Investigation of a hyperbranched polyurethane as a solid-state phase change material

As phase change material (PCM), doped hyperbranched polyurethane (D-HBPU) was synthesized by PEG6000 via two steps: at first take tetrafunctional pentaerythritol as soft segment, and then use liquefaction-modified isocyanate, and chain extender as hard segment. This material’s molecular structure, phase change behaviors, thermal-resistant property, and crystalline morphology had been investigated by Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analyses, wide-angle X-ray diffraction (WAXD), and polarization optical microscopy (POM). Results showed that D-HBPU was a typical solid-state PCM with better heat storage property and thermal resistance when compared with hyperbranched polyurethane. The transition enthalpy and the main decomposition temperature of D-HBPU were 125.0 J/g and 437 °C, respectively. From WAXD and POM, the results suggested that D-HBPU has good crystallization.     

聚乙二醇/氯化聚丙烯相变材料的制备

制备了一种骨架为氯化聚丙烯、支链为聚乙二醇的新型相变材料。利用聚乙二醇从结晶态到无定形态之间的转变,实现该材料的储能和释能,整个过程中材料始终保持为固体。叙述了该相变材料的合成方法,讨论了它的相变机理及特点等。结果表明,该新型固态相变材料相变焓较大,具有很好的应用价值和发展前途。     

Influence of SiO2 pore structure on phase change enthalpy of shape-stabilized polyethylene glycol/silica composites

Polyethylene glycol (PEG2000)/silica (SiO₂) composites with various weight percentages of PEG were prepared as solid–liquid shape-stabilized phase change materials using sol–gel method. In the composite, PEG and SiO₂ were chosen as the phase change substance and the supporting material, respectively. The composites were characterized by differential scanning calorimetry and scanning electron microscope. The pore structure of the SiO₂ matrix with removal of PEG was studied using N₂ adsorption analysis. The phase change enthalpy of PEG in the composite was determined. It was lower than the theoretical value, and decreased with the increase of PEG content. PEG in the composite was strongly confined during the phase transition, and the confinement effect was related with the pore structure of the silica matrix. By correlating the phase change enthalpy with the average pore diameter of the SiO₂ matrix by employing a confined phase change model with a constraint layer, the effect of the pore structure on phase transition of PEG was quantitatively evaluated. The phase change enthalpy of PEG in the composite depended on the average pore diameter of the SiO₂ matrix, the pore geometrical shape, and the thickness of the PEG constraint layer.     

Discontinuities in Amplitudes of Particles Micromotions Characterizing Phase Transitions in Liquid State

The behavior of amorphous solids below Vogel's (T _V ) or the glass transition (T _g ) temperature, as well as the solid-liquid transition, have been analyzed taking into consideration the anharmonicity of motion of microparticles forming the amorphous bodies. The T _g transition is explained within the logical association of this transition with the higher-temperature transitions, which can eventually involve the process of particle release into the gas phase through the process of a sudden vibrational amplitude growth. It follows from the mathematical solution of the anharmonicity problems that the pulses and the double amplitudes will always be present in aliquid matrix. The T _g temperature is considered as the boundary point for the liquid state at which the dynamical microcracks of a solid state matrix start to proceed. The processes at T _g are accompanied by appearance of new, highly agitated spots and the first microcracks (vacancies) filled up with the ‘semi-evaporated’ particles. In the mechanical sense, these vacancies form a new particle species characterized by quite different properties (different thermal expansion coefficient) as compared with the particles of the original matrix. It is assumed that a number of new mechanical units are growing up to the critical temperature when the original liquid frame, bonding the particles to lower amplitudes, is completely destroyed. The approach proposed does not contradict the traditional views reflected in the famous Adam-Gibbs-Di Marzio or WLF approaches, but allows a different approach to these theories. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature-composition-degree of compositional ordering phase diagram of lead scandoniobate-scandotantalate solid solutions

It was established that the temperature of the ferroelectric phase transition in crystals of compositionally ordered solid solutions of lead scandoniobate—scandotantalate with high and low degrees of ordering S depends linearly on the composition x. These data were used to calculate the T-x-S phase diagram of these solid solutions for the first time. Pis’ma Zh. Tekh. Fiz. 24, 88–93 (December 12, 1998)     

Wetting transition of grain boundaries in the Sn-rich part of the Sn–Bi phase diagram

The microstructural evolution of tin-rich Sn–Bi alloys after the grain boundary wetting phase transition in the (liquid + β-Sn) two-phase region of the Sn–Bi phase diagram was investigated. Three Sn–Bi alloys with 30.6, 23, and 10 wt% Bi were annealed between 139 and 215 °C for 24 h. The micrographs of Sn–Bi alloys reveal that the small amount of liquid phase prevented the grain boundary wetting transition to occur during annealing close to the solidus line. The melted area of the grain boundary triple junctions and grain boundaries increased with increasing the annealing temperature. When the amount of liquid phase exceeded 34 wt% during annealing, increasing temperature has not affected the wetting behavior of grain boundaries noticeably and led only to the increase of the amount of liquid phase among solid grains in the microstructure. The XRD results show that the phase structure and crystallinity remained unchanged after quenching from various annealing temperatures.     

