for the electrical contacts or electrodes industry, as well as the high temperature resistance and high thermal conductivity for the plasma-oriented wall material. This work prepared two kinds of W-Cu functionally graded material. Three layers with the copper content in the range of35～87vol.%. Six layers with the copper content in the range of35～94.1vol.%and nine layers with the copper content in the range of35～96.2vol.%. By using optimized tape casting and plasma activated sintering (PAS) processing above, The whole sample is dense. The intermediate layer combines tightly. And the volume fraction of the W varied gradiently. The properties of the W-Cu functionally gradient material were then studied in detail. The results show that W-Cu functionally graded material achieved good structure and distributed compactly. The thermal conductivity along the thickness direction of the three layers W-Cu functionally gradient material with the distribution index of1was301.8W/(m·K), and the electrical conductivity was0.366×108S/m. The thermal conductivity along the thickness direction of the six layers W-Cu functionally gradient material with the distribution index of1was309.2W/(m·K), and the electrical conductivity was0.378×108S/m. The thermal conductivity along the thickness direction of the nine layers W-Cu functionally gradient material with the distribution index of1was310.1W/(m·K), and the electrical conductivity was0.381×108S/m. With the layers increase, the performance of the W-Cu FGM was improved. This work successfully developed the application aspect for the W-Cu functionally gradient material in the high-tech areas of electronic packaging, integrated circuit, defense industry and aerospace.
引文
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