Adjustable thermal expansion in La(Fe,Si)_(13)-based conductive composites by high-pressure synthesis

Providing a suitable contact interface, where a high conductivity material with a desirable coefficient of thermal expansion(CTE) adjoins the target micro-electric devices, is very crucial to optimize the properties and service life of the relevant instruments. Regrettably, a high conductivity, low thermal expansion and relatively inexpensive material is very rare. Composites, fortunately, can offer a method to design materials with adjustable properties by mixing two or more diverse constituents. In this paper, high conductivity composites with adjustable thermal expansion were successfully prepared by a high-pressure synthesis. The composites are based on combining La(Fe,Si)_(13)-based compounds, the materials showing a giant, isotropic negative thermal expansion(NTE) properties, within Cu matrix. The La(Fe,Si)_(13)-based compounds were used to adjust the CTE of the composites, while the Cu phase is in charge of tuning the thermal conductivity properties. Thus, by changing the relative amount of the two components, the composites with high conductivities and adjustable CTE were achieved. Furthermore, the thermal expansion and magnetic properties of the composites were investigated by a physical property measurement system. The present results highlight the potential applications of the Cu-based high conductivity composites with room-temperature NTE properties in the thermal contacts to various semiconductor and microelectronic devices.     

氧化物材料负热膨胀机理

氧化物的负热膨胀性（ＮＴＥ）是由于在其开放式的骨架结构中,有二配位的桥氧键产一横向热运动,以及引起的多面体热摆动和耦合作用使非键合Ｍ－Ｍ１键距变短,导致单胞体积缩小;各向同性ＮＴＥ氧化物在复合材料及其相关技术领域具有更广泛的应用价值。     

Temperature-induced valence transition and associated lattice collapse in samarium fulleride

The different degrees of freedom of a given system are usually independent of each other but can in some materials be strongly coupled, giving rise to phase equilibria sensitively susceptible to external perturbations. Such systems often exhibit unusual physical properties that are difficult to treat theoretically, as exemplified by strongly correlated electron systems such as intermediate-valence rare-earth heavy fermions and Kondo insulators, colossal magnetoresistive manganites and high-transition temperature (high-T(c)) copper oxide superconductors. Metal fulleride salts-metal intercalation compounds of C60--and materials based on rare-earth metals also exhibit strong electronic correlations. Rare-earth fullerides thus constitute a particularly intriguing system--they contain highly correlated cation (rare-earth) and anion (C60) sublattices. Here we show, using high-resolution synchrotron X-ray diffraction and magnetic susceptibility measurements, that cooling the rare-earth fulleride Sm2.75C60 induces an isosymmetric phase transition near 32 K, accompanied by a dramatic isotropic volume increase and a samarium valence transition from (2 + epsilon) + to nearly 2 +. The negative thermal expansion--heating from 4.2 to 32 K leads to contraction rather than expansion--occurs at a rate about 40 times larger than in ternary metal oxides typically exhibiting such behaviour. We attribute the large negative thermal expansion, unprecedented in fullerene or other molecular systems, to a quasi-continuous valence transition from Sm(2+) towards the smaller Sm((2+epsilon)+), analogous to the valence or configuration transitions encountered in intermediate-valence Kondo insulators like SmS (ref. 3).     

负热膨胀化合物材料ZrW2O8的机理与制备技术

以ZrW2O8为典例，介绍了新型功能材料——负热膨胀材料的最新研究进展；列举了几种负热膨胀材料的物理性质和热力学参量；叙述了ZrW2O8的α相、β相和γ相之间的转变关系和过程；介绍了该类新材料的负热膨胀机理及刚性单元模式和受抑软性模式两种理论模型；系统探讨了该类材料的固相合成和液相合成技术，对比分析了各自的特点与存在的不足，在此基础上结合研究工作提出了该类材料的研究方向、     

Li_(0.025)Na_(0.0975)NbO_3的低温热电性及其测量

在300K至20K的温废范围内,研究了Li_(0.025)Na_(0.975)NbO_3晶体的热电性。在降温和升温过程中,热电系数分别在180K附近和260K附近显示峰值。这表示晶体中发生了低温相变而且该相变有很大的热滞(约80K)。观测了热电电荷随时间的变化。在降温和升温过程中,热电响应分别在180K附近和260K附近随时间改变极性。这一现象被认为是相变过程中两相共存的结果。     

In situ small angle X-ray scattering investigation of the thermal expansion and related structural information of carbon nanotube composites

In-situ thermal expansion tests on a series of carbon nanotube bucky-paper composites were performed with direct heating within a synchrotron SAXS source. The impact of the samples density and morphology as well as the chemistry and degree of decoration of the carbon nanotubes on the scattering patterns were investigated and correlated to the materials macro-properties. The results demonstrate that simple densification techniques, such as acetone dipping or gold electroless deposition, could reduce greatly the displacements of the carbon nanotubes within the structure and lead to more thermally stable material.     