Novel concept for the aggregation structure of fatty acid monolayers on the water surface and direct observation of molecular arrangements in their monolayers

The aggregation structure of fatty acid monolayers on water subphases of different pH’s was investigated by means of transmission electron microscopy. Fatty acid monolayers exhibited the phase transition from an amorphous state to a crystalline one by surface compression in the case of a highly dissociated state of hydrophilic groups, whereas they did not show the phase transition in the case of a slightly dissociated state. The aggregation structure of monolayers on the water surface was systematically classified into “the crystalline monolayer”, “the amorphous monolayer” and “the compressing crystallized monolayer” with respect to thermal and chemical (intermolecular repulsive) factors. Molecular-resolution images of fatty acid molecules in the monolayers on mica substrate were successfully observed with an atomic force microscope (AFM) for the first time. The AFM image of a lignoceric acid monolayer prepared at a surface pressure of 5mNm⁻¹ showed a two-dimensional periodic structure with locally disordered molecular arrangements. Also, the nondestructive AFM image observation was successful for a stearic acid monolayer which was prepared by a multistep creep method, indicating that a high mechanical stability of the monolayer is inevitably required for the nondestructive AFM observation.     

A density-driven phase transition between semiconducting and metallic polyamorphs of silicon

Amorphous and crystalline forms of silicon are well-known, tetrahedrally coordinated semiconductors. High-pressure studies have revealed extensive polymorphism among various metallic crystal structures containing atoms in six-, eight- and 12-fold coordination. Melting silicon at ambient or high pressure results in a conducting liquid, in which the average coordination is greater than four (ref. 3). This liquid cannot normally be quenched to a glass, because of rapid crystallization to the diamond-structured semiconductor. Solid amorphous silicon is obtained by synthesis routes such as chemical or physical vapour deposition that result in a tetrahedrally bonded semiconducting state. It has long been speculated that the amorphous solid and the liquid could represent two polymorphic forms of the amorphous state that are linked by density- or entropy-driven transformations. Such polyamorphic transitions are recognized to occur among several different types of liquid and glassy systems. Here we present experimental evidence for the occurrence of a density-driven polyamorphic transition between semiconducting and metallic forms of solid amorphous silicon. The experiments are combined with molecular dynamics simulations that map the behaviour of the amorphous solid on to that of the liquid state.     

Synthesis and characterization of polydioxolane polyurethane ionomer

A series of novel sodium salts of sulfonated polyurethane ionomers with polydioxolane as soft segment was successfully synthesized and their properties were characterized by means of Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and complex impedance analysis. As the ionization level increased, the compatibility of the hard segments and soft segments was improved and the glass transition region of soft segment became broader. The effects of ionization level, temperature, chemical components of soft segment on ionic conductivity of ionomers were investigated in detail. Adjusting the ionization level and incorporating PEG soft segment into the PDXL-PU ionomers, a new type of solid polymer electrolyte material with good mechanical properties and high ionic conductivity could be obtained.     

Quantitative determination of the volume fraction of intergranular amorphous phase in sintered silicon nitride

In this paper, a commercial liquid phase sintered silicon nitride with Y₂O₃ and Al₂O₃ additives is investigated. The material is characterised by a large, and stable, internal friction peak near 1150 °C. The peak is linked with the glass transition in intergranular amorphous volumes, whose presence is confirmed with transmission electron microscopy. An estimate of the volume fraction of the amorphous phase is calculated from the difference in stiffness below and above the glass transition temperature. The procedure relies on accurate Young’s modulus data, which were obtained with the impulse excitation technique (IET). The amount of amorphous pocket phase was estimated at 12.4 vol.%. For the first time, microstructural evidence supporting this estimate is obtained, using analysis of scanning electron microscopy (SEM) images. The rather large amount of amorphous matter explains the limited high temperature potential of the material, which was primarily and successfully developed for wear applications.     

