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.     

Entwicklung der Konnektivität der Phasen in einer kurzfaserverstärkten Aluminium‐Kolbenlegierung unter Kriechbelastung

Developing connectivity of the phases in a short fibre reinforced aluminium piston alloy subjected to creep The evolution of the micro‐structure during creep of an AlSi12CuMgNi piston alloy with 15 vol% of Al₂O₃ short fibres is investigated by means of synchrotron micro‐tomography. The results reveal a 3D morphology of the rigid phases in the composite: the eutectic‐Si, the short fibres and the Fe‐ and Ni‐rich intermetallic particles, which form an interconnected hybrid reinforcement. The connectivity of these phases increases during creep exposure at 300 °C due to the diffusion induced ripening of Si and of the intermetallic particles. The hybrid reinforcement reaches almost complete percolation after 6400 h of creep exposure. The fibre orientation analysed by three‐dimensional Fast Fourier Transformation does not indicate any reorientation of the fibres along the load direction. The formerly observed strengthening effect during creep exposure is attributed to the increasing load carrying capacity of the interconnected hybrid reinforcement. The analysis of creep damage during secondary creep stage shows the increase of the void volume fraction by a factor of 2 with respect to the void content from processing, while the number of voids per volume remains practically constant. The voids are located at interfaces of the rigid phases and not within the α‐aluminium matrix.     

Neuartige Faserverbunddrähte für die Leistungselektronik

Novel fibre reinforced wires for power electronics The use of power electronics within the scope of mechatronic applications as well as the increasing integration of components lead to increased requirements concerning their mechanical and thermal reliability. Today contact making in power electronics is mostly done by aluminum thick wire bonding. This process is highly productive, however the life time of power electronic components is meanwhile predominantly limited by the durability of these wire bonds. The thermal mismatch between the wire material and the connected components is one cause. A new starting point, in order to improve the reliability, is the application of new fibre reinforced metal matrix composite (MMC) wires with increased reliability under thermo‐mechanical stress. In the context of a research project MMC bond wires of different material combinations and arrangements were manufactured. Aluminum wires with copper fiber reinforcement as well as Copper wires containing FeNi36 fibre reinforcement have successfully be drawn to a final diameter of 300 μm. The fibre reinforcements should reduce the coefficient of thermal expansion and improve the mechanical strength. By aluminium copper MMC the electrical conductivity is increased as well. Measurements of the produced MMC wires confirmed these expectations. The manufacturing of the MMC took place on the basis of wire material of different diameters. These wires were stacked in capsules in different arrangements and material combinations. Subsequently, the capsules were either hot‐isostatically pressed or directly extruded. In such a way produced composites have been manufactured by rotary swaging and wire drawing into bond wires and after that tested.     

Interfacial design of Cu/SiC composites prepared by powder metallurgy for heat sink applications

In order to dissipate the heat generated in electronic packages, suitable materials must be developed as heat spreaders or heat sinks. Metal matrix composites (MMCs) offer the possibility to tailor the properties of a metal (Cu) by adding an appropriate reinforcement phase (SiC) to meet the demands for high thermal conductivities in thermal management applications. Copper/SiC composites have been produced by powder metallurgy. Silicon carbide is not stable in copper at the temperature needed for the fabrication of Cu/SiC. The major challenge in development of Cu/SiC is the suppression of this reaction between copper and SiC. Improvements in bonding strength and thermo-physical properties of the composites have been achieved by a vapour deposited molybdenum coating on SiC powders to control the detrimental interfacial reactions.     

High performance carbon-carbon composites

Carbon-carbon composites rank first among ceramic composite materials with a spectrum of properties and applications in various sectors. These composites are made of fibres in various directions and carbonaceous polymers and hydrocarbons as matrix precursors. Their density and properties depend on the type and volume fraction of reinforcement, matrix precursor used and end heat treatment temperature. Composites made with thermosetting resins as matrix precursors possess low densities (1.55–1.75g/cm³) and well-distributed microporosity whereas those made with pitch as the matrix precursor, after densification exhibit densities of 1.8–2.0g/cm³ with some mesopores, and those made by the CVD technique with hydrocarbon gases, possess intermediate densities and matrices with close porosities. The former (resin-based) composites exhibit high flexural strength, low toughness and low thermal conductivity, whereas the latter (pitch- and CVD-based) can be made with very high thermal conductivity (400–700 W/MK) in the fibre direction. Carbon-carbon composites are used in a variety of sectors requiring high mechanical properties at elevated temperatures, good frictional properties for brake pads in high speed vehicles or high thermal conductivity for thermal management applications. However, for extended life applications, these composites need to be protected against oxidation either through matrix modification with Si, Zr, Hf etc. or by multilayer oxidation protection coatings consisting of SiC, silica, zircon etc.     

