Dynamic light scattering in solid polymers

The viscosity of an amorphous polymeric solid above its glass transition [T _g (T,P)] increases as the temperature of the solid is decreased or the pressure is increased. Under changes in temperature or pressure, molecular subunits in the polymeric solid undergo configurational changes. Such changes or relaxations have a distribution of relaxation strengths and times. As the solid is cooled or as the hydrostatic pressure on the solid is increased, the relaxation strengths increase and the relaxation times increase. These changes in relaxation or dynamic properties are very dramatic as the empirical T _g is approached. Near T _g the polymeric solid is no longer in volume equilibrium; continued cooling or pressuring at a time rate faster than the average relaxation time will produce a polymeric glass. This glass is a nonequilibrium, amorphous solid. If the glass is held at a fixed temperature and pressure very close to, but below, T _g , the volume of the glass will be observed to relax to its equilibrium value. For temperatures and pressures well below T _g , equilibrium is a much more conjectural concept since the relaxation times become extremely long. It has been proposed^1,2 that there is a characteristic temperature T _g at which an amorphous polymer undergoes a second-order transition to an equilibrium glass with zero configurational entropy (i.e., a noncrystallizable solid).     

Super-pressed and super-cooled (?) P(EO)20LiBETI

High pressure conductivity measurements have been carried out on P(EO)₂₀LiBETI from 295 K to 368 K. The decrease of electrical conductivity with pressure is larger in the partially crystalline phase (low temperature and low pressure or high temperature and high pressure) than in the fully amorphous phase (high temperature and low pressure). It is found that if the phase transition is approached from the crystalline phase (decreasing pressure), the pressure of the phase transition varies from 0 to 0.23 GPa as the temperature increases from 336 K to 358 K. The shift of the phase transition temperature with pressure is approximately the same as the shift of the glass transition temperature with pressure for pure PEO. This can be understood in terms of the defect diffusion model. If the material is above 336 K and is in the fully amorphous phase, after pressure is increased above the critical pressure, the material remains in the amorphous phase for extended periods of time before transforming to the partially crystalline phase. This is reminiscent of a super-pressed state but may be an indication of slow crystallization kinetics.     

Polycrystalline semiconductors

Generally substances are more stable in a crystalline than in a glassy state. Therefore, to form a glass, crystallization must be bypassed. Under certain conditions, the melts of many substances can be cooled to the glass state. Whether or not the melt of a given material forms a glass is determined principally by a set of factors which can be specified to some extent in the laboratory, namely, the cooling rate, - T, the liquid volume, v], and the seed density, ps and upon a set of materials constants: the reduced crystal–liquid interfacial tension, α the fraction, f, of acceptor sites in the crystal surface, and the reduced glass temperature, T_rg . The glass-forming tendency will be greater the larger are - T and T_rg and the smaller are v]. ps, and f. The number and variety of substances which have been prepared in a glassy or ‘amorphous solid’ form have been greatly increased with techniques in which the material is condensed from solution on to a surface held well below its glass temperature. There are at least some glass formers in every category of material, according to bond type, i.e. covalent, ionic, metallic, van der Waals or hydrogen. However, it is not established whether or not every substance can be put into a glass form.     

FTIR spectroscopy and calorimetry of the amorphous state

These investigations are directed at identifying the microscopic polymer characteristics which determine the properties and relaxation behavior of amorphous polymers. In addition to the molecular structure, the number of holes (free volume) and the fraction of flexed bonds (i. e., gauche states) can be used as a starting point for the specification of the states of the glass. In addition, the effect of pressure is used to influence the number of holes and the fraction of flexed bonds frozen in the glass. Fourier transform infrared spectroscopy (FTIR) has been used to follow the trans to gauche populations as a function of temperature and pressure.     

Amorphous nanoparticles — Experiments and computer simulations

The data obtained by both experiments and computer simulations concerning the amorphous nanoparticles for decades including methods of synthesis, characterization, structural properties, atomic mechanism of a glass formation in nanoparticles, crystallization of the amorphous nanoparticles, physico-chemical properties (i.e. catalytic, optical, thermodynamic, magnetic, bioactivity and other properties) and various applications in science and technology have been reviewed. Amorphous nanoparticles coated with different surfactants are also reviewed as an extension in this direction. Much attention is paid to the pressure-induced polyamorphism of the amorphous nanoparticles or amorphization of the nanocrystalline counterparts. We also introduce here nanocomposites and nanofluids containing amorphous nanoparticles. Overall, amorphous nanoparticles exhibit a disordered structure different from that of corresponding bulks or from that of the nanocrystalline counterparts. Therefore, amorphous nanoparticles can have unique physico-chemical properties differed from those of the crystalline counterparts leading to their potential applications in science and technology.     

