Dielectric properties of polymer-based microheterogeneous insulator

The study of the dielectric behavior of polymer-based insulating materials used in high voltage equipment is presented in this work. The materials tested are microheterogeneous in nature with mineral particles (calcium fluoride) dispersed in a semicrystalline polymeric matrix (polytetrafluoroethylene). The dielectric properties are analyzed in a wide frequency (or time) range by a combination of two techniques: the short time response is assessed by measuring the complex permittivity in the frequency domain (20 Hz–1 MHz), and the long term response is studied by measuring the polarization current in the time domain (1–600 s) thus covering more than eight decades in frequency or time. The modelling used to describe microheterogeneous dielectrics is shown to fit the experimental data. However, exposure to moisture is shown to induce strong relaxational effects. These effects vanish in a reversible way by drying treatment (i.e. removal of water molecules). These additional relaxations are discussed in term of conductivity effects occurring at the interface matrix-filler particles. When mechanical and dielectric spectroscopy studies are compared, it is to be underlined: (i) that the presence of water molecules has no significant effects on internal friction spectra; and (ii) solid state transformations in the non polar polymeric matrix induce strong mechanical and non significant dielectric relaxations. This is an example of complementary relaxation spectroscopy techniques when the coupling between the atomic motions and the excitation is not the same.     

Temperature dependence of Young's modulus in Si3N4-based ceramics: roles of sintering additives and of SiC-particle content

The temperature dependence of Young's modulus has been investigated by ultrasonic-echography in the 20–1400 °C temperature range for different oxynitride glasses, silicon nitride ceramics, and SiC/Si₃N₄ particulate composites. Glasses exhibit a transition between a slow softening (elastic) regime and a rapid softening one which almost coincides with the glass transition range, and is located between 840 and 920 °C. The transition observed in ceramics is located between 1080 and 1150 °C and accounts for the behavior of the intergranular amorphous films. The higher the Y/Al ratio or the SiC content, the higher the transition temperature, and the smaller the softening rate above the transition range. Following the theories of thermally activated flow phenomena and of hierarchically constrained dynamics for glass relaxation, an expression for the correlation factor (b) was derived from the softening rate above the transition temperature. The estimated values for b range between 0.5 and 0.7 and are consistent with previously reported values, as obtained by stress relaxation or by mechanical spectroscopy techniques.     

Relaxation of rapidly quenched metallic glasses: Effect of the relaxation state on the slow low temperature dynamics

The relaxation spectrum of rapidly quenched Mg₆₅Cu₂₅Y₁₀ metallic glass ribbons is studied by mechanical spectroscopy at temperatures below and around the glass transition. The comparison between hyper-quenched and relaxed samples is used to examine the origin of the low temperature “excess wing” of internal friction commonly observed in mechanical spectroscopy of metallic glasses. The results show that the excess wing can be attributed to access of the system to the broad α-relaxation process while evidence of secondary relaxations is not found. This suggests that in this glassy system the activation energies of structural relaxation and low temperature deformation are directly related to the activation energy of the main relaxation process of the glassy state.     

Study of relaxations in epoxy polymer by thermally stimulated depolarization current (TSDC) and dielectric relaxation spectroscopy (DRS)

Thermally stimulated depolarization current (TSDC) and dielectric relaxation spectroscopy (DRS) techniques were employed to study the relaxations and the conductivity phenomena in epoxy-based polymer. In addition to the primary α relaxation process associated with the glass–rubber transition, significant interfacial relaxation and ionic conduction process have been revealed. The ac conductivity is temperature and frequency dependent and shows a dc plateau at low frequencies. Above the glass transition temperature, dc conductivity is described by a Vogel–Tamman–Fulcher–Hesse (VFTH) equation while it shows Arrhenius behaviour at higher temperatures.     