石蜡/膨胀石墨复合相变材料耦合热管电池热管理性能研究

为了改善相变材料用于电池热管理导热率较低的问题,制备了不同含量的石蜡（Paraffin, PA）-膨胀石墨（Expanded Graphite, EG）复合相变材料,在相变储能单元中使用PA-EG复合相变材料包裹热源并耦合热管增强其散热能力。使用方形发热源模拟高密度能量电池充放电时的发热,在不同发热功率下探究不同EG含量的PA-EG复合相变材料耦合热管的热管理性能。实验结果表明,发热功率较低时,各相变材料热管理性能相当,发热功率较高时,更高的EG含量将带来更高的导热率以及更好的热管理效果。在EG含量为5%时,PA-EG复合相变材料导热率可提高至纯石蜡的6.36倍,实验中最大降温幅度可达7.6%,相变储能单元中温差峰值为纯石蜡散热系统温差峰值的37.7%。当EG含量较低时,PA-EG复合相变材料未熔融时呈现固态,导热率呈现各向同性,但当PA熔融后,PA-EG复合相变材料总体呈现液态,出现较强的自然对流换热,将导致储能单元内温度分布不均,容易导致电池表面温度不均产生应力,因此EG含量不宜低于5%。     

Y4Si2O7N2-BN复合材料抗热震性能研究

系统地研究了Y4Si2O7N2_BN复合材料的抗热震性能.计算表明,Y4Si2O7N2_30vol.％BN复合材料具有较高的热应力断裂抵抗因子R1值.进一步的实验证明该复合材料的临界热震温差为850～900K.第二相h-BN在热震条件下的层间劈裂愈合机制对复合材料保持较高的残余强度有利.     

Thin-film thermoelectric devices with high room-temperature figures of merit

Thermoelectric materials are of interest for applications as heat pumps and power generators. The performance of thermoelectric devices is quantified by a figure of merit, ZT, where Z is a measure of a material's thermoelectric properties and T is the absolute temperature. A material with a figure of merit of around unity was first reported over four decades ago, but since then-despite investigation of various approaches-there has been only modest progress in finding materials with enhanced ZT values at room temperature. Here we report thin-film thermoelectric materials that demonstrate a significant enhancement in ZT at 300 K, compared to state-of-the-art bulk Bi2Te3 alloys. This amounts to a maximum observed factor of approximately 2.4 for our p-type Bi2Te3/Sb2Te3 superlattice devices. The enhancement is achieved by controlling the transport of phonons and electrons in the superlattices. Preliminary devices exhibit significant cooling (32 K at around room temperature) and the potential to pump a heat flux of up to 700 W cm-2; the localized cooling and heating occurs some 23,000 times faster than in bulk devices. We anticipate that the combination of performance, power density and speed achieved in these materials will lead to diverse technological applications: for example, in thermochemistry-on-a-chip, DNA microarrays, fibre-optic switches and microelectrothermal systems.     

Preparation and performances of bulk porous Al foams impregnated with phase-change-materials for thermal storage

Shape-stabilized phase change materials (PCMs) exhibit desirable thermal storage properties and are used for energy conservation systems. This study is focused on the preparation and thermal-/dynamic-mechanical properties of the bulk porous Al foams impregnated with organic PCMs such as paraffins and stearic acid. The results indicated that the new shape-stabilized PCMs composites could be produced by a simple and eco-friendly constant-pressure impregnation method. The filling fraction of PCMs was approximately more than 80% for the impregnated samples with vacuuming or without vacuuming. The shape-stabilized PCMs/Al-foam composites exhibited considerable latent thermal storage potential for good interface, desirable phase-change temperature range and latent heat values (72.9 kJ/kg for the paraffin/Al-foam composite and 66.7 kJ/kg for the stearic acid/Al-foam composite). Moreover, the impregnation of PCMs improved the dynamic-compression yield strength, durability and resistance to oxidation of Al foams and made its ideal energy-saving materials.     

植物结构铝基复合材料的制备及热学性能

为研制新型复合材料,将植物结构引入到金属基复合材料的制备中.该文以柳桉木材为模板,先将其转变为多孔碳,再通过铝合金和硅树脂的浸渍,制备了具有木材结构的Al/C、Al/(SiC+C)两种铝基复合材料,并通过扫面电镜、热膨胀仪和导热仪对复合材料的微观结构、热膨胀性能及导热性能进行了研究,建立了导热模型.结果发现该复合材料的结构由所选模板的结构决定,这与以往结构完全由人为控制的金属基复合材料不同;并且其热膨胀系数明显低于铝合金,导热系数(98.2和95.4 W·m~(-1)·K~(-1))远高于由木材转化的多孔碳(2.22 W·m~(-1)·K~(-1)).     

