Structural instability of metallic glasses under radio-frequency-ultrasonic perturbation and its correlation with glass-to-crystal transition of less-stable metallic glasses

It has been reported that the structural stability is significantly deteriorated under radio-frequency-ultrasonic perturbation at relatively low temperatures, e.g., near/below the glass transition temperature T(g), even for thermally stable metallic glasses. Here, we consider an underlying mechanism of the ultrasound-induced instability, i.e., crystallization, of a glass structure to grasp the nature of the glass-to-liquid transition of metallic glasses. Mechanical spectroscopy analysis indicates that the instability is caused by atomic motions resonant with the dynamic ultrasonic-strain field, i.e., atomic jumps associated with the beta relaxation that is usually observed for low frequencies of the order of 1 Hz at temperatures far below T(g). Such atomic motions at temperatures lower than the so-called kinetic freezing temperature T(g) originate from relatively weakly bonded (and/or low-density) regions in a nanoscale inhomogeneous microstructure of glass, which can be straightforwardly inferred from a partially crystallized microstructure obtained by annealing of a Pd-based metallic glass just below T(g) under ultrasonic perturbation. According to this nanoscale inhomogeneity concept, we can reasonably understand an intriguing characteristic feature of less-stable metallic glasses (fabricated only by rapid melt quenching) that the crystallization precedes the glass transition upon standard heating but the glass transition is observable at extremely high rates. Namely, in such less-stable metallic glasses, atomic motions are considerably active at some local regions even below the kinetic freezing temperature. Thus, the glass-to-crystal transition of less-stable metallic glasses is, in part, explained with the present nanoscale inhomogeneity concept.     

Surface and bulk morphology of cold-crystallized poly(ethylene terephthalate)

The morphology/habit of crystals of cold-crystallized poly(ethylene terephthalate) (PET) has been evaluated using scanning and transmission electron microscopy and using atomic force microscopy. The combination of different preparation and analysis techniques allowed assessing the structure at the nanometer scale of films of PET at both the surface and the bulk. It is found that crystals formed on heating the amorphous glass to a temperature higher than the glass transition temperature are of lamellar shape in the bulk and almost isometric habit at the surface. This finding is explained by different rates of nucleation/crystallization in the bulk and at the surface, being supported by the observation of nanometer-scale surface heterogeneities after quenching PET to ambient temperature before crystallization was initiated by heating.     

SeTe8Sb1非晶合金的玻璃转变

The process of glass transition on the non-crystal SeTe_8Sb_1 has been researched using low temperature differential scanning calorimetry (DSC). The following conclusions are deduced: this process is a second-order phase transformation, the stronger endotherm displayed in it is imputed to hysteresis of the transition. The temperature of glass transition in heating process decrease with reduction of the cooling rate while forming glass. The activation energy of glass transition quickly increases from 174 kJ·mol. (at quenching rate ~10~4 K·min~(-1)) to 399 kJ·mol~(-1). (at cooling rate ~10 K·min~(-1)) So that the glass obtained at lower cooling rate is more stable than that obtained at higher rate.     

Rapid bonding of Pyrex glass microchips

A newly developed vacuum hot press system has been specially designed for the thermal bonding of glass substrates in the fabrication process of Pyrex glass microchemical chips. This system includes a vacuum chamber equipped with a high-pressure piston cylinder and carbon plate heaters. A temperature of up to 900 degrees C and a force of as much as 9800 N could be applied to the substrates in a vacuum atmosphere. The Pyrex substrates bonded with this system under different temperatures, pressures, and heating times were evaluated by tensile strength tests, by measurements of thickness, and by observations of the cross-sectional shapes of the microchannels. The optimal bonding conditions of the Pyrex glass substrates were 570 degrees C for 10 min under 4.7 N/mm(2) of applied pressure. Whereas more than 16 h is required for thermal bonding with a conventional furnace, the new system could complete the whole bonding processes within just 79 min, including heating and cooling periods. Such improvements should considerably enhance the production rate of Pyrex glass microchemical chips. Whereas flat and dust-free surfaces are required for conventional thermal bonding, especially without long and repeated heating periods, our hot press system could press a fine dust into glass substrates so that even the areas around the dust were bonded. Using this capability, we were able to successfully integrate Pt/Ti thin film electrodes into a Pyrex glass microchip.     