聚变堆结构应用SiC/SiC热导率的研究进展

得益于其优异的高温性能和韧性断裂行为以及聚变环境下的低诱导辐射,SiC/SiC复合材料在聚变堆结构应用方面有着巨大潜力.由于传统SiC/SiC复合材料的热导率难以达到要求,近年来越来越多的研究机构将研究重点放在提高该材料的热导率上.介绍了聚变堆结构应用背景下提高SiC/SiC复合材料热导率的途径及方法,综述了世界范围内该领域的研究进展,并在此基础上展望了SiC/SiC复合材料热导率的研究前景.     

Effects of Fibre Geometry and Volume Fraction on the Flexural Behaviour of Steel‐Fibre Reinforced Concrete

Abstract: This work aims in studying the mechanical behaviour of concrete, reinforced with steel fibres of different geometry and volume fraction. Experiments include compression tests and four‐point bending tests. Slump and air content tests were performed on fresh concrete. The flexural toughness, flexural strength and residual strength factors of the beam specimens were evaluated in accordance with ASTM C1609/C1609M‐05 standard. Improvement in the mechanical properties, in particular the toughness, was observed with the increase of the volume fraction of steel‐fibres in the concrete. The fibre geometry was found to be a key factor affecting the mechanical performance of the material.     

Microstructure and Ablation Behavior of W Coating Prepared by Atmospheric Plasma Spraying for Zr/Cu Infiltrated C/C Composites

To improve the ablation resistance of carbon/carbon (C/C) composites, W coating prepared by atmospheric plasma spraying (APS) for Zr/Cu infiltrated C/C composites are fabricated, the Zr/Cu infiltrated C/C composites are prepared by reactive melt infiltration (RMI). The microstructural features of the composites are examined by scanning electron microscopy coupled with energy dispersive spectrometry (SEM‐EDS), X‐ray diffraction (XRD), and transmission electron microscopy (TEM). The ablation resistance of the W coated Zr/Cu infiltrated composites are tested in the oxyacetylene torch environment at heat flux of 4186 kW m^?2 for 150 s. The results show that the diameter of ablation center and line ablation rate (LAR) of W coated composites are about 3.92 mm and 5.16 × 10^?3 mm s^?1. Compared to pure C/C and the Zr/Cu infiltrated composites, the diameters of ablation center of W coated composites are reduced by 18.00% and 15.52%, the LAR of W coated composites decreases by 74.52% and 23.55%. Overall, the W coated composites depict good ablation property due to the high melting point of W coating, which can be resistant to high‐temperature oxidation and ablation, the sublimation of WO₃ carries away the heat.

     

Numerical and analytical calculation of the orthotropic heat transfer properties of fibre reinforced materials

This paper is on the investigation of the orthotropic heat transfer properties of unidirectional fibre reinforced materials. The orthotropic effective thermal conductivity of such composite materials is investigated based on two different approaches: the finite element method as a representative for numerical approximation methods and an analytical method for homogenised models based on the solution of the respective boundary value problem. It is found that fibre reinforced composites possess strong orthotropic heat transfer properties, which are getting more distinctive with increasing deviation of the thermal conductivities of matrix and reinforcements. Furthermore, the effect of small perturbations of the periodic configuration of fibres in the matrix on the thermal conductivity is investigated.     

Thermal and electrical properties of FeNi/Cu composites

Abstract

Powder metallurgy FeNi/Cu composites with low thermal expansivity and high electrical (thermal) conductivity were fabricated. The effects of Cu content, FeNi particle size, sintering temperature, and rolling reduction on the coefficient of thermal expansion (CTE) and electrical (thermal) resistivity were investigated. The results show that the CTE and electrical resistivity were affected by the volume fraction of the components, the particular properties of the FeNi alloy, diffusion between the FeNi particles and the Cu particles, and the distribution of the Cu particles and the FeNi particles. The experiments indicated that the FeNi/Cu composites could be used as heat sinks in welding type bolt silicon rectifier tubes.     

Thermal conductivity of Al–SiC composites with monomodal and bimodal particle size distribution

The thermal conductivity of aluminum matrix composites having a high volume fraction of SiC particles is investigated by comparing data for composites fabricated by infiltrating liquid aluminum into preforms made either from a single particle size, or by mixing and packing SiC particles of two largely different average sizes (170 and 16 μm). For composites based on powders with a monomodal size distribution, the thermal conductivity increases steadily from 151 W/m K for particles of average diameter 8 μm to 216 W/m K for 170 μm particles. For the bimodal particle mixtures the thermal conductivity increases with increasing volume fraction of coarse particles and reaches a roughly constant value of 220 W/m K for mixtures with 40 or more vol.% of coarse particles. It is shown that all present data can be accounted for by the differential effective medium (DEM) scheme taking into account a finite interfacial thermal resistance.     