Enhancing thermoelectric performance of Cu-modified Bi0.5Sb1.5Te3 by electroless plating and annealing

With the capacity of energy conversion from heat to electricity directly, thermoelectric materials have been considered as an alternative solution to global energy crisis. In this work, Cu modified Bi_(0.5)Sb_(1.5)Te_3(BST)composites are prepared by a facile electroless plating Cu method, spark plasma sintering, and annealing. The annealed 0.22 wt.%Cu/BST has an enhanced peak Figure of Merit(z T) of ~ 0.71 at 573 K with high average z T of0.65 in the wide temperature range between 300 and 573 K. Due to the significant increase of electrical conductivity and low lattice thermal conductivity, the annealed 0.22 wt.%Cu/BST shifts peak z T to high temperature, and shows 492% enhancement than that of pristine BST with z T of 0.12 at 573 K. Through detailed structural characterization of the annealed 0.22 wt.%Cu/BST, we found that Cu can dope into BST matrix and further form Cu2 Te nanoprecipitates, dislocations, and massive grain boundaries, leading to a low lattice thermal conductivity of 0.30 Wm-1 K-1 in the annealed 0.22 wt.%Cu/BST. Such enhanced peak z T in high-temperature and high average z T in the wide temperature range shows that the electroless plating Cu method and annealing can improve the thermoelectric performance of commercial BST and expand the applicability of Bi_2Te_3 thermoelectric materials in the power generations.     

Investigation on regenerator temperature inhomogeneity in Stirling-type pulse tube cooler

Regenerator is one of the most crucial components to pulse tube cooler (PTC) and thermoacoustic engine. As such regenerator is scaled up to high-power, the thermal and hydrodynamic communication transverse to the acoustic axis gets weaker and weaker. Under this condition, any unsymmetric factor could cause serious instability to the cooler or engine, which degrades their performance. Investigation has been carried out on a high-power two-stage thermal-coupled U-shape Stirling-type PTC. By detailed circumferential temperature measurements along the middle heat exchanger and second stage regen-erator, a kind of temperature inhomogeneity caused by unsymmetric pre-cooling effect of inter-stage thermal bridge was found in the lower part of the regenerator of the PTC. The temperature inhomoge-neity originating from the middle heat exchanger of the second stage regenerator amplified itself in the lower part of the regenerator and then internal streaming formed. The maximal radial temperature dif-ference could reach 30―40 K. Experimental results show that the temperature inhomogeneity intensi-fies with increased pre-cooling power and its direction can be reversed by changing the pre-cooling effect of the first stage PTC to heating effect by using external thermal load. This research shows that it is important to maintain the heating or cooling effects of heat exchangers uniform in high-power re-generative coolers and engines.     

Nonlinear conductive properties and scaling behavior of conductive particle filled high-density polyethylene composites

The blends prepared by incorporation of carbon black (CB) or graphite powder (GP) into high-density polyethylene (HDPE) matrix have been novel and extensively applied polymeric positive temperature coefficient (PTC) composites. A phenomenological model was proposed on the basis of the GEM equation and the dilution effect of filler volume fraction due to the thermal volume expansion of the polymer matrix. Accordingly, the contribution of the thermal expansion of the matrix to the jump-like PTC transition of the composites was quantitatively estimated and a mechanical explanation was given. It was proved that the contribution of the volume expansion to PTC effect decreased for HDPE/CB composites crosslinked through electron-beam irradiation. Furthermore, the influences of the filler content, temperature and crosslinking on the self-heating behavior as well as the nonlinear conduction characteristics at electrical-thermal equilibrium state were examined. Based on the electric-field and initial resistivity dependence of the self-heating temperature and resistance dependence of the critical field, the mechanisms of the self-heating of the polymeric PTC materials were evaluated. The intrinsic relations between macroscopic electrical properties and microscopic percolation network at electrical-thermal equilibrium state were discussed according to the scaling relationship between the self-heating critical parameter and the conductivity of materials.     

