填料种类对炭/石墨材料性能和微观结构的影响

分别利用炭黑、石油焦、针状焦和天然石墨粉为填料,煤沥青为黏结剂,经模压成型(150MPa,10min)、炭化(1300℃,1h)和石墨化(2300℃)制备炭/石墨材料.考察了填料类型对最终炭/石墨材料物理性能和微观结构的影响.研究结果表明:利用炭黑为填料所制材料具有较高的机械强度,但其导热和导电性能相对较差;经石墨化后(2300℃),其抗弯和抗压强度分别达到88.0和173.2MPa.而以天然石墨粉为填料所制材料具有较好的导热和导电性能,在室温下其导热率达到278W/m · K;另外,其抗弯和抗压强度分别达到51.1和90.2MPa.微观结构分析表明,以天然石墨粉为填料所制得的材料具有最大的微晶尺寸和高度的取向性.     

Carbon black/graphite nanoplatelet/rubbery epoxy hybrid composites for thermal interface applications

Hybrid composites were developed by dispersing carbon black (CB) nanoparticles and graphite nanoplatelets (GNPs) at 4–6 and 12–14 wt%, respectively, into rubbery epoxy resin. SEM analysis showed that CB particles improved the dispersion of GNPs in the hybrid composite. The thermal conductivity of 4 wt% CB/14 wt% GNP-15/rubbery epoxy hybrid composite, 0.81 W/m K, is ca. four times higher than that of rubbery epoxy. When silane-functionalised, the fillers reduced the viscosity of the hybrid dispersion and made the hybrid composite highly electrically insulating. Nevertheless, filler functionalisation decreased the composite’s thermal conductivity by only 16.6%. Compression testing showed that the hybrid fillers increased the compressive modulus and strength of rubbery epoxy by nearly two and three times, respectively. Overall, the hybrid composites with their thermal paste-type morphology, low viscosity, high compliance, improved thermal conductivity and, when fillers are functionalised, low electrical conductivity makes them promising materials as thermal interface adhesives.     

Physicomechanical and thermophysical properties of SiC-based ceramics

Fine-grain SiC-based ceramics have been produced via infiltration of molten silicon into preforms fabricated from SiC and graphite powders, with a phenol-formaldehyde resin as a binder. The materials thus prepared have a density of 2.70–3.15 g/cm³, dynamic modulus of elasticity from 200 to 400 GPa, compressive strength from 800 to 1900 MPa, bending strength from 150 to 315 MPa, thermal expansion coefficient (KTE) of 4.1 × 10⁻⁶ K⁻¹, and thermal conductivity of 140–150 W/(m K). Their properties are compared to those of known silicon carbide materials fabricated by other processes. The results indicate that the density and physicomechanical properties of the silicon carbide ceramics depend little on the fabrication process and are determined primarily by the SiC content. Increasing the SiC content from 20 to 99.5 wt % increases the density of the ceramics from 2.2 to 3.15 g/cm³ and leads to an exponential rise in their physicomechanical parameters: an increase in modulus of elasticity from 95 to 430 GPa, in compressive strength from 120 to 4200 MPa, and in bending strength from 70 to 410 MPa. The thermal conductivity of the ceramics depends very little on the fabrication process, falling in the range 100–150 W/(m K) over the entire range of SiC concentrations. Their KTE decreases slightly, from 4.3 × 10⁻⁶ to 2.4 × 10⁻⁶ K⁻¹, as the SiC content increases to 99–100 wt %.     

Mechanical energy dissipation using carbon fiber polymer–matrix structural composites with filler incorporation