Thermodynamics and structural relaxation in Ce-based bulk metallic glass-forming liquids

Thermodynamics, kinetics and structural relaxation of Ce-based bulk metallic glass-forming liquid were investigated in the glass transition region by calorimetric measurements. The differences in thermodynamic functions were calculated between the supercooled liquid and crystalline state of the Ce-based alloys. Structural relaxation was studied by heating rate dependence of glass transition temperature. In terms of fragility parameter m, the Ce-based alloys were stronger liquid than other metallic glass-forming liquids. The correlation of the excellent glass-forming ability of Ce-based alloys with the thermodynamic property (Gibbs free energy) and the kinetic property (m) was discussed. The structural relaxation from glass state to the equilibrium supercooled state was well described by Tool-Narayanaswamy-Moynihan (TNM) model using the parameters derived from the calorimetric measurements.     

Effect of crystallinity on the impact toughness of a La-based bulk metallic glass

The influence of crystallinity on the ductile–brittle transition in a bulk lanthanum-based metallic glass was investigated. The degree of crystallization in the metallic glass, which was processed through the arc-melting and water-quenching route, was systematically altered by varying the annealing time above the glass transition temperature. The resulting amorphous/crystalline microstructures were characterized by XRD, DSC, and TEM techniques. Instrumented impact test results show a significant decrease, by ∼90%, in impact toughness with the introduction of a small percentage of crystallinity. This decrease in toughness was also associated with a distinct change in fracture morphology, from a ductile vein-type fracture in the bulk glass to intergranular cleavage fracture in the crystalline material. The observed fracture transition was rationalized by recourse to the mechanism of stress relaxation due to viscous flow. For this purpose, variations in elastic modulus and dynamic viscosity with respect to the crystallinity were measured using a dynamic mechanical analyzer. The characteristic relaxation times were computed from the viscosity data and were used to explain the ductile–brittle transition. Microscopic mechanisms responsible for the fracture transition are also discussed.     

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF SiC W/BAS (BaAl 2Si 2O 8) GLASS CERAMIC COMPOSITE

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Ti53Cu27Ni12Hf3Al7Si3B1大块金属玻璃的制备及性能研究

用单辊甩带法和铜模铸造法制备了新型Ti基大块金属玻璃Ti53Cu27Ni12Hf3Al7Si3B1.DSC、DTA研究表明该合金具有较高的热力学稳定性,其第1晶化温度Tx1、玻璃转变温度Tg、过冷液相区间ΔTx以及约化转变温度Trg分别为705、750、45、0.63 K.压缩性能研究其压缩断裂强度、弹性模量和压缩塑性变形量分别为2 304 MPa、120 GPa和1.1%.研究发现,具有复杂拓扑结构的原子配比提高了Ti53Cu15Ni18.5Al7Hf3Si3B0.5大块金属玻璃的力学性能和热力学性能.     

Ti20Zr20Hf20Cu20Be20高熵非晶合金应力弛豫行为研究

高熵非晶合金力学弛豫行为的研究,对于理解玻璃转变、塑性变形、弛豫机理等科学问题和拓展其工程应用极为关键。本文采用应力分析方法对Ti20Zr20Hf20Cu20Be20高熵非晶合金条带进行了研究,旨在揭示高熵非晶合金应力弛豫行为。通过分析其在恒定应变下较宽时间窗口和温度窗口内的应力衰减过程,发现在低于Tg的玻璃态下,高熵非晶合金中存在着弛豫解耦现象,分别是慢弛豫和快弛豫过程。其中慢弛豫呈现扩展指数衰减模式,弛豫时间存在对温度的Arrhenius动力学依赖关系,与长程原子重排运动有关,快弛豫过程对应于微观局部内应力的逐步消散过程。不论该高熵非晶合金变形处于弹性阶段还是发生了屈服,应力弛豫过程受应变的影响都较小。本研究揭示了高熵非晶合金中新的弛豫解耦现象和与之相关的独特动力学机制,拓宽了我们对高熵非晶合金弛豫动力学过程以及其本征特性的认知。     

