Thermische Ausdehnung,innere Spannungen und Porenverteilung in AlSiCp Metallmatrix‐ verbundwerkstoffen

Thermal expansion, internal stresses and porosity distribution in AlSiCp MMC AlSi7Mg/SiC/70p (AlSiC) is used for heat sinks because of its good thermal conductivity combined with a low coefficient of thermal expansion (CTE). These properties are important for power electronic devices where heat sinks have to provide efficient heat transfer to a cooling device. A low CTE is essential for a good surface bonding of the heat sink material to the isolating ceramics. Otherwise mismatch in thermal expansion would lead to damage of the bonding degrading the thermal contact within the electronic package. Therefore AlSiC replaces increasingly copper heat sinks. The CTE mismatch between isolation and a conventional metallic heat sink is transferred into the metal matrix composite (MMC). The stability of the external and internal interface bonding is essential for the heat sink function of AlSiC. In situ thermal cycling (RT – 400 °C) measurements of an AlSi7Mg/SiC/70p MMC are reported yielding the pore volume fraction and internal stresses between the matrix and the reinforcements in function of temperature. The changes in pore volume fractions are determined by synchrotron tomography and residual stresses by synchrotron diffraction at ESRF‐ID15A. The measurements show a relationship between thermal expansion, residual stresses and pore formation in the MMC. The results obtained from the in situ measurements reveal a thermo elastic range with inversion of the dominant tensile stresses in the matrix into compressive up to 200 °C followed by plastic matrix deformation reducing the volume of pores during heating. A reverse process takes place during cooling from 500 °C starting with elastic matrix strains converting into tensile stresses increasing the pore volume fraction again. Below 200 °C, the CTE behaves again according to thermo elastic calculations. Damage like in low cycle fatigue could be observed after multiple extreme cooling‐heating cycles between –100 °C and +400 °C, which increase the volume fraction and the size of the voids.     

Void formation in metal matrix composites by solidification and shrinkage of an AlSi7 matrix between densely packed particles

Particle reinforced metals are developed as heat sink materials for advanced thermal management applications. Metal matrix composites combine the high thermal conductivity of a metal with a low coefficient of thermal expansion of ceramic reinforcements. SiC and carbon diamond particle reinforced aluminum offer suitable thermal properties for heat sink applications. These composites are produced by liquid metal infiltration of a densely packed particle preform. Wettability, interface bonding strength and thermal mismatch are critical for void formation which leads to thermal fatigue damage under operation. The evolution of voids in AlSiC and AlCD has been studied by in-situ high resolution synchrotron tomography during matrix solidification. Large irregularly shaped matrix voids form during eutectic solidification. These voids help alleviate thermal expansion mismatch stresses by visco-plastic matrix deformation during cooling to RT after solidification, if sufficient interface bonding strength is assumed.     

Zirconium tungstate/cyanate ester nanocomposites with tailored thermal expansivity

The dimensional stability of polymer matrix composites can be enhanced by reducing the mismatch in the coefficient of thermal expansion (CTE) between the high CTE polymer matrix and low CTE fiber reinforcements, which leads to development of residual stresses and matrix microcracking. A potential strategy to diminish these residual stresses involves development of polymer nanocomposites with well dispersed nanoparticles that reduce the extent of mismatch in CTE. In this work, we explore the potential for development of bulk polymer nanocomposites with tailored thermal expansivity through incorporation of zirconium tungstate nanoparticles that are characterized by a negative CTE in a unique low viscosity bisphenol E cyanate ester (BECy) thermosetting polymer matrix. Incorporation of up to 10 vol.% whisker-like nanoparticles, synthesized by a hydrothermal method, results in a 20% reduction in the CTE of the polymer matrix. However, the nanoparticles exert a dramatic catalytic effect on the cure reaction of BECy resin and subsequently decrease the onset temperature of the glass transition for the cured polymer network, at high filler loadings.     

Carbon fiber-reinforced cyanate ester/nano-ZrW2O8 composites with tailored thermal expansion

Fiber-reinforced composites are widely used in the design and fabrication of a variety of high performance aerospace components. The mismatch in coefficient of thermal expansion (CTE) between the high CTE polymer matrix and low CTE fiber reinforcements in such composite systems can lead to dimensional instability and deterioration of material lifetimes due to development of residual thermal stresses. The magnitude of thermally induced residual stresses in fiber-reinforced composite systems can be minimized by replacement of conventional polymer matrices with a low CTE, polymer nanocomposite matrix. Zirconium tungstate (ZrW(2)O(8)) is a unique ceramic material that exhibits isotropic negative thermal expansion and has excellent potential as a filler for development of low CTE polymer nanocomposites. In this paper, we report the fabrication and thermal characterization of novel, multiscale, macro-nano hybrid composite laminates comprising bisphenol E cyanate ester (BECy)/ZrW(2)O(8) nanocomposite matrices reinforced with unidirectional carbon fibers. The results reveal that incorporation of nanoparticles facilitates a reduction in CTE of the composite systems, which in turn results in a reduction in panel warpage and curvature after the cure because of mitigation of thermally induced residual stresses.     