NANOSTRUCTURED Zr-BASED ALLOYS-PHASE FORMATION AND MECHANICAL PROPERTIES

Newly developed nanostructured Zr/Ti-Al-TM multiphase alloys can provide a large bandwidth of interesting properties, such as mechanical properties. Bulk materials with nanocrystalline/ amorphous and (nano)quasicrystalline/ amorphous microstructure with different volume fractions of nanophases and with different grain sizes can be obtained by slowly cooling the melt as well as by solid state reactions. Multiphase structures are realized either by partial de-vitrification of bulk glass-forming alloys or by defined addition of inert compounds upon alloying. Special preparation techniques e.g. copper mould casting and subsequent controlled annealing and mechanical alloying combined with hot consolidation of powders are described. The phase formation and transformation processes and the thermal stability of such materials in dependence on alloy composition and processing parameters are discussed in detail. Currently, the exploration of properties with respect to potential applications of these nanostructured alloys is still at the beginning. First investigations on the contributions of different phases/ volume fractions to the overall mechanical behaviour will be shown. At room temperature, the deformation behaviour of amorphous/crystalline bulk samples is governed by contributions of all existing phases yielding a high strength of the material.     

Synthesis and characterization of NaAlSi_2O_6 jadeite under 3.5 GPa

The high pressure and high temperature(HPHT) method is successfully used to synthesize jadeite in a temperature range of 1000℃–1400℃ under a pressure of 3.5 GPa. The initial raw materials are Na_2SiO_3·9H_2O and Al_2(SiO_3)_3.Through the HPHT method, the amorphous glass material is entirely converted into crystalline jadeite. We can obtain the good-quality jadeite by optimizing the reaction pressure and temperature. The measurements of x-ray diffraction(XRD),scanning electron microscopy(SEM), Fourier-transform infrared(FTIR) and Raman scattering indicate that the properties of synthesized jadeite at 1260℃ under 3.5 GPa are extremely similar to those of the natural jadeite. What is more, the results will be valuable for understanding the formation process of natural jadeite. This work also reveals the mechanism for metamorphism of magma in the earth.     

Elementary considerations on the local symmetry in optoelectronic materials and their phase change behavior

Crystalline structures of elemental solids can be rationalized in terms of the competition between ions and electrons: ions try to increase local symmetry and thus packing fraction, while electrons want to reduce it. If the latter win, layered structures, network, or molecular solids form and the opening of an electronic gap is favoured. In this work, it will be discussed how this competition can affect the thermodynamic behavior of phase change materials (PCMs), in particular that of Ge-Sb-Te alloys: their technologically relevant metastable crystalline structures can be derived from (hypothetical, metallic) simple cubic crystals near half-filling via a symmetry breaking, such as a Peierls distortion in Sb-rich PCMs or ordering of chemical species onto sublattices on the GeTe-Sb₂Te₃ pseudo-binary line, leading to the formation of σ-bonded networks. Local symmetry and density become even smaller and the gap opens up even more in the glass, for example, when the group IV element germanium undergoes a coordination change from (distorted) octahedral in the crystal to tetrahedral. This coordination change leaves the σ-bonded network intact, as will be demonstrated by analysis of first-principle simulations. Based on local symmetry arguments, simple rules for the number of electron holes and/or vacancies in metastable crystalline structure of PCMs can be derived and the response of Ge-Sb-Te alloys to pressure be predicted: crystalline alloys will amorphise under pressure when there are more Te than Ge atoms and increase their conductivity. Conversely, disordered alloys will crystallize if the number of Ge atoms exceeds that of Te. The possibility to switch the latter PCMs reversibly with pressure will be discussed. Lastly, unusual relaxation dynamics of PCMs are identified from first-principle calculations: when a solid is streched to its amorphisation point, the ionic energy (which is minimized in the crystal) increases with time as opposed to the dominating electronic energy. At the same time, coordination statistics become increasingly distinct with age from those in the crystal, i.e., the glass initially relaxes away from the crystalline phase.     