聚乙二醇基复合相变材料的制备与热性能研究

利用聚乙二醇(PEG)为相变材料、以羟丙基甲基纤维素为分子骨架,采用4,4-二苯基甲烷二异氰酸酯作为交联剂,用化学接枝法成功合成了一种新型复合相变材料。采用红外光谱、差示扫描量热仪、热重仪、扫描电镜和X射线衍射仪对该复合相变材料的化学结构、相变性能、热稳定性、微观形貌和晶体结构等性能进行了表征。结果表明:该复合相变材料的相变过程表现为固-固相变的性质,其相变温度在309~323.2K范围内,相变焓值在89.8~106.8J/g之间。可见,通过化学接枝法得到的复合相变材料具有较好的相变行为,且克服了聚乙二醇在相变过程中的泄露问题。     

Liquid-liquid phase transition in supercooled silicon

Silicon in its liquid and amorphous forms occupies a unique position among amorphous materials. Obviously important in its own right, the amorphous form is structurally close to the group of 4-4, 3-5 and 2-6 amorphous semiconductors that have been found to have interesting pressure-induced semiconductor-to-metal phase transitions. On the other hand, its liquid form has much in common, thermodynamically, with water and other 'tetrahedral network' liquids that show density maxima. Proper study of the 'liquid-amorphous transition', documented for non-crystalline silicon by both experimental and computer simulation studies, may therefore also shed light on phase behaviour in these related materials. Here, we provide detailed and unambiguous simulation evidence that the transition in supercooled liquid silicon, in the Stillinger-Weber potential, is thermodynamically of first order and indeed occurs between two liquid states, as originally predicted by Aptekar. In addition we present evidence to support the relevance of spinodal divergences near such a transition, and the prediction that the transition marks a change in the liquid dynamic character from that of a fragile liquid to that of a strong liquid.     

AC impedance and dielectric spectroscopic studies of Mg<Superscript>2 + </Superscript> ion conducting PVA–PEG blended polymer electrolytes

Polyvinyl alcohol (PVA)–polyethylene glycol (PEG) based solid polymer blend electrolytes with magnesium nitrate have been prepared by the solution cast technique. Impedance spectroscopic technique has been used, to characterize these polymer electrolytes. Complex impedance analysis was used to calculate bulk resistance of the polymer electrolytes. The a.c.-impedance data reveal that the ionic conductivity of PVA–PEG–Mg(NO₃)₂ system is changed with the concentration of magnesium nitrate, maximum conductivity of 9·63 × 10^− 5 S/cm at room temperature was observed for the system of PVA–PEG–Mg(NO₃)₂ (35–35–30). However, ionic conductivity of the above system increased with the increase of temperature, and the highest conductivity of 1·71 × 10^− 3 S/cm was observed at 100°C. The effect of ionic conductivity of polymer blend electrolytes was measured by varying the temperature ranging from 303 to 373 K. The variation of imaginary and real parts of dielectric constant with frequency was studied.     

Photochemical Control of Dark Conductivity - A New Approach to Devices Based on Molecular Crystals

Thermal analysis of Ag(DM)₂, where DM = 2,5-dimethyl-N,N’- dicyanoquinonediimine, clarified that the salt had an insulating amorphous phase ( ≥ 155°C). Characterization of this and related solid states of Ag(DM)₂ indicated that a photo-induced process should be essential in controlling the number of carriers and thus conduction behavior of the salt by illumination. In fact, while heating could do nothing but make the salt insulating when the sample temperature exceeded 155°C, ultraviolet-visible light illumination ( < 155°C) could gradually change the properties to be semiconducting with retaining the crystal lattice (average structure).     

Diffusion processes inβ-Zr(Al) phase: a thermodynamic approach

The diffusion behaviour in theβ -Zr(Al) solid solution phase was investigated in the temperature range 1203-1323 K using a thermodynamic approach. The Boltzmann-Matano relation used for determining interdiffusion coefficients and the Darken’s equations used for evaluating the intrinsic diffusion coefficients from velocity of movement of markers in diffusion couples were modified suitably. The composition dependent thermodynamic interdiffusion coefficients were evaluated using chemical potential gradient. Composition and temperature dependence of the thermodynamic interdiffusion coefficients were also established. The thermodynamic intrinsic diffusion coefficients of Al and Zr and their temperature dependence were determined using the modified Darken’s equations.     

Structure–property relationship of shape memory polyurethane cross-linked by a polyethyleneglycol spacer between polyurethane chains

Shape memory polyurethane (SMPU) cross-linked by a polyethyleneglycol (PEG) spacer at its side was compared with a linear SMPU and the one randomly cross-linked by glycerol. The SMPU was composed of 4,4′-methylenebis(phenylisocyanate) (MDI), polytetramethyleneglycol-2000 (PTMG-2000), 1,4-butanediol (BD), glycerol, and PEG-200 as a spacer. PEG-200 connected the glycerol hydroxyl groups of two polyurethane chains via the connecting agent, MDI. T_m of the soft segment increased with the increase of cross-linking content. Cross-link density measured by the swelling experiment increased in proportion to the glycerol content. A surprising increase in maximum stress compared to the linear SMPU was attained without any sacrifice in strain. Remarkably, the stress–strain curve revealed that the PEG cross-linked SMPU exhibited a similar behavior and superior tensile mechanical properties to natural rubber. Storage modulus also increased and loss tangent became broad in distribution over the temperature range after PEG cross-linking. Shape recovery rate went up to 96.8%, and shape recovery speed was three times faster than that of linear SMPU.     