An experimental study on the in situ strength of SiC fibre in unidirectional SiC/Al composites

In order to investigate the effect of strain rate and high temperature exposure on the mechanical properties of the fibre in the unidirectional fibre reinforced metal-matrix composite, in situ SiC fibre bundles are extracted from two kinds of SiC/Al composite wires, which are heat-treated at two different temperatures (exposed in the air at 400 and 600 °C for 40 min after composition). Tensile tests for these two fibre bundles are performed at different strain rates (quasi-static test: 0.001 s⁻¹, dynamic test: 200, 700, and 1200 s⁻¹) and the stress–strain curves are obtained. The experimental results show that their mechanical properties are rate-dependent, the modulus E, strength σ_b and unstable strain _b (the strain corresponding to σ_b) all increase with increasing strain rate. Compared with the mechanical properties of the original SiC fibre, those of the two in situ fibres degrade to some extent, the degradation of the in situ fibre extracted from the composite wire exposed at 600 °C (hereafter referred to as in situ fibre 2) is more serious than that of the in situ fibre extracted from the composite wire exposed at 400 °C (hereafter referred to as in situ fibre 1). The mechanism of the degradation is investigated. A bi-modal Weibull statistical constitutive equation is established to describe the stress–strain relationship of the two in situ fibre bundles. The simulated stress–strain curves agree well with the experimental results.     

Analysis of Strain and Stress in Ceramic,Polymer and Metal Matrix Composites by Raman Spectroscopy

Raman scattering is a unique tool providing information on the structure and short‐range order of matter. Stress‐induced Raman shifts can be used to determine the stress/strain in films, fibres, particulate composites and, more generally, in any phase a few microns or more in scale. Quantitative results follow from a wavenumber calibration as a function of tensile strains or pressures applied to reference fibres or crystals. Furthermore, if the material is coloured, (near) resonant Raman scattering occurs, which enhances the scattered light intensity and simplifies the spectra – especially for harmonics – but drastically reduces the analysed volume (in‐depth penetration ∼10–100 nm). This paper discusses the effective and potential advantages/drawbacks of Raman micro‐spectrometry technique. The procedures to improve the sensitivity, the legibility and the reliability will be addressed. Examples will be chosen among (aramid, C, SiC) fibre‐ reinforced ceramic (CMCs), polymer (PMCs) or metal matrix (MMCs) composites.     

Interfacial microstructure and its effect on thermal conductivity of SiCp/Cu composites

Thermal conductivity of SiCp/Cu composites was usually far below the expectation, which is usually attributed to the low real thermal conductivity of matrix. In the present work, highly pure Cu matrix composites reinforced with acid washed SiC particles were prepared by the pressure infiltration method. The interfacial microstructure of SiCp/Cu composites was characterized by layered interfacial products, including un-reacted SiC particles, a Cu–Si layer, a polycrystalline C layer and Cu–Si matrix. However, no Cu₃Si was found in the present work, which is evidence for the hypothesis that the formation of Cu₃Si phase in SiC/Cu system might be related to the alloying elements in Cu matrix and residual Si in SiC particles. The thermal conductivity of SiCp/Cu composites was slightly increased with the particle size from 69.9 to 78.6 W/(m K). Due to high density defects, the real thermal conductivity of Cu matrix calculated by H–J model was only about 70 W/(m K). The significant decrease in thermal conductivity of Cu matrix is an important factor for the low thermal conductivity of SiCp/Cu composites. However, even considered the significant decrease of thermal conductivity of Cu matrix, theoretical values of SiCp/Cu composites calculated by H–J model were still higher than the experimental results. Therefore, an ideal particle was introduced in the present work to evaluate the effect of interfacial thermal resistance. The reverse-deduced effective thermal conductivities of ideal particles according to H–J model was about 80 W/(m K). Therefore, severe interfacial reaction in SiCp/Cu composites also leads to the low thermal conductivity of SiCp/Cu composites.     