Thermo-physical properties and TEM analysis of silver based MMCs utilizing metallized multiwall-carbon nanotubes

Metal matrix composites with embedded multiwall-carbon nanotubes (MWNT) are attractive because MWNTs exhibit high intrinsic thermal conductivity. Thus to improve the thermal conductivity of a metal matrix, silver matrix composites with MWNT were prepared by “chemical” mixing, different active elements were introduced enhancing the bonding between inclusions and matrix. The evolution of the thermal conductivity and the coefficient of thermal expansion CTE as a function of the MWNT concentration and the presence of active elements cobalt, molybdenum or nickel in the silver matrix in Ag–X/MWNT composites are presented. A transition from weak to strong matrix/MWNT bonding is observed by adding active elements, the latter leading concomitantly to an increase in thermal conductivity and a decrease in CTE. The thermal conductivity was found to increase by up to 10% for a composition of 0.2 wt.% MWNT and cobalt as active element and a 6% decrease in CTE compared to a pure silver reference.     

Thermomechanical properties of copper–carbon nanofibre composites prepared by spark plasma sintering and hot pressing

Several types of carbon nanofibres (CNF) were coated with a uniform and dense copper layer by electroless copper deposition. The coated fibres were then sintered by two different methods, spark plasma sintering (SPS) and hot pressing (HP). The Cu coating thickness was varied so that different volume fraction of fibres was achieved in the produced composites. In some cases, the CNF were pre-coated with Cr for the improvement the Cu adhesion on CNF. The results show that the dispersion of the CNF into the Cu matrix is independent of the sintering method used. On the contrary, the dispersion is directly related to the efficiency of the Cu coating, which is tightly connected to the CNF type. Overall, strong variations of the thermal conductivity (TC) of the composites were observed (20–200 W/mK) as a function of CNF type, CNF volume fraction and Cr content, while the coefficient of thermal expansion (CTE) in all cases was found to be considerably lower than Cu (9.9–11.3 ppm/K). The results show a good potential for SPS to be used to process this type of materials, since the SPS samples show better properties than HP samples even though they have a higher porosity, in applications where moderate TC and low CTE are required.     

Use of zirconium diboride-copper as an electrode in plasma applications

Frequent replacement of electrodes, due to their high wear rate, is an undesired feature of most thermal plasma processes. Hence, the discovery of a high spark-resistive tool, ZrB2-Cu, is of interest. Performance evaluation of this metal matrix ceramic (MMC) employed electrical discharge machining (EDM), where steel is used as the cathode workpiece and the MMC is used as the anode tool. Compared with the performance of copper and graphite tools, ZrB2-Cu yields the highest workpiece removal rate,; and the lowest tool wear rate at high plasma heat flux conditions, resulting in an extremely low wear ratio. Energy dispersive spectroscopy shows deposition of workpiece materials (Fe, Cr, Ni and S) on the ZrB2-Cu surface after EDM. This is due to the difference between the surface temperature of the tool and the workpiece. Scanning electron microscopy and elemental mapping analysis reveal that the composite electrode erodes by a combination of dominant evaporation and melting of the metal phase, negligible melting and thermal spalling in the ceramic phase, quick refreezing of the metal phase back to the surface, and deposition of the workpiece (steel) on the tool surface. Most of the heat is conducted through the Cu phase, reducing thermal stress in the ceramic phase. This causes lower surface temperatures for the molten ZrB2 matrix; hence, the Cu tends to refreeze quickly near the surrounding ceramic matrix.     