BRITTLE-DUCTILE TRANSITION OF ANNEALEMBRITTLED Fe_(75)Si_(10)B_(15)METALLIC GLASS

Studies were made of the effect of mechanical pulverization on relaxation,crystallizationand brittle-ductile transition of the anneal-embrittled Fe_(75)Si_(10)B_(15) metallic glass rib-bon.Results show that the Curie temperature,T_C,decreases and the total enthalpy ofrelaxation increases gradually with variation of pulverized time.DSC traces reveal anextra exothermic peak,T_X_1,and a distinct glass transition endothermic peak,T_g_1,withincreasing pulverized time,T_C,T_X_1,and T_g_1 decrease simultaneously,and the exother-mic peak area corresponding to T_X_1 increases gradually.The surface slip-steps of flakyparticles and their corresponding shear are produced by pulverizing the pre-embrittledamorphous glass.The lost ductility of the glass may restore during annealing.     

Ni对Mg-Cu-Tb非晶合金形成及力学性能的影响

利用熔体铜模喷铸法制备出直径为3 mm的Mg65Cu25-xNixTb10(x=0,5,10)非晶合金。利用X射线衍射、差热分析、压缩实验分析和扫描电镜分析了添加Ni元素对Mg-Cu-Tb非晶合金形成能力及力学性能的影响。研究表明:随着Ni含量的增加,合金的玻璃转变温度Tg增大;开始结晶温度Tx降低;过冷液相区宽度ΔTx减小,约化玻璃转变温度Trg从0.562降至0.530,非晶形成能力逐渐降低。压缩实验结果表明:当Ni含量增加到5%时可以明显提高Mg-Cu-Tb-Ni非晶合金的断裂强度。     

Zr43Cu50Al7块体非晶合金的结构弛豫和晶化研究

在不同加热速率下应用差示扫描量热法(DSC)研究了Zr43Cu50Al7块体非晶合金的结构弛豫现象和晶化行为,得到了满意的晶化曲线.由晶化曲线得到弛豫峰与玻璃转变温度(Tg)、晶化起始温度(Tx)和晶化峰温度(Tp).Zr43Cu50Al7非晶合金的过冷液相区ΔTx达76.2 K,具有较强的玻璃形成能力.对所得到的弛豫峰作了初步的研究,并运用Kissinger法和Deloy法分别计算出玻璃转变激活能Eg、晶化起始激活能Ec、晶化峰的激活能Ep和晶化阶段激活能Ex.结果表明Zr43Cu50Al7块体非晶合金具有良好的热稳定性.     

The synthesis and properties of Zr-based metallic glasses and glass-matrix composites

In this article, the formation of metallic glass composites in the system Zr/Ti-Al-Cu-Ni by partial devitrification or by blending with second-phase particles through solid-state processing is discussed with respect to the effect of second phases on the thermal stability of the glassy matrix and on mechanical properties. The composites exhibit no significant reduction of the supercooled liquid region as compared to the particle-free metallic glass. The viscosity of the supercooled liquid increases with an increasing volume fraction of particles. The mechanical behavior was characterized by microhardness measurements and constant compression rate tests. At room temperature, there is a significant increase in yield strength with an increasing volume fraction of crystalline phases. At temperatures around the glass transition, the influence of these nanoscaled particles is of minor importance. Rather, the homogeneous flow of the composites is determined by Newtonian viscous flow of the amorphous matrix. This opens a promising route for easy shaping of complex parts of bulk metallic glasses at temperatures that are above T_g. For more information, contact J.H. Ecker, IFW Dresden, Institute of Metallic Materials, P.O., Box 27 00 16, Dresden D-01171, Germany; telephone 49-351-4659-602; fax 49-351-4659-541; e-mail j.eckert@ifw-dresden.de.     