SiC颗粒增强AI基复合材料拉伸性能与断裂机理的研究

研究了颗粒增强铝基复合材料的室温拉伸性能与断裂机理。结果表明,由于为形的区域化及残余应力的存在,使应用传统诉测量均匀材料强度的方法来测量颗粒增强金属基复全材料（ＰＲＭＭＣｓ）强度会产生一定的误差,断口分析显示,ＰＲＭＭＣｓ的断裂也属于ＭＮＧ断裂模式。     

内埋CFRC材料的混凝土梁的应变主动调节

利用碳纤维水泥基（CFRC）材料的电热效应，通过对埋置于普通混凝土梁中的CFRC材料通电加热后产生的热膨胀可以实现对混凝土梁应变或挠度变形的主动调节和控制．实验验证了这个方案的可行性．根据弹性力学推导的理论值与混凝土梁挠度变形实测值非常吻合．通过改变内埋CFRC材料的输入功率和通电时间可以控制CFRC材料的升温速率和终点温度，继而控制混凝土梁的变形速率和最大变形值．     

热力射流温度场及温度应力数值模拟研究

热力射流破岩技术是指利用高温介质诸如超临界水对岩石进行快速局部加热达到破碎岩石的目的。由于岩石基质热导率很低，因此会在岩石表面形成温度应力。当温度应力超过岩石的强度，会在岩石内部形成微裂缝，且裂缝不断扩展最终使得岩石表面发生热裂解，热裂解作用导致岩石表面从本体脱落从而使得岩石破碎。基于热-固耦合理论建立了热裂解钻井模型，利用Crank-Nicolson差分方法求解得到了热裂解过程中井底岩石温度场和温度应力的分布规律。结果表明，在热裂解钻井过程中，岩石受热部分温度迅速升高，在径向和轴向方向上产生温度梯度；受热部分体积膨胀在径向方向上受到压应力作用，在轴向方向上发生屈曲，受到剪应力作用。研究成果对热裂解钻井的现场应用具有十分重要的指导意义。     

Extreme damping in composite materials with negative-stiffness inclusions

When a force deforms an elastic object, practical experience suggests that the resulting displacement will be in the same direction as the force. This property is known as positive stiffness. Less familiar is the concept of negative stiffness, where the deforming force and the resulting displacement are in opposite directions. (Negative stiffness is distinct from negative Poisson's ratio, which refers to the occurrence of lateral expansion upon stretching an object.) Negative stiffness can occur, for example, when the deforming object has stored (or is supplied with) energy. This property is usually unstable, but it has been shown theoretically that inclusions of negative stiffness can be stabilized within a positive-stiffness matrix. Here we describe the experimental realization of this composite approach by embedding negative-stiffness inclusions of ferroelastic vanadium dioxide in a pure tin matrix. The resulting composites exhibit extreme mechanical damping and large anomalies in stiffness, as a consequence of the high local strains that result from the inclusions deforming more than the composite as a whole. Moreover, for certain temperature ranges, the negative-stiffness inclusions are more effective than diamond inclusions for increasing the overall composite stiffness. We expect that such composites could be useful as high damping materials, as stiff structural elements or for actuator-type applications.     

Stabilization of Leidenfrost vapour layer by textured superhydrophobic surfaces

In 1756, Leidenfrost observed that water drops skittered on a sufficiently hot skillet, owing to levitation by an evaporative vapour film. Such films are stable only when the hot surface is above a critical temperature, and are a central phenomenon in boiling. In this so-called Leidenfrost regime, the low thermal conductivity of the vapour layer inhibits heat transfer between the hot surface and the liquid. When the temperature of the cooling surface drops below the critical temperature, the vapour film collapses and the system enters a nucleate-boiling regime, which can result in vapour explosions that are particularly detrimental in certain contexts, such as in nuclear power plants. The presence of these vapour films can also reduce liquid-solid drag. Here we show how vapour film collapse can be completely suppressed at textured superhydrophobic surfaces. At a smooth hydrophobic surface, the vapour film still collapses on cooling, albeit at a reduced critical temperature, and the system switches explosively to nucleate boiling. In contrast, at textured, superhydrophobic surfaces, the vapour layer gradually relaxes until the surface is completely cooled, without exhibiting a nucleate-boiling phase. This result demonstrates that topological texture on superhydrophobic materials is critical in stabilizing the vapour layer and thus in controlling--by heat transfer--the liquid-gas phase transition at hot surfaces. This concept can potentially be applied to control other phase transitions, such as ice or frost formation, and to the design of low-drag surfaces at which the vapour phase is stabilized in the grooves of textures without heating.     

具有温度稳定的法布里-珀罗标准具设计

为了使法布里-珀罗标准具有较好的热稳定性,从而制作性能较好的梳状滤波器,可采用特别的晶体制作标准具.通过分析表明,温度对标准具的影响归因于材料的热膨胀和折射率的温度效应.如果晶体的热膨胀系数和折射率温度系数在1个或1个以上方向上的值符号相反,则光束在晶体中某些方向上的光程与温度无关.据此提出了用晶体材料制作具有热稳定性的法布里-珀罗标准具的方法,并以晶体-LiSAF为例,进行具体计算和说明.结果表明,用晶体可以设计出效果很好的具有温度稳定的法布里-珀罗标准具.