Continuous carbon fiber composites with enhanced mechanical energy dissipation (vibration damping) under flexure are provided by incorporation of fillers between the laminae. Exfoliated graphite (EG) as a sole filler is more effective than carbon nanotube (SWCNT/MWCNT), halloysite nanotube (HNT), or nanoclay as sole fillers in enhancing the loss tangent, if the curing pressure is 2.0 (not 0.5) MPa. The MWCNT, SiC whisker, and HNT as sole fillers are effective for increasing the storage modulus. The combined use of a storage modulus-enhancing filler (CNT, SiC whisker, or HNT) and a loss tangent-enhancing filler (EG or nanoclay) gives the best performance. With EG, HNT, and 2.0-MPa curing, the loss modulus is increased by 110%, while the flexural strength is decreased by 14% and the flexural modulus is not affected. With nanoclay, HNT, and 0.5-MPa curing, the loss modulus is increased by 96%, while the flexural strength and modulus are essentially not affected. The filler incorporation is more effective for crossply than unidirectional composites. The highest fraction of mechanical energy dissipated is 11%. The loss tangent enhancement is primarily contributed by the innermost interlaminar interfaces, indicating shear deformation dominance in damping. The filler incorporation increases the interlaminar interface thickness, which remains below ~10 μm.     

Effect of Grain Size on the Thermal Expansion of Isotropic Synthetic Graphites

The relationship is analyzed between the thermal expansion coefficient of synthetic graphites and the grain size of their filler. It is shown by examining a large number of commercial graphites of different classes that their thermal expansion coefficient increases from 2 × 10⁻⁶ to (7–8) × 10⁻⁶ K⁻¹ as the grain size decreases from 3000 to 1 μm. The strength of synthetic graphites also increases with increasing grain size and correlates with thermal expansion. The likely reason for the increase in thermal expansion coefficient is the better contact between neighboring microvolumes of the material. A model is proposed for the thermal expansion of synthetic graphites which considers microvolumes 0.1 to 0.5 μm in size, with a thermal expansion coefficient on the order of 8.3 × 10⁻⁶ K⁻¹, only a fraction of the microvolumes being involved in thermal-expansion transfer. The fraction of such microvolumes decreases with increasing grain size. __________ Translated from Neorganicheskie Materialy, Vol. 41, No. 12, 2005, pp. 1456–1462. Original Russian Text Copyright ? 2005 by Samoilov, Shilo.     

Fabrication and characterization of nano filler-filled epoxy composites for underfill application

The present research aims to fabricate and characterize different nano filler types and filler loadings in epoxy composites for underfill application. The nano filler types were synthetic diamond (SD), boron nitride (BN), and silica (S). The filler loadings which were considered in the study were varied from 1 to 4 vol%. Sonication process was used to facilitate filler dispersion. The results showed that BN had a good flow ability, with higher flow rates than the other filler types. The thermal conductivity of the composites increased with the addition of fillers, and higher thermal conductivity value is observed in SD system. The coefficient of thermal expansion (CTE) of composites decreased with the addition of filler with lower CTE value shown by BN system. Generally, SD showed higher flexural strength and flexural modulus compared with BN and S. A high filler loading also resulted in decreased flexural strength but increased flexural modulus.     

Effect of Cristobalite and Quartz on the Properties of Gypsum Bonded Investment

Generally the gold investment material consists of cristobalite,quartz and plaster.The physical property of gold investment materials depends on its thermal expansion coefficients,compressive strength,and particles size distribution.Since the thermal expansion coefficient of cristobalite and quartz are 2.6×10^-6/℃and 2.32×10^-6/℃respectively,the composition ratio of each components influence the thermal and physical properties of gold investment materials.For the clinical applications,it is necessary to improve the properties of gold investment materials such as homogeneous size distribution and thermal expansion coefficients.In the present study,effect of inorganic fillers such as cristobalite and quartz on gold alloy investment was investigated to improve the properties of it.The compressive strength and thermal expansion coefficients of the specimens were evaluated.The results showed that cristobalite and quartz were homogeneously distributed by milling. The optimum compressive strength was obtained at the ratio of 42：22 cristobalite and quartz,respectively.     

Thermal behaviour of inorganic geopolymers and composites derived from sodium polysialate

Inorganic polymers based on alumina and silica polysialate units were synthesised at room temperature from metakaolinite and sodium silicate in a highly alkaline medium, followed by curing and drying at 65 °C. When properly cured, these polymers exhibit remarkable thermal stability; after losing their hydration water at about 200 °C, they retain their X-ray-amorphous tetrahedral Al and Si network up to the onset of melting at 1300 °C. A small amount of mullite and corundum formed at 1200-1300 °C may result from the presence of a trace of unreacted metakaolinite. Similar experiments with poorly-curing formulations containing higher Na and Si contents show that their unpolymerised components form crystalline nepheline (NaAlSiO₄) at 800 °C, prior to melting at about 1100 °C.A series of geopolymer composites were prepared containing 10-20 vol.% of various granular inorganic fillers ranging from waste demolition materials through mineral tailings to engineering ceramics. The physical and thermal properties (bulk density, compressive strength and thermal expansion) of these composites were measured. The thermal expansion is influenced by the properties of the filler, but all the samples showed only slight expansion up to ∼800 °C on the first heating cycle. Microcracking of the composite bodies during drying can be minimised by the addition of a small amount of glycerol.     