Calorimetric and dielectric studies of ferroelectric sodium nitrite confined in a nanoscale porous glass matrix

Heat-capacity measurements of the sodium nitrite confined in a nanoscale porous glass matrix show that the intermediate incommensurate phase, present in the bulk, has disappeared and that the first-order ferroelectric transition becomes suppressed and gradual. The ferroelectric transition temperature is shifted considerably to lower temperatures. Two noncritical dielectric modes were observed; however, the observed giant growth of the dielectric constant on heating through the transition temperature TC is shown to be mainly due to the electrode polarization effect.     

Tri-alpha-naphthylbenzene as a crystalline or glassy matrix for matrix-assisted laser desorption/ionization: a model system for the study of effects of dispersion of polymer samples at a molecular level

Tri-alpha-naphthylbenzene (TalphaNB) can exist as either a crystalline or glassy solid at ambient temperatures, making it a unique matrix in matrix-assisted laser desorption/ionization (MALDI) spectroscopy. Electrosprayed TalphaNB is crystalline and has a melting point of 180 +/- 2 degrees C, as measured by differential scanning calorimetry (DSC). A glass of TalphaNB is obtained upon heating above the crystalline melting point with a glass transition temperature of 68 +/- 2 degrees C having no remaining crystallinity. MALDI samples containing mass fraction 1% polystyrene (PS) are run in both the crystalline and amorphous states. In the crystalline state, there is a strong spectrum typical of PS, but upon melting and quenching to the glassy state, the MALDI signal disappears. If the transparent, amorphous sample is treated with 1-butanol, it becomes white, and the MALDI signal returns. DSC shows that the 1-butanol treatment leads to the return of some of the crystallinity. Small angle neutron scattering (SANS) shows that the crystalline state has large aggregations of PS while the amorphous state has molecularly dispersed PS molecules. MALDI gives strong signals only when there are large aggregations of polymer molecules, with individually dispersed molecules producing no signal.     

碳纳米管-Mg65Cu25Gd10非晶复合材料玻璃转变的动力学性质

制备了Mg65Cu25Gd10大块非晶合金及其碳纳米管（CNTs）复合材料，对两种材料进行了不同扫描速率下的差热扫描量热分析，研究了加入CNTs对材料玻璃转变和晶化动力学效应的影响。结果表明：加入CNTs后，复合材料的玻璃转变和晶化行为仍然具有动力学效应，但加入的CNTs减小了材料晶化行为对升温速率的依赖程度；同时，加入CNTs加大了材料发生玻璃转变时需要克服的能量势垒，增大了峰值温度时的晶化反应速率常数，从而降低了材料的玻璃形成能力（GFA）；对CNTs降低GFA的原因也进行了探讨。     

The role of interfacial chemistry and interactions in the dynamics of thermosetting polyurethane–multiwalled carbon nanotube composites at low filler contents

Acid-oxidized multiwalled carbon nanotubes (MWCNTs) were introduced into a polyurethane (PU) matrix at low filler levels (0.01–0.25 wt%) through either van der Waals or covalent interactions, and their glass transition dynamics using dynamic mechanical analysis and laser-interferometric creep rate spectroscopy was investigated. The nanocomposites reveal substantial impact on the PU glass transition dynamics, which depends on the nanotube content and type of interfacial interactions. The pronounced dynamic heterogeneity within the glass transition covering 200 °C range and the displacement of main PU relaxation maxima from around 0 to 80–140 °C were registered. The results are treated in the framework of chemical inhomogeneity, constrained dynamics effects, and different motional cooperativities. The peculiariaties of the glass transition dynamics in the composites are reflected in their dynamic and static mechanical properties, in particular a two- to threefold increase in modulus and tensile strength for the covalent interfacial interaction of MWCNTs with PU.     