Mechanical relaxations of a (Zr77.5Ti22.5)55(Ni48Cu52)21.25Be23.75 amorphous alloy studied using dynamic mechanical analysis

The dynamic mechanical properties of a (Zr_77.5Ti_22.5)₅₅(Ni₄₈Cu₅₂)_21.25Be_23.75 amorphous alloy were investigated by frequency-dependent elastic moduli and isothermal multi-frequency measurements. The frequency-dependent loss modulus showed a relaxation behavior resulting from a glass transition, and the variation of the peak frequency was related to the Arrhenius equation. Isothermal multi-frequency measurement data were used to construct the master curves of the elastic moduli and tan δ by applying the time-temperature superposition principle. The temperature dependence of the shift factor was found to follow the Arrhenius relationship, and the activation energies for the low temperature relaxation and glass transition were approximately 156.6kJ/mol and 554kJ/mol, respectively. The glass transition temperature (T _g) was manifested by the crossover region of the shift factor dependence, and from the relationship between the shift factors and the temperature aboveT _g), the fragility index of this alloy was estimated.     

Thermally induced transitions of polarons to bipolarons in poly(3-dodecylthiophene)

The influence of thermal activation on polaron interactions of poly(3-dodecylthiophene) (PDDT) was studied. It was found that when an undoped PDDT sample was heated above 450 K and subsequently cooled to under the glass transition temperature, there were strong spin–spin interactions. When the polaron concentration was increased by FeCl₃ doping, all polarons including the immobile ones started to interact mutually. Changes in its internal microstructure, caused by thermal activation of the sample, followed by its freezing can generate an amplification of polaron interactions, giving rise to diamagnetic inter- and intra-chain bipolarons. As a result of strong spin–spin interactions the doped polymeric sample of PDDT displays diamagnetism even at room temperature. The diamagnetic properties depend on the consecutive structural relaxation of PDDT, caused either by immediate temperature changes, or by long-term standing at room temperature.     

Synthesis and characterization of polyaniline–polycarbonate composites prepared by an emulsion polymerization

Electrically conductive polymer composite, polyaniline (PANI)–dodecylbenzene sulfonic acid (DBSA)/polycarbonate (PC), was prepared by an inverted emulsion polymerization in which DBSA played both roles of surfactant and dopant. Fourier transform infrared (FT-IR) spectroscopy for the composite showed the existence of hydrogen bonding between PANI and PC which caused the increase of glass transition temperature with PANI content. Moreover, the electrical conductivity increased around the glass transition temperature because the PANI chains contacted more frequently and facilitated the electron transfer through the hydrogen bonding between PANI and PC. In addition, it was proved by mechanical property that tensile strength of the composite decreased with PANI content because PANI functioned as a defect in PC matrix and the tensile modulus continuously increased because PANI had a higher rigidity of molecules.     

Misch metal based metallic glasses

In this paper, a glass-forming range of metallic glasses based on Ce-rich misch metal (Mm) was pinpointed in Mm-Al-Co composition map by melt spinning. The thermal analysis indicated that the wide supercooled liquid region (above 60 K) can be found out in a large composition range in Mm-Al-Co system. The investigation of the glass-forming ability (GFA) in this system indicated a glassy composition with a larger supercooled liquid region wouldn’t be the glassy former with higher GFA. The reduced glass transition temperature is a better indicator to explore metallic glasses with high GFA. Furthermore, the mechanical properties of Mm₆₅Al₁₀Co₂₅ bulk glassy samples were evaluated in a compressive measurement. The obvious advantages of the Mm-based BMGs with high GFA, good mechanical properties and low material cost make these BMGs hopeful to be applied in the future.     