Mechanical properties of a diamond–copper composite with high thermal conductivity

Using pressureless infiltration of copper into a bed of coarse (180 μm) diamond particles pre-coated with tungsten, a composite with a thermal conductivity of 720 W/(m K) was prepared. The bending strength and compression strength of the composite were measured as 380 MPa. As measured by sound velocity, the Young's modulus of the composite was 310 GPa. Model calculations of the thermal conductivity, the strength and elastic constants of the copper–diamond composite were carried out, depending on the size and volume fraction of filler particles. The coincidence of the values of bending strength and compressive strength and the relatively high deformation at failure (a few percent) characterize the fabricated diamond–copper composite as ductile. The properties of the composite are compared to the known analogues — metal matrix composites with a high thermal conductivity having a high content of filler particles (~ 60 vol.%). In strength and ductility our composite is superior to diamond–metal composites with a coarse filler; in thermal conductivity it surpasses composites of SiC–Al, W–Cu and WC–Cu, and dispersion-strengthened copper.     

Influence of glass microspheres on selected properties of polylactide composites

Comparison of some changes occurring in polylactide (PLA) due to its modification by glass filler being in the form of microspheres (GM) was the objective of the present study. Mechanical and thermal properties, density, mass flow rate, and were determined. In addition, there were examined surface free energy and changes in the surface geometrical structure of sample fractures. It was found that PLA as modified with GM exhibited the enhanced longitudinal modulus of elasticity, flexural modulus, and mass flow rate. The impact strength and flexural strength did not change. The tensile strength and tensile strain at break decreased. The used glass filler did not affect essentially the thermal properties of PLA. The prepared composites exhibited uniform distribution of the dispersed phase in the polymer matrix and adequate adhesion at the interface between the two components. Substantial changes in the properties of the surface layer were observed, mainly in surface free energy.     

Effects of filler shape and size on the properties of silver filled epoxy composite for electronic applications

Epoxy composites filled with nano- and micro-sized silver (Ag) particulate fillers were prepared and characterized based on flexural properties, coefficient of thermal expansion, dynamic mechanical analysis, electrical conductivity, and morphological properties. The influences of these two types of Ag fillers, especially in terms of their sizes and shapes, were investigated. Silver nanoparticles were nano-sized and spherical, while silver flakes were micron-sized and flaky. It was found that the flexural strength of the epoxy composite filled with silver flakes decreased, while the flexural strength of the epoxy composite filled with silver nanoparticles showed an optimum value at 4 vol.% before it subsequently dropped. Both silver composites showed improvement in flexural modulus with increasing filler loads. CTE value indicated significant decrements in filled samples compared to neat epoxy. Results on the electrical conductivity of both systems showed a transition from insulation to conduction at 6 vol.%.     

Enhanced conductivity behavior of polydimethylsiloxane (PDMS) hybrid composites containing exfoliated graphite nanoplatelets and carbon nanotubes

Polydimethylsiloxane (PDMS) hybrid composites consisting of exfoliated graphite nanoplatelets (xGnPs) and multiwalled carbon nanotubes functionalized with hydroxyl groups (MWCNTs-OH) were fabricated, and the effects of the xGnP/MWCNT-OH ratio on the thermal, electrical, and mechanical properties of polydimethylsiloxane (PDMS) hybrid composites were investigated. With the total filler content fixed at 4 wt%, a hybrid composite consisting of 75% × GnP/25% MWCNT-OH showed the highest thermal conductivity (0.392 W/m K) and electrical conductivity (1.24 × 10⁻³ S/m), which significantly exceeded the values shown by either of the respective single filler composites. The increased thermal and electrical conductivity found when both fillers are used in combination is attributed to the synergistic effect between the fillers that forms an interconnected hybrid network. In contrast, the various different combinations of the fillers only showed a modest effect on the mechanical behavior, thermal stability, and thermal expansion of the PDMS composite.     