Incorporation of halloysite nanotubes into PVDF matrix: Nucleation of electroactive phase accompany with significant reinforcement and dimensional stability improvement

Poly(vinylidene fluoride)/halloysite nanotubes (PVDF/HNTs) nanocomposites were prepared via melt compounding. Electroactive β- and γ-phases of PVDF were nucleated by the HNTs due to electrical interaction between negatively charged surface of the HNTs and CH₂ groups of the PVDF. The ends and surface defects of the HNTs were also responsible for the formation of γ-phase, since long trans conformations with gauche defects were induced at the two regions via the formation of hydrogen bonding. In addition to nucleation of the electroactive phase, the HNTs were found to reinforce the PVDF matrix and improve its dimensional stability, as evidenced by the substantially increased tensile strength and Young’s modulus, and the remarkably decreased coefficient of thermal expansion (CTE). The improved tensile property and reduced CTE were attributed to the uniformly dispersed HNTs and good interfacial interaction between HNTs and PVDF matrix via hydrogen bonding.     

高温处理对3D C/SiC复合材料热膨胀性能的影响

研究了不同高温处理前后3D C/SiC复合材料热膨胀系数(CTE)的变化规律,从材料内部热应力变化及结构改变的角度定性地分析了其变化机理。研究发现,3D C/SiC复合材料的热膨胀系数受界面热应力的影响,其变化规律是纤维和基体相互限制、相互竞争的结果;高温处理可提高材料的热稳定性,并通过改变界面热应力及材料内部结构,来影响材料热膨胀系数的变化规律;通过增加基体裂纹来降低复合材料的低温热膨胀,但不影响其变化规律;通过改变材料内部结构,使热应力发生变化并重新分布,对复合材料的高温热膨胀产生显著影响。但高温处理没有改变3D C/SiC复合材料的基体裂纹愈合温度(900℃)。      

Effect of heat treatment of carbon nanofibres on electroless copper deposition

Cu is a well known heat sink material due to its high thermal conductivity. However, its coefficient of thermal expansion (CTE) is high. One of the most promising solutions for reducing it is to reinforce copper with carbon nanofibres (CNF) because of their low CTE. To exploit the properties of the CNFs a good dispersion of the reinforcement within the matrix must be achieved. One of the processing methods used to obtain a homogeneous CNF distribution is coating the CNF with Cu using electrochemical deposition. In this paper, the effect of the carbon structure on electroless deposition technique is studied. Different CNF have been compared: herringbone (HB), platelet (PL) and longitudinally aligned (previously heat treated) (LAHT). Herringbone and Platelet CNF were heat treated at 2750 °C for 30′ which resulted in a structure resembling graphite with loops at the fibre surface. These loops are responsible for an enhancement of the copper coating. It is shown that the Cu coverage in electroless deposition is high for the graphene plane and poor at the edges of the plane.     

Development of light composite materials with low coefficients of thermal expansion

Abstract

Composite materials based on aluminium are used in different fields where weight, thermal expansion, and thermal stability are key requirements. The aim of the present study was to develop a universal method and scientific approach for evaluating the design of lightweight, Al matrix composites with low coefficients of thermal expansion (CTE) and high dimensional stability, and to produce such composites using the vacuum plasma spray (VPS)process. The methodology is general and could be applied to other composite systems. The VPS-produced Al and Al alloy 6061 based composites were reinforced with a variety of ceramic particles including Si₃N₄, B₄C, TiB₂, and 3Al₂O₃.2SiO₂. These composites have low CTE values ((12–13)×10^-6 K^-1), similar to that of steel, and high dimensional stability (capable of keeping dimensions stable with changes in temperature). They have low porosity (98–99%dense) and a uniform distribution of the strengthening particles. Hot rolling of the VPS-formed composites, followed by heat treatment, resulted in a significant improvement in the mechanical properties. Deformed and heat treated 6061 based composites, containing 20 wt-%TiB₂ and 40 wt-%3Al₂O₃?2SiO₂, showed excellent mechanical properties (ultimate tensile strength 210–250 MPa, elongation >4%).     

Thermal expansion in metal/lithia-alumina-silica (LAS) composites

Lithia-alumina-silica (LAS) with metallic dispersions offers a new approach toward near-zero, isotropic, thermal expansion composites. The metallic phase contributes a positive coefficient of thermal expansion (CTE) to the negative CTE of the glass/ceramic matrix. In addition, the metal will increase the electrical and thermal conductivities over those of the matrix alone. The LAS system offers tailorable negative CTEs and light weight compared to other negative CTE ceramics. The most negative CTE phase is crystalline -eucryptite, whose proportion in an initially glassy matrix can be controlled by heat treatment. Dispersed metal powders were both hot-pressed and cold-pressed and sintered together with LAS matrices prepared by sol gel methods. Super Invar powder was studied for its minimal CTE mismatch, while titanium powders offered a compromise between light weight and low CTE. An ultralow-expansion (ULE) glass- and linear variable differential transducer (LVDT)-based differential dilatometer was developed for rapid screening of compositions, while a double-laser Michelson interferometer was used for precise near-zero CTE measurements. The reinforced -eucryptite glass/ceramic matrix exhibited both a U-shaped L/L curve with temperature and some thermal hysteresis, depending on the fabrication and heat treatment sequences. The temperature of the zero-CTE portion of this curve was found to change with increasing titanium powder content. Results are also given for mixtures of Super Invar powders in ULE glass and -eucryptite matrices. Negative CTEs in a LAS matrix above ambient temperatures were more difficult to obtain than below, although the use of petalite (high-silica LAS) appears promising.Paper presented at the Ninth International Thermal Expansion Symposium, December 8–10, 1986, Pittsburgh, Pennsylvania, U.S.A.     