Thermo-physical Properties of Continuous Carbon Fiber Reinforced Copper Matrix Composites

Continuous carbon fiber reinforced copper matrix composites with 70％(volume fraction)of carbon fibers prepared by squeeze casting technique have been used for investigation of the coefficient of thermal expansion(CTE)and thermal conductivity．Thermo-physical properties have been measured in both, longitudinal and transversal directions to the fiber orientation．The results showed that Cf／Cu composites may be a suitable candidate for heat sinks because of its good thermo-physical properties e．g．the low CTE(4.18×10-6／K)in longitudinal orientation and(14．98×10-6／K)in transversal orientation at the range of 20-50℃，a good thermal conductivity(87．2 W／m·K)in longitudinal orientation and(58．2 W／m·K)in transversal orientation．Measured CTE and thermal conductivity values are compared with those predicted by several well-known models．Eshelby model gave better results for prediction of the CTE and thermal conductivity of the unidirectional composites．     

Fabrication process and thermal properties of SiCp/Al metal matrix composites for electronic packaging applications

The fabrication process and thermal properties of 50–71 vol% SiC_p/Al metal matrix composites (MMCs) for electronic packaging applications have been investigated. The preforms consisted with 50–71 vol% SiC particles were fabricated by the ball milling and pressing method. The SiC particles were mixed with SiO₂ as an inorganic binder, and cationic starch as a organic binder in distilled water. The mixtures were consolidated in a mold by pressing and dried in two step process, followed by calcination at 1100 °C. The SiC_p/Al composites were fabricated by the infiltration of Al melt into SiC preforms using squeeze casting process. The thermal conductivity ranged 120–177 W/mK and coefficient of thermal expansion ranged 6–10 × 10^–6/K were obtained in 50–71 vol% SiC_p/Al MMCs. The thermal conductivity of SiC_p/Al composite decreased with increasing volume fraction of SiC_p and with increasing the amount of inorganic binder. The coefficient of thermal expansion of SiC_p/Al composite decreased with increasing volume fraction of SiC_p, while thermal conductivity was insensitive to the amount of inorganic binder. The experimental values of the coefficient of thermal expansion and thermal conductivity were in good agreement with the calculated coefficient of thermal expansion based on Turner's model and the calculated thermal conductivity based on Maxwell's model.     

用K2ZrF6溶液处理碳化硅涂层碳纤维制造碳/铝复合丝

在碳纤维表面用CVD法涂覆碳化硅涂层,接着用K₂ZrF₆溶液处理,干燥后通过700℃的熔融铝制得碳/铝复合丝。复合丝的强度约为复合准则强度的70%。当碳纤维的体积分数为54%时,复合丝的强度达到1200MPa。用扫描电镜观察复合丝的断口发现界面结合状态良好。用X射线衍射分析了碳化硅-K₂ZrF₆一铝在高温下的反应情况,表明K₂ZrF₆的良好的改善润湿的作用是由于它与铝和碳化硅之间发生了强烈的化学反应。     

Thermal expansion characteristics of cast Cu based metal matrix composites

Like any other metal/alloy, copper and its alloys also soften at elevated temperatures. Reinforcing with ceramic or carbon fibres is one of the suggested solutions to overcome this. Very limited literature is available on Cu based metal matrix composites (MMCs); none of these pertain to liquid phase fabrication. Hence, a systematic investigation was carried out on MMCs based on copper, with alumino-silicate fibres and carbon fibres as reinforcements. The MMCs thus produced exhibit a uniform distribution of reinforcement in the matrix. Coefficient of thermal expansion (CTE) values are lower than that of pure copper.     

Die Entwicklung von Strukturmaterialien für die Kernfusion

The Development of Structural Materials for Fusion Reactors Structural materials for the First Wall and breeding blankets of future fusion reactors will be exposed to intense neutron irradiation and thermal wall loading. Fusion‐specific selection criteria for the proper choice of materials are primary damage parameters, a minimum of produced radioactivity (low activation materials) and also conventional properties like strength and corrosion resistance. Three major material groups are under discussion: ferritic‐martensitic 7–12%Cr steels, SiC‐fiber‐enforced compound materials of type SiC_f/SiC and specific vanadium‐based alloys. A short status of development and a survey on necessary further research work is given to fulfil the material requirements for the construction of the next fusion reactor devices. Finally the necessity for an appropriate 14 MeV neutron source as test bed for the material development is mentioned.     

石墨表面TiC涂层对高定向石墨/Cu复合材料热导率和抗弯强度的影响

以高温盐浴法对天然鳞片石墨粉体(GF)进行表面TiC镀层处理,然后采用真空热压烧结法制备TiCGF/Cu复合材料,研究了粉体表面涂层和GF体积分数对复合材料微观结构、热导率及抗弯强度的影响。系列测试结果表明:随着GF体积分数的降低以及粉体表面TiC镀层的形成,TiC-GF/Cu复合材料平行于GF片层方向的热导率有所降低,抗弯强度有所提升。其中在GF的体积分数占TiC-GF/Cu复合材料70%时,这种变化最为明显,平行于GF片层方向的TiC-GF/Cu复合材料热导率下降幅度最大,从676W/(m·K)下降到526 W/(m·K)。同时,TiC-GF/Cu复合材料的微观结构进一步说明,GF表面的TiC涂层对GF/Cu复合材料的断裂模型起着重要的作用。