The properties of epoxy resin coated silica fillers composites

Epoxy resin coated silica fillers composites with high percentage of filler loading, such as 80 to 95 vol.% are able to be produced by a mechanical mixing technique. The advantages of high filler loading of theses materials are noted from the thermal and flexural modulus. Apparently, the materials exhibit low coefficient of thermal expansion (CTE) at as low as or below 10 ppm/°C and high flexural modulus of above 20 GPa. In general, these promising characteristics fulfill the requirement to be used as substrate materials in electronic packaging applications.     

基于SLS的高强度低密石墨陶瓷复合隔热材料快速制备

本文率先利用选择性激光烧结技术快速制备了高强度石墨陶瓷复合隔热材料,重点研究了二次固化、真空压力浸渍、碳化和高温烧结等后处理工艺以及材料配方组成对其密度、抗压强度和导热系数的影响。研究发现加入适量的硅粉和可膨胀石墨可以对石墨陶瓷复合隔热材料的密度、抗压强度和导热系数进行调控,采取合适后处理工艺路线可以改变石墨陶瓷复合隔热材料的综合性能。最终实现了低密度(<1.2 g/cm^3)、高抗压强度(>10 MPa)、低的导热系数(<2 W/(m·K))和耐高温(>1650℃)等多个性能指标的统一,满足了工业应用需求。     

Effect of matrix glass transition on reinforcement efficiency of epoxy-matrix composites with single walled carbon nanotubes,multi-walled carbon nanotubes,carbon nanofibers and graphite

This study compares the mechanical and thermal properties of glassy and rubbery epoxy–matrix composites reinforced with 1 and 4 wt.% single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), graphite, and carbon nanofibers (CNFs). The tensile modulus of most glassy composites was higher than that of the epoxy and increased with higher filler concentration and 4% graphite/epoxy and 4% SWCNT/epoxy exhibited approximately the same highest tensile modulus. The elongation of glassy composites was significantly lower than that of the epoxy and decreased with increasing filler loading. Most rubbery composites showed a higher tensile modulus and elongation than the epoxy and the modulus increased with rising filler content and 4% SWCNT/epoxy showed the highest tensile modulus and tensile strength. In the rubbery regime, glassy and rubbery composites displayed a higher storage modulus than the corresponding epoxy and 4 wt.% SWCNT/epoxy composites showed a 300% improvement in storage modulus compared to the epoxy.     

Relationship between structure and thermal fatigue in cast iron

Abstract

Thermal fatigue of a material is determined by rupture stress, the elasticity modulus, heat conductivity, and thermal expansion. In addition to thermal expansion, one has to consider also the volume changes as a result of phase transformations. It is known that high rupture stress and high heat conductivity result in high resistance to thermal fatigue. A high Young's modulus and high thermal expansion give low resistance to thermal fatigue. Cast iron is a composite material, consisting mostly of graphite, ferrite, and cementite. The graphite can occur in a number of different morphologies. It can be spherical, as in ductile cast iron, it can be flakelike, as in flake cast iron, but it can also be rodlike, as in vermicular or undercooled graphite. Many of the properties important for thermal fatigue are influenced by the shape of the graphite. By using various models to explain the properties of composite materials, the changes in the properties of cast iron as a function of graphite shape are analysed. The analytical results are compared with experimental results. It is shown that the elasticity modulus and thermal expansion are lowest for flake graphite and that thermal conductivity is highest for this material. The conclusion is that grey cast iron has a better resistance to thermal fatigue than vermicular as well as nodular cast irons, in spite of its lower rupture stress.

MST/783     

复合型散热硅橡胶研究

研究了氧化铝和氮化硼粒子用量对硅橡胶热膨胀系数、热稳定性及热导率的影响.发现加入两种填料均显著降低了橡胶热膨胀系数,提高了热稳定性及热导率;试验测试热导率和理论计算值差距较大,用Agari模型分析了两种填料对硅橡胶热导率差异影响的原因.按照一定比例混合这两种填料所得混合填料填充硅橡胶可获得最大热导率及较佳性能.所制备的复合硅橡胶具有很好的物理性能,是作为散热使用的弹性导热垫片的理想材料.     