Nanostructured bulk glassy Cu_(60)Zr_(30)Ti_(10) alloy with high strength and good ductility in cast state

1　INTRODUCTIONSinceaseriesofmetallicbulkglassyalloyswithexpectedmechanicalpropertiesandhighglassformingabilityhavebeenfound[13] ,particularattemptshavebeendonetoimprovethemechanicalpropertiesoftheglassyalloystomeetengineeringapplications .Bycrystallizingthemetallicglasses ,nanostructuredal loyscontainingnanocrystallinephasesandglassyma trixwereobtainedandthemechanicalpropertiesofthealloyswereenhancedbyhomogeneousdispersionofnanoscalemetallic particlesintheAl [4 ,5] ,Fe Ni [6 ] andMg [7]…     

High Temperature Deformation Behavior of High Strength and Toughness Ti-Ni Base Bulk Metallic Glass CompositesEI北大核心CSCD

Room-temperature brittleness and strain-softening during deformation of bulk metallic glasses, and limited processability of shape memory alloys have been stumbling blocks for their advanced functional structural applications. To solve the key scientific problems, a new shape memory bulk metallic glass based composite, through the approach using transformation-induced plasticity (TRIP) effect of shape memory alloys to enhance both ductility and work-hardening capability of metallic glasses, and superplasticity of bulk metallic glass in supercooled liquid region to realize near net forming, was developed in this work. And the Ti-Ni base bulk metallic glass composites (BMGCs) rods were prepared by the levitation suspend melting-water cooled Cu mold process. Microstructure, thermal behavior, mechanical properties and high temperature deformation behavior of the alloy were investigated. The results show that the as-cast alloy microstructure consists of amorphous matrix, undercooled austenite and thermally-induced martensite. Besides, the size of the crystal phase precipitated on the amorphous matrix in-creases from the surface to the inside. The alloy exhibits excellent comprehensive mechanical properties at room temperature. The yield strength, fracture strength and the plastic strain of alloy are up to 1286 MPa, 2256 MPa and 12.2%, respectively. Under compressive loading in the supercooled liquid region, the composite exhibits approximate Newtonian behavior at lower strain rate in higher deformation temperature, and the optimum deformation temperature is T>480 degrees C and the intersection part with supercooled liquid region (SLR). When the temperature is 560 degrees C and the strain rate is 5x10(-4) s(-1), the stress sensitivity index m and the energy dissipation rate Psi are 0.81 and 0.895, respectively. Furthermore, the volume of activation is quantified to characterize the rheological behavior.     

Deformation and flow in bulk metallic glasses and deeply undercooled glass forming liquids—a self consistent dynamic free volume model

Bulk glass forming metallic liquids such as those of the Zr–Ti–Ni–Cu–Be Vitreloy alloy family have been shown to have Newtonian Viscosity which is well described by the Vogel–Fulcher–Tamann (VFT) equation over roughly 15 orders of magnitude from the high temperature equilibrium melt to the deeply undercooled liquid near and below the experimentally observed glass transition. Experiments have also shown flow becomes non-Newtonian and ultimately unstable against spatial localization into shear bands as the strain rate at a given temperature is increased. This transition from homogeneous to inhomogeneous flow and flow localization has been discussed by several authors and attributed to the influence of strain softening, strain rate sensitivity, and thermal softening.which collectively result in the destabilization of the uniform flow field. The present paper presents a simple self consistent model of uniform steady state flow which is based on the tradition Free Volume Model of the glass transition, the VFT-equation, and a simple treatment of free volume production and annihilation during flow. The model is used to analyze the flow data and shown to give a simple one-parameter fit to experimental steady state Newtonian and non-Newtonian flow data over a broad range of temperatures and strain rates. The model gives simple analytic expressions for the steady state constitutive flow law, the strain rate sensitivity exponent (SRSE), and an implicit equation for the strain rate and temperature dependent viscosity, which is solved numerically. An approximate analytic expression for the non-Newtonian effects is proposed. Generalizing the model to time dependent flow, it is argued that observed shear localization and serrated flow in bulk glass forming liquids arises primarily from transient response phenomena.