Effect of filler geometry on coefficient of thermal expansion in carbon nanofiber reinforced epoxy composites

This study involves the investigation of the geometry effect of nano-fillers on thermally induced dimensional stability of epoxy composites by experimentally evaluating the linear coefficient of thermal expansion (CTE). Carbon nanofibers (CNF) were chosen as the filler in epoxy matrix to investigate the effect of an aspect ratio on the CTE of the nanocomposites at three different volume fractions of 0.5, 1, and 2% of the nano-filler. The composites were fabricated using a mechanical mixing method. The CTE values were evaluated by measuring thermal strains of the composites and also compared with a micromechanics model. It was observed that the composites with short CNF (average L/d = 10) show better thermal stability than one of the composites with long CNF (average L/d = 70), and the thermal stability of the composites was proportional to the volume fraction of the filler in each composite. In addition, the CTE of mutliwalled carbon nanotubes (MWNT) reinforced epoxy composites was evaluated and compared with the CTE of the CNF reinforced composites. Interestingly, the MWNT reinforced composites show the greatest thermal stability with an 11.5% reduction in the CTE over the pure epoxy. The experimental data was compared with micromechanics model.     

Thermally conducting aluminum nitride polymer-matrix composites

Thermally conducting, but electrically insulating, polymer-matrix composites that exhibit low values of the dielectric constant and the coefficient of thermal expansion (CTE) are needed for electronic packaging. For developing such composites, this work used aluminum nitride whiskers (and/or particles) and/or silicon carbide whiskers as fillers(s) and polyvinylidene fluoride (PVDF) or epoxy as matrix. The highest thermal conductivity of 11.5 W/(m K) was attained by using PVDF, AlN whiskers and AlN particles (7 μm), such that the total filler volume fraction was 60% and the AlN whisker–particle ratio was 1:25.7. When AlN particles were used as the sole filler, the thermal conductivity was highest for the largest AlN particle size (115 μm), but the porosity increased with increasing AlN particle size. The thermal conductivity of AlN particle epoxy-matrix composite was increased by up to 97% by silane surface treatment of the particles prior to composite fabrication. The increase in thermal conductivity is due to decrease in the filler–matrix thermal contact resistance through the improvement of the interface between matrix and particles. At 60 vol.% silane-treated AlN particles only, the thermal conductivity of epoxy-matrix composite reached 11.0 W/(m K). The dielectric constant was quite high (up to 10 at 2 MHz) for the PVDF composites. The change of the filler from AlN to SiC greatly increased the dielectric constant. Combined use of whiskers and particles in an appropriate ratio gave composites with higher thermal conductivity and low CTE than the use of whiskers alone or particles alone. However, AlN addition caused the tensile strength, modulus and ductility to decrease from the values of the neat polymer, and caused degradation after water immersion.     

Recent progress in graphene-reinforced aluminum matrix composites

Recent years witnessed a growing research interest in graphene-reinforced aluminum matrix composites(GRAMCs).Compared with conventionalreinforcements of aluminum matrix composites(AMCs),graphene possesses manyattractive characteristics such as extremely high strength and modulus,unique self-lubricating property,high thermal conductivity(TC)and electrical conductivity(EC),andlow coefficient of thermal expansion(CTE).A lot of studies have demonstrated that theincorporation of graphene into Al or Al alloy can effectively enhance mechanical andphysical properties of the Al matrix.The purpose of this work is aimed to trace recentdevelopment of GRAMCs.Initially,this paper covers a brief overview of fabricationmethods of GRAMCs.Then,mechanical,tribological,thermal and electrical properties ofrecently developed GRAMCs are presented and discussed.Finally,challenges andcorresponding solutions related to GRAMCs are reviewed.     