EXAFS方法在金属玻璃结构研究中的误差分析

本文对EXAFS方法在金属玻璃结构研究中所得径向距离的误差进行了分析。所做的模拟计算表明当前流行的关于这个问题的误差理论不能圆满地解释这一现象。本文分析了玻璃态和晶态的相移函数,指出在相似的体系中玻璃态和晶态相移函数与k的关系采用线性函数近似时,其斜率可能是不同的,而且无序度相差越大,两者的偏差可能也越大。在EXAFS数据处理过程中,没有考虑这种偏差的影响是造成EXAFS方法结果中径向距离误差的一个原因。本文还提出了一个修正参考样品(晶态)相移函数斜率偏差的方法,并将它应用于Cu₅₅Zr₄₅金属玻璃的EXAFS数据处理中,所得结果与用X射线异常散射方法测定的结果符合得很好。 关键词：     

非晶物质中的临界现象

非晶态材料有着复杂的原子结构(短程有序、长程无序)和特殊的物理性质,其临界现象和相变问题一直受到学术界关注.非晶合金,又称为金属玻璃,是一种新型的非晶态材料,具有很高的强度和优异的弹性.从微观的角度来看,非晶合金可以看作是一个多粒子系统.临界现象的研究对认识和理解多粒子系统之间的相互作用有深刻的意义.本文主要讨论非晶合金中的临界现象,包括非晶合金从制备过程、微观结构到宏观的力学性能以及磁性方面存在的临界现象,并分析这些临界现象之间的内在联系,进而深入理解非晶合金的微观结构对其宏观性质的影响.这为认识非晶合金的形成本质,提高服役可靠性,探索具有实际应用价值的非晶合金提供理论依据.     

非晶纤维的制备和力学行为

将块体材料制备成微纳米纤维时,其力学性能会得到进一步的提高,甚至具备块体材料所没有的力学行为.非晶态材料可经过熔体拉丝一次性成型而得到所需尺寸的均匀纤维,纤维表面质量好,其制备过程相对简单且节能.由于非晶材料短程有序、长程无序的结构,具备优异的力学性能,所以非晶纤维有着广泛的应用前景和基础研究价值.本文对能制备成非晶纤维且有优异力学性能的材料做了简单介绍,对非晶纤维的制备方法及其成型物理机制、非晶纤维的力学行为及其物理机制进行了综述,最后总结了非晶纤维的制备和力学行为的研究中存在的问题,对非晶纤维的发展前景做了展望.     

Amorphisation in solid metallic systems

The formation of amorphous alloys by a solid state reaction without any rapid quenching is reviewed. The crystal to glass transition is driven by the large negative heat of mixing of the crystalline reactants. Kinetic constraints assure the formation of an amorphous phase instead of the crystalline equilibrium phases. A comparison with other recently developed methods, like ion beam mixing, and a comparison of some physical properties between differently prepared amorphous alloys of the same composition are given.     

Neutron diffraction study of the interaction of iron with amorphous fullerite

The amorphous fullerite C₆₀ has been prepared by mechanical activation (grinding in a ball mill), and its interaction with iron during sintering of powders with 0–95 at % Fe has been studied. After sintering in the range 800–1200°C under a pressure of 70 MPa, the samples have nonequilibrium structures different from the structures of both annealed and quenched steels. In this case, the carbon phase, i.e., amorphous fullerite, undergoes a polyamorphous transition to amorphous graphite. It has also been shown that the interaction of amorphous fullerite with iron is weaker compared to crystalline fullerite or crystalline graphite.     

Nanocrystalline iron-based alloys investigated by Mössbauer spectrometry

Nanocrystalline alloys exhibit great fundamental and technological interests because of their microstructural properties, and their excellent soft magnetic properties. ⁵⁷Fe Mössbauer spectrometry is a well suitable technique to investigate Fe-based nanocrystalline alloys: its local probe behaviour permits to elucidate the nature of hyperfine interactions at different resonating iron nuclei and to distinguish their immediate atomic surroundings. We review on the recent Mössbauer developments performed on first FeCuMBSi and then FeCuBSi nanocrystalline alloys. From Mössbauer studies, one can estimate the crystalline (i.e., amorphous) fraction, the Si-content in Fe--Si nanocrystalline grains emerging from amorphous alloys of the first series, the temperature dependence of magnetic behaviours of both crystalline and amorphous phases; finally, we present a novel fitting procedure applied to FeCuBSi nanocrystalline alloys which result from bcc-Fe crystalline grains embedded in an amorphous matrix. In this case, the hyperfine structure is able to model the intergranular phase.     