Novel method to detect the volumetric glass --> liquid transition at high pressures: glycerol as a test case

We report a novel method of detecting the glass --> liquid transition at high pressures, which comprises measuring the relative volume change incurred upon heating glassy samples into the liquid state. We show data on glycerol in the pressure range 0.050-1.00 GPa to demonstrate the viability of the method. The reversible glass --> liquid transition is observed by means of a kink in the relative volume change on heating the sample isobarically, which is attributed to the glass --> liquid transition temperature Tg. This kink can only be observed in the second and subsequent heating cycles since it is superposed by a compaction in the first heating cycle. The isobaric thermal expansivity beta, which is closely related to the first derivative of this curve, shows the features expected for a glass --> liquid transition, including a sharp rise of beta(glass) in a narrow temperature interval to beta(viscous liquid) and an accompanying overshoot effect. Both Tg and the size of the overshoot effect vary in accordance with theory upon changing the ratio of cooling to heating rates. From the shape of this curve the onset, inflection, overshoot peak, and endpoint of the glass --> liquid transition can be extracted, which can be employed to calculate the reduced glass transition width as a measure for the fragility of the liquid. Comparison with literature data allows quantifying the accuracy of the liquid's thermal expansivity beta to be at least +/-10%, while the error in beta is significantly larger for the expansivity of the glassy state. The reproducibility of the glass --> liquid transition temperature Tg is better than +/-2 K. Our glycerol data confirms literature studies showing a nonlinear increase of Tg with increasing pressure (approximately 35 K/GPa on average), which is accompanied by an increase in fragility.     

Configurational specific heat of molecular liquids by modulated calorimetry

The specific heat of a liquid varies as its structure and molecular vibrational frequencies vary with the temperature. We report the magnitude of the structural or configurational part C(p,conf) for five molecular liquids by measuring their dynamic and the apparent specific heats, and find that the unrelaxed or vibrational specific heat, of the equilibrium liquid, is not greatly different from that of the nonequilibrium glass. Therefore, the vibrational part of the specific heat C(p,vib) does not change substantially when a glass becomes an ultraviscous liquid. This contradicts the inference that there is a large sigmoid-shape (discontinuous) increase in C(p,vib) as the structure of a glass kinetically unfreezes on heating above its T(g), and further that C(p,conf) is 20%-50% of the net C(p) change at the glass transition.     

Assessment of the parameters influencing the fiber characteristics of electrospun poly(ethyl methacrylate) membranes

This work evaluates the influence of electrospinning process parameters on the mean diameter and standard deviation of fiber diameters in electrospun poly(ethyl methacrylate) (PEMA). Processing conditions were selected using Taguchi’s statistical method. Oriented and unoriented electrospun mats with good mechanical properties were produced and demonstrated with tensile stress–strain diagrams. Differential scanning calorimetry (DSC) experiments showed that the polymer chains were forced into non-equilibrium conformations due to electrospinning. Enthalpic recovery during a heating scan is shown by an endothermic peak in the initial DSC heating scan that disappears in subsequent heating scans. An increase in the glass transition temperature with respect to PEMA films shows that the polymer is not equilibrated by heating above glass transition. Cell attachment was tested with MC3T3-E1 pre-osteoblastic cells cultured for short time periods on the electrospun mats. It is shown that the cells present less extended morphology with more diffuse perimetral focal adhesions than cells cultured on flat substrates. A tendency of cells to align in the direction of the substrate fibers in oriented electrospun membranes was also found.     

Preparation and characterization of Nd3+ doped sodium leadbismuthate glass

Nd(3+) doped unconventional sodium leadbismuthate glass is prepared through the melt quenching method. The amorphous nature of the glass is confirmed through the X-ray diffraction study. The differential thermal study was performed to identify the glass transition and approximate glass thermal stability measurements. To identify the local structure of the glass, Fourier transform infrared spectral analysis was also carried out. By performing the Judd-Ofelt theory, the theoretical and experimental oscillator strengths were calculated.     