沸石胶凝增强复合包装板材的制备及性能

目的 为了拓展固废资源化的应用,本研究以生物质废料（秸秆粉、废木屑）为基体,以预处理后的污水污泥焚烧灰作为填料,采用热压成型制备了一种沸石胶凝增强复合包装板材。方法 通过XRD、SEM等方法表征预处理前后填料的元素组成、晶体结构、微观形貌等,并研究预处理填料对制备复合包装板材性能的影响趋势以及增强机制。结果 添加质量分数为22.5%的预处理后的填料制备的复合板材,其弯曲强度为27.73MPa,压缩强度为44.38MPa,在力学性能、隔热性能和吸水厚度膨胀率等方面均明显优于未经过预处理的对照组,满足GB/T7284—2016《框架木箱》、GB/T23898—2009《木托盘用人造板》等国家标准的要求。复合板材性能的增强主要归因于预处理引起填料相组成及结构的变化。由于方沸石晶相的生成,填料对自身和基体的吸附性、易分散性得到极大改善。结论 经过预处理的填料制备的沸石胶凝增强复合包装板材性能优异,在木包装领域及建筑领域具有广阔的应用前景。该研究可为大宗固体废弃物资源化利用提供高附加值的解决方案,还可以为缓解林产资源短缺的现状提供新的思路。     

Effect of Grain Size on the Modulus of Elasticity and Strength of Synthetic Graphites

The effect of the grain size of the filler on the mechanical properties (compressive, bending, and tensile strength and modulus of elasticity) of synthetic graphite is analyzed using data for commercial structural graphites. As the mean particle size of the filler (_av) decreases from 3000 to 1 m, the modulus of elasticity increases, on the average, from 10 to 15 GPa, and the compressive, bending, and tensile strength increases by about one order of magnitude. The Griffith equation is used to evaluate the size of defects that initiate fracture (c _c) in different types of graphites. It is shown that the factors determining the critical defect size depend on the particle size of the filler. For _av > 150 m, c _c is comparable to _av or _max. In the range 30 < _av < 150 m, c _c is equal to or greater than _max. In graphites with _av < 30 m, c _c far exceeds _max and, presumably, corresponds to the particle size of the molding powder.     

Thermal-Diffusivity Measurements of Conductive Composites Based on EVA Copolymer Filled With Expanded and Unexpanded Graphite

In this research, the thermal diffusivity of composites based on ethylene- vinyl acetate (EVA) copolymer filled with two kinds of reinforcement graphite materials was investigated. The reinforcement graphite fillers were untreated natural graphite (UG) and expanded graphite (EG). Composite samples up to 29.3 % graphite particle volumetric concentrations (50 % mass concentration) were prepared by the melt- mixing process in a Brabender Plasticorder. Upon mixing, the EG exfoliates in these films having nanosized thicknesses as evidenced by TEM micrographs. Thus, the thermal diffusivity and electrical conductivity of composites based on the ethylene-vinyl acetate matrix filled with nanostructuralized expanded graphite and standard, micro-sized graphite were investigated. From the experimental results it was deduced that the electrical conductivity was not only a function of filler concentration, but also strongly dependent on the graphite structure. The percolation concentration of the filler was found to be (15 to 17) vol% for micro-sized natural graphite, whereas the percolation concentration of the filler in nanocomposites filled with expanded graphite was much lower, about (5 to 6) vol%. The electrical conductivity of nanocomposites was also much higher than the electrical conductivity of composites filled with micro-sized filler at similar concentrations. Similarly, the values of the thermal diffusivity for the nanocomposites, EG-filled EVA, were significantly higher than the thermal diffusivity of the composites filled with micro-sized filler, UG-filled EVA, at similar concentrations. For 29.3 % graphite particle volumetric concentrations, the thermal diffusivity was 8.23 × 10^?7 m² · s^?1 for EG-filled EVA and 6.14 × 10^?7 m² · s^?1 for UG-filled EVA. The thermal diffusivity was measured by the flash method.