不透明玻璃显现出的曙光——块体金属玻璃的发现与应用

在足够高的冷却速度下，如同其他大多数物质一样，金属合金溶体在冷冷却到室温的过程中能够经过玻璃化转变过程变成非晶态固体——金属玻璃。金属玻璃因其具有许多优异和独特的物理、化学和力学性能而一直受到很大的关注。在过去，由于玻璃形成能力的限制，金属合金只能制成厚度为数十数米的薄带状金属玻璃，因而其应用范围受到极大的限制。通过对金属合金的组成、溶体的过冷与稳定性及玻璃形成能力的关系研究，人们用常规的方法在较低的冷却速度下就能在许多金属合金体系中形成三给尺度都达毫米至数厘米的块体金属玻璃，这为金属玻璃获得广泛的应用奠定了基础。     

铀基非晶合金的发展现状

铀基非晶合金是非晶家族中的特殊成员,受限于铀元素的高活性与放射性特点,目前这类非晶材料的研究极不充分.本文结合非晶合金的最新发展动态简要介绍了铀基非晶发展历史,较系统地总结了本团队的最新铀基非晶研究工作:首先较详细地介绍了新型铀基非晶的制备技术、成分体系、形成规律与晶化行为,澄清了其形成机制与热稳定性;结合高分辨电镜分析展示了其微观结构特点;采用纳米压痕技术揭示了这类非晶的微纳力学性能;利用电化学测试方法评估了其耐腐蚀性能.这些结果丰富了非晶材料的内涵,有助于深化对非晶物理基础科学问题的理解,并推动新型铀合金材料的发展,为这种材料的潜在工程应用奠定了基础.     

Ionographic patterns with amorphous/crystalline contrast

Ion irradiation of thin layers of crystalline semiconductors induces a phase transition to the amorphous state. The concomitant optical contrast between unirradiated, crystalline, and irradiated, amorphous, material may be used for pattern fabrication in the submicron range. This process will be explained by the example of silicon single-crystal layers on sapphire.International Patents pending     

The influence of short-range order on electronic properties of transition metals: II. Amorphous metals

Local electronic properties have been calculated for the amorphous transition metals Nb, Mo, and Tc using a cluster model which accounts for the topology of the local structure. Trends in the density of states observed in metallic glasses are reproduced, and the behaviour of the averaged electron-phonon coupling strength correlates well with the behaviour of the superconducting transition temperature. The ratios of the critical temperatures in the amorphous and the crystalline state are reproduced correctly. The results show that the change in the critical temperature on going from the crystalline to the amorphous state can be explained by the change in short-range order.     

The influence of short-range order on electronic properties of transition metals: II. Amorphous metals

Local electronic properties have been calculated for the amorphous transition metals Nb, Mo, and Tc using a cluster model which accounts for the topology of the local structure. Trends in the density of states observed in metallic glasses are reproduced, and the behaviour of the averaged electron-phonon coupling strength correlates well with the behaviour of the superconducting transition temperature. The ratios of the critical temperatures in the amorphous and the crystalline state are reproduced correctly. The results show that the change in the critical temperature on going from the crystalline to the amorphous state can be explained by the change in short-range order.     

非晶InGaZnO薄膜成分配比对透明性和迁移率的影响

利用固相反应法制备了富铟含量在不同成分配比下的高质量InGaZnO陶瓷靶材,采用脉冲激光沉积法,在基片温度为20℃、氧压为1Pa条件下,在石英玻璃衬底上生长了非晶InGaZnO薄膜,并对薄膜进行X射线衍射、透射吸收光谱、拉曼光谱与霍尔效应测试。通过对InGaZnO薄膜的测试表征,在较低温度条件下,铟含量较高的薄膜样品保持了非晶结构、可见光的高透明性和高电子迁移率,InGaZnO薄膜有望应用于电子器件。