二元脂肪酸/生物基SiO_2复合相变材料的制备与性能

为了获得适用于建筑领域的相变材料和相变温度,选用月桂酸(LA)和棕榈酸(PA)共混制备的二元低共熔脂肪酸(LA-PA)作为储能材料,废弃稻草和稻草灰提取的生物基SiO_2(b-SiO_2)粉末作为载体,采用熔融浸渗法制备了LA-PA/b-SiO_2定形相变储能材料。采用FTIR、XRD、比表面测试、SEM、DSC、TGA对LA-PA/b-SiO_2复合相变材料的结构与性能进行分析。结果表明:LA-PA和b-SiO_2并不是简单的物理相互作用;LA-PA被束缚在b-SiO_2多孔网络中,从而在固相变为液相时,相变材料不会泄露。通过XRD分析可得,随着b-SiO_2含量的增加,LA-PA/b-SiO_2复合相变材料的结晶度降低。由DSC和TGA分析可知,LA-PA/b-SiO_2复合相变材料具有良好的相变性质和热稳定性,相变焓在67.36~146.0J/g之间。     

On the Order–Disorder Surface Phase Transition and Critical Temperature of Pure Metals Originating from BCC, FCC, and HCP Crystal Structures

The excess surface Gibbs energy and surface tension of pure liquid metals (originating from bcc, fcc, and hcp solid metals) of ordered and disordered surface structures are compared in this paper. It is shown that at a special temperature T ^* an order–disorder surface phase transition is expected in all liquid metals from a low-temperature ordered surface state to a high-temperature disordered surface state. This surface phase transition is similar to the first-order bulk solid–liquid phase transition (melting). The values of T ^* appear in the temperature interval between the melting point and the critical point of metals. Critical temperatures of metals are estimated from the equation for high-temperature disordered surfaces.     

Relevance of liquid state to solid state properties

We outline in this talk the beginning of a new programme to study physical properties of crystalline solids. It is based on considering the latter, a broken symmetry phase, in terms of the higher symmetry liquid phase. The solid is a calculable perturbation on the fluid. This is exactly opposite to the standard approach which relates mechanical properties to the behaviour of defects (mainly dislocations) etc., in an otherwise perfect crystalline solid. However, most other broken symmetry phases (e.g. ferromagnets) are discussed starting from a symmetric Hamiltonian or a free energy functional, and earlier work by one of the authors shows that the liquid-solid transition is well described, qualitatively and quantitatively, by this approach. On the other hand, defect theories of melting have a long record of nonsuccess. In the first part of the talk, the density wave theory of freezing will be outlined, and it will be shown how properties such as Debye Waller factor, entropy change of freezing etc. can be calculated with no or one free parameter. The problem of calculating shear elastic constants and dislocation core structures as well as energies in terms only of observable liquid state properties will be set up, and results presented. The method will be contrasted with zero temperature ‘atomistic’ models which obscure the essential dependence on structure and flounder in a mass of detail. The concluding part will describe further proposed applications, some suggestive experimental results extant in the literature, and some speculations. Only a summary is presented.     

Elastic properties of powders during compaction. Part 1: Pseudo-isotropic moduli

The elastic moduli of powdered materials undergoing uniaxial compaction was investigated, paying particular attention to effects of solid phase material properties and initial particle shape. Elastic properties were characterised by the isotropic elastic moduli Poisson’s ratio and Young’s modulus, calculated from elastic wave speeds measured in the axial (pressing direction). To isolate material property effects, three different ductile metal powders (copper, stainless steel, and aluminium) with equivalent particle shape (spheroidal) were tested. Comparison with similar measurements for a brittle spheroidal powder (glass) illustrated that solid phase yield mechanism affects the evolution of pore character, and hence bulk elastic properties of the powder compact. Pore character was also studied separately by comparing copper powders with differing particle shapes (spheroidal, irregular, and dendritic). For all powders, Young’s modulus increased monotonically with compaction (reducing porosity). For the ductile spheroidal powders, differences in evolution of Young’s modulus with compaction were accounted for by solid phase elastic properties. The different morphology copper powders showed an increase in compact compliance as particle (pore) ruggedness increased. Poisson’s ratio followed a concave porosity dependence: decreasing in the initial stages of compaction, then increasing as porosity approached zero. Comparison between powders indicated the initial decrease in Poisson’s ratio was insensitive to solid phase material properties. However, as the compact approached solid phase density, the Poisson’s ratio—porosity locus diverged towards corresponding solid phase values for each particle material, indicating an influence of solid phase elastic properties.