Verbundwerkstoffe mit Metallmatrix

Metal Matrix Composites Conventional metallic materials have been tailored in the past close to their ultimate properties. New technological requirements ask for further improved materials. Metal-matrix composites (MMC) promise to reach this goal. MMC can be described as materials whose microstructures comprise a continuous metallic phase (the matrix) into which a second phase has been artificially introduced during processing, as reinforcement. Presently the interest in MMC is primarily focused on light alloys reinforced with fibrous or particulate phase to achieve major jumps in selected mechanical properties or thermal stability. This new interest is mainly related to the fact that ceramic based reinforcement constituents became recently available, which are comparatively inexpensive. Al₂O₃ or SiC-based fibres, whiskers and particles, but also carbon fibres are used to reinforce aluminium, magnesium or titanium matrix alloys.     

Study on the thermal expansion properties of C/C composites

Two different woven (2D and 3D) carbon/carbon composites (C/C) and a block carbon have been prepared by chemical vapor infiltration (CVI). The effects of the density and porosity of composites, preform architectures and heat treatment on the thermal expansion properties of the C/C composites were investigated. It is revealed that the coefficient of thermal expansion (CTE) of C/C composites is negative below 100 °C, and the CTE values are inversely proportion to its porosity. Comparing with 2D C/C composites, 3D C/C composites have a better thermal stability. Heat treatment can increase the thermal stability of composites by changing interfacial thermal stress. The thermal expansion behavior of C/C composites is considered as the result of interaction between fibers and matrix.     

Zum Einfluss des Schweißens auf das Kriechverhalten moderner Stähle für die thermische Energieerzeugung

The influence of welding on creep behaviour of modern steels for thermal power generation Un‐ and low alloyed ferritic/bainitic Chromium steels as well as high alloyed ferritic/martensitic 9–12 % Chromium steels are widely used for high temperature components in thermal power generation. Welding in all its variety is the major repair and joining technology for such components. The weld thermal cycle has significant influence on the base material microstructure and its properties. The Heat Affected Zone is often regarded as the weakest link during high temperature service. While weldments of un‐ and low alloyed ferritic Chromium steels can show significant susceptibility to Reheat Cracking in the coarse grained heat affected zone, weldments of high alloyed ferritic Chromium steels generally fail by Type IV Cracking in the fine grained heat affected zone during long term service. In this paper the influence of the weld thermal cycle on the base material microstructure is described. Long‐term creep behaviour of weldments is directly related to the main failure mechanisms in creep exposed ferritic weldments and implications for industries using heat resistant ferritic steels are shown.     

Three-dimensional carbon nanotube based polymer composites for thermal management

In this study, the porous multiwall carbon nanotube (MWCNT) foams possessing three-dimensional (3D) scaffold structures have been introduced into polydimethylsiloxane (PDMS) for enhancing the overall thermal conductivity (TC). This unique interconnected structure of freeze-dried MWCNT foams can provide thermally conductive pathways leading to higher TC. The TC of 3D MWCNT and PDMS composites can reach 0.82 W/m K, which is about 455% that of pure PDMS, and 300% higher than that of composites prepared from traditional blending process. The obtained polymer composites not only exhibit superior mechanical properties but also dimensional stability. To evaluate the performance of thermal management, the LED modulus incorporated with the 3D MWCNT/PDMS composite as heat sink is also fabricated. The composites display much faster and higher temperature rise than the pristine PDMS matrix, suggestive of its better thermal dissipation. These results imply that the as-developed 3D-MWCNT/PDMS composite can be a good candidate in thermal interface for thermal management of electronic devices.     

Properties of high reinforcement-content aluminum matrix composite for electronic packages

Aluminum matrix composites reinforced by a high volume fraction of ceramic particles provide a novel solution to electronic packaging technology, because of their high thermal conductivity, compatible and tailorable coefficient of thermal expansion (CTE) with chips or substrates, low weight, enhanced specific stiffness, and low cost. In this paper, SiC-particle-reinforced aluminum matrix composites are fabricated by the cost-effective squeeze-casting technology, and their microstructure characteristics, thermo-physical, and mechanical properties are investigated. The reinforcement volume fraction is as high as 70% and composites with linear CTE of 6.9–9.7×10^–6 °C^–1 and thermal conductivity of 120–170 W m^–1 °C^–1 are produced. The composites can be electric-discharge machined, ground, and electric-spark drilled. An electroless nickel layer is plated on the composite by the conventional procedures. Finally, their potential applications in electronic packaging and thermal management are illustrated via prototype examples.