Nonmonotonic temperature dependence of heat capacity through the glass transition within a kinetic model

The heat capacity of a supercooled liquid subjected to a temperature cycle through its glass transition is studied within a kinetic model. In this model, the beta process is assumed to be thermally activated and described by a two-level system. The alpha process is described as a beta relaxation mediated cooperative transition in a double well. The overshoot of the heat capacity during the heating scan is well reproduced and is shown to be directly related to delayed energy relaxation in the double well. In addition, the calculated scan rate dependencies of the glass transition temperature T(g) and the limiting fictive temperature T(f) (L) show qualitative agreement with the known results. Heterogeneity is found to significantly reduce the overshoot of heat capacity. Furthermore, the frequency dependent heat capacity has been calculated within the present framework and found to be rather similar to the experimentally observed behavior of supercooled liquids.     

Studies on new hybrid materials prepared by both Diels–Alder and Michael addition reactions

New polyurethane chemically crosslinked networks containing silica were synthesized by both Diels–Alder polymerization and Michael addition reaction. Structural characterization of the products was evidenced by proton nuclear magnetic resonance and attenuated total reflectance in conjunction with Fourier transform infrared spectroscopy techniques. Differential scanning calorimetry was used to demonstrate the thermally remendable character of the materials obtained through Diels–Alder polymerization. The influence of increasing silica content on the glass transition temperatures was studied. It was observed that the glass transition temperatures increased with increasing silica content. Absolute heat capacities and crosslinking densities were determined for the thermoreversible materials. A comparison between materials obtained through Diels–Alder process and Michael addition method was studied. A kinetic study was conducted via an isoconversional method. Morphological studies were conducted by atomic force microscopy technique.     

Transient dynamics of cold-rolled and subsequently thermally rejuvenated atactic-polystyrene using broadband dielectric spectroscopy

The effect of plastic deformation on the molecular dynamics of atactic polystyrene (a-PS) was studied by broadband dielectric relaxation spectroscopy (BDRS), Fourier-transform infrared spectroscopy (FTIR) and polarized-light microscopy. Sheets of a-PS have been subjected to cold rolling, that is, mechanical rejuvenation, followed by a quenching step and fast heating above its glass-transition temperature, resulting in thermal rejuvenation. Cold rolling revealed, in addition to the known α- and γ(I)-relaxations, four hitherto unknown relaxation processes (II, III, IV and V). Using the framework of craze formation and multiplicity of the glass transition (E. Donth, G. H. Michler, Colloid Polym. Sci. 1989, 267, 557–567), supported by an activation-enthalpy/entropy analysis (Starkweather, W. Howard, Macromolecules 1981, 14, 1277–1281), the following physical picture emerges: (a) processes I and II represent local conformation transitions γ referring to chains of two different degrees of stretching (T/G-ratio); and (ii) processes III and IV were identified as helix-inversion processes of T₂G₂ helices as reported earlier for syndiotactic-rich PS—an assignment supported by FTIR results. Finally, the relaxation V could be attributed to the onset of the fibrillar glass transition (within crazes), leading to stress release by collapse of the fibrils and hence dying out of process V. Polarized-light microscopy confirmed the creation of oriented structures and internal stresses upon cold rolling, and their removal upon thermal rejuvenation.     

Recent developments in thermal analysis of polymers: Calorimetry in the limit of slow and fast heating rates

This review focuses on new insights into the crystal melting transition and the amorphous glass transition of polymers that have been gained through recent advances in thermoanalytical methods. The specific heat capacity can now be studied under two extreme limits, that is, under quasi‐isothermal conditions (limit of zero heating rate) and, at the other end of the scale, under rapid heating conditions (heating rates on the order of thousands of degrees per second), made possible through nanocalorimetry. The reversible melting, and multiple reversible melting, of semicrystalline polymers is explored using quasi‐isothermal temperature modulated differential scanning calorimetry, TMDSC. The excess reversing heat capacity, above the baseline, measured under nearly isothermal conditions is attributed to locally reversible surface melting and crystallization processes that do not require molecular nucleation. Observations of double reversible melting endotherms in isotactic polystyrene suggest existence of two distinct populations of crystals, each showing locally reversible surface melting. The second subject of the review, nanocalorimetry, is utilized to study samples of small mass under conditions of very fast heating and cooling. The glass transition properties of thin amorphous polymer films are observed under adiabatic conditions. The glass transition temperature appears to be independent of film thickness, and is observed even in ultra‐thin films. Recrystallization and reorganization during rapid heating are studied by nanocalorimetry of semicrystalline polymers. The uppermost endotherm seen under normal DSC scanning of poly(ethylene terephthalate) is caused by reorganization, and vanishes under the rapid heating conditions (3000K/s) provided by nanocalorimetry. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 629–636, 2005     

Real-time monitoring of Yb vapor density using an extended cavity violet diode laser

Based on laser absorption spectroscopy (LAS), we developed a vapor density monitor for controlling the vaporization rate of Yb using a tunable diode laser. The laser source consisted of an extended cavity violet diode laser which has an emission wavelength of 398.8 nm coincident with the Yb absorption transition line, 6s(2) 1S(0)-6s6p 1P(1). The light emitted from the diode laser was transmitted across an atomic vapor column generated by heating the Yb metal, while the laser frequency was scanned across the atomic transition line. By comparing the amount of incident light to the amount of light transmitted after the light passed through the vapor column, the vapor density was determined using the Beer's law. From the experimental results, we demonstrated that the diode-laser-based LAS operated successfully for the real-time monitoring of the Yb vapor density.     

Non-isothermal crystallisation kinetics study on Se90−xIn10Sbx (x = 0, 1, 2, 4, 5) chalcogenide glasses

The non-isothermal crystallisation kinetics of Se_90?xIn₁₀Sb_x (x = 0, 1, 2, 4, 5) chalcogenide glasses prepared by a conventional melt quenching technique was studied using the differential scanning calorimetry (DSC) measurement at different heating rates 5, 7, 10 and 12 °C min^?1. The values of the glass transition temperature T _g and the crystallisation temperature T _c are found to be composition and heating rate dependent. The activation energy of glass transition E _g, Avrami index n, dimensionality of growth m and activation energy of crystallisation E _c have been determined from different models.     

择优掺杂的La-Ca-Na-Mn-O体系的庞磁电阻效应

利用溶胶-凝胶法制备出择优掺杂的名义组分为La_(1-x)(Ca_(1-y)Na_y)_xMnO_3(LCNM)的多晶样品.xRD测试表明样品均为钙钛矿结构.电输运测试表明所有样品均发生金属-绝缘体转变和顺磁一铁磁转变(转变温度分别用T_m和T_c表示),转变温度随Na含量的增加而增加,但是由于Ca,Na离子间价态的不同引起的磁不均匀性,同时含Ca和Na的样品电输运曲线在T_m温度以下出现肩型鼓包,同时表现出增宽的顺磁·铁磁转变过程,磁电阻测量发现所有样品均表现出庞磁电阻(CMR)效应,但是由于磁不均匀性,y约为0.25的样品表现出两个CMR峰.     

金属Cu液固转变及晶体生长的分子动力学模拟

采用分子动力学模拟研究了液态Ｃｕ在不同冷却速度下的凝固特点,模拟采用ＥＡＭ作用势,计算了不同温度,不同冷却速度下Ｃｕ的偶相关函数,结果表明ＥＡＭ作用势能很好地描述液态Ｃｕ的结构特征,当冷却速度较快时,液Ｃｕ形成非晶;当冷却速度较慢时,液Ｃｕ形成晶体,分析了不同冷却速度下体系的相变热力学及相变动力学过程,最后采用液固两层构型法,描述了Ｃｕ晶体的生长过程。