Signature of a type-a glass transition and intrinsic confinement effects in a binary glass-forming system

We study dynamically highly asymmetric binary mixtures comprised of small methyl tetrahydrofuran (MTHF) molecules and polystyrene. Combined use of dielectric spectroscopy, ^{2}H nuclear magnetic resonance, incoherent quasielastic neutron scattering, and depolarized dynamic light scattering allows us to selectively probe the dynamics of the components in a broad dynamic range. It turns out that the mixtures exhibit two glass transitions in a wide concentration range although being fully miscible on a macroscopic scale. In between both glass transition temperatures, the dynamics of the small molecules show strong confinement effects, e.g., a crossover from Vogel-Fulcher to Arrhenius behavior of the time constants. Moreover, the dynamical behavior of small molecules close to the slow matrix is consistent with mode coupling theory predictions for a type-A glass transition, which was expected from recent theoretical and simulation studies in comparable systems.     

Time and amplitude dependences of the damping decrement and shear modulus upon irreversible structural relaxation in a Zr-Cu-Ni-Al-Ti bulk metallic glass

The time dependences of the irreversible relaxation of the damping decrement and the shear modulus of a Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk metallic glass are investigated using an inverse torsion pendulum in the range from room temperature to ～650 K. The spectrum of activation energies of irreversible structural relaxation is evaluated from the results obtained. Analysis of the amplitude dependences of the damping decrement and the shear modulus allows the conclusion that the relaxation centers responsible for the amplitude dependence differ from those associated with the irreversible structural relaxation at temperatures below and in the vicinity of the glass transition point.     

The response of the dielectric susceptibility to an external electric field around the phase transition temperature in Li:KTaO3

Experimental results of measurements of dielectric susceptibility vs applied electric field at various temperatures on 3.5 and 8% Li: KTaO₃ are presented. It is found that there is an enhancement of the dielectric susceptibility as a function of increasing applied electric field at temperatures below ～ 90 K for the 8% Li crystal and at temperatures below ～ 60 K for the 3.5% Li crystal. The results are consistent with a slightly first-order phase transition.     

Kinetics study of a-Se80Te20−xPbx using non-isothermal crystallization

The kinetics of crystallization in Se₈₀Te_20−xPb_x (x=0, 2, 6 and 10) glasses is studied by non-isothermal method using differential scanning calorimetry (DSC). DSC is performed at different heating rates of 5, 10, 15 and 20 K/min. The values of glass transition and crystallization temperatures are found to be composition and heating-rate dependent. From the heating-rate dependence of the glass transition and crystallization temperatures, the activation energy of crystallization (ΔE_c) and order parameter (n) are calculated.     

Thermal expansion of polymers subjected to low-amplitude temperature cycling

The temperature dependence of the thermal expansion of polymers subjected to small harmonic temperature oscillations about certain base temperatures near the glass transition range is considered. The conformational component of the expansion as a function of temperature is calculated in terms of the conformational transition kinetics. The temperature dependences of the expansion and the expansion vs. oscillation phase shift are analyzed with allowance for the vibrationally anharmonic component. The thermal expansion of polyvinylacetate at base temperatures of 295–320 K, oscillation frequencies of 0.3 and 0.1 Hz, and an oscillation amplitude of 0.8 K are studied experimentally. The measured and calculated data for the expansion and expansion vs. temperature phase shift are shown to be in good agreement.     

A specific heat study of natural haematite around the Morin transition and the effects of entrapped water

The Morin transition in samples of natural haemitite has been examined by specific heat and Mössbauer measurements. Specific heat measurements on as-received samples reveal an anomaly at ～ 273.2 K and irregular behaviour at higher temperatures. However, heat treatment of samples demonstrated that these effects were due to water trapped inside the natural specimens.No anomaly due to the Morin transition was observed in the specific heat although the expected changes in the Mössbauer hyperfine parameters were observed for the different magnetic phases below and above the Morin transition. The results are consistent with a coexistence of the two phases and a spread of temperatures ～ 40 K over which the transitions take place throughout the material of purity 99.6 wt%.     

Glass Transition Temperature of Water： from Simulations of Diffusion and Excess Entropy

We report a computer simulation study of the glass transition of water with SP2 potential. The temperature dependences of the diffusion coefficient and the excess entropy on cooling process are calculated. It is found that both the diffusion coefficient and the excess entropy show a break point at 160K. Our results support the viewpoint that the glass transition temperature is 160K. According to the calculated viscosity, we obtain a fragility index of water to be 326, which is much larger than the value accepted before.     

不同频率声子参与的氟玻璃中Ho3+离子的多声子弛豫

测量了掺Ho3+离子的氟玻璃的吸收光谱,激发光谱、发射光谱,10K～660K不同温度下的激发态寿命及氟玻璃的拉曼光谱.根据Judd-ofelt理论计算了辐射跃迁几率,进而计算了不同温度下的无辐射联迁几率.利用Huang-Rhys多声子跃迁理论分析了无辐射跃迁几率与温度的关系.在分析中考虑到不同频率声子的参与,并考虑到黄昆因子,得到与实验很好的符合.由理论和实验的拟合中估计了黄昆因子的大小,讨论了黄昆因子在无辐射跃迁中的作用.     

1.5μm处CO2与CO高温线强的实验分析与理论计算

本文在采用乘积近似方法计算二氧化碳、一氧化碳分子总的配分函数(其中分子的振动配分函数采用谐振子近似, 转动配分函数采用非刚性转子模型, 并考虑了离心扭曲修正)的基础上, 利用所得配分函数和振动跃迁矩平方的实验值以及Herman-Wallis系数, 计算了1.5 μm 附近二氧化碳30012–00001跃迁带和一氧化碳3–0跃迁带在300–6000 K温度范围内部分温度下的吸收线强; 为验证计算方法和结果的准确性, 在基于可调谐二极管激光吸收光谱技术搭建的高温测量系统中, 对300–800 K温度范围内部分谱线线强进行了测量, 并把计算结果、测量结果及HITRAN数据库中对应数据进行了对比, 发现相对偏差小于3%, 证明了本方法的有效性, 同时计算及测量所得高温线强数据可对HITRAN数据库进行有效的校正和补充.     

Thermoplastic Elastomer Phase Changing Materials for Stiffness Modulation

Abstract

Four natural rubber-based phase changing materials (PCMs) were synthesized and evaluated with the goal of achieving controlled stiffness modulation at elevated temperatures (～100-120?°C). The phase changing was achieved through either the glass transition or melt transition of one of four phase changing fillers: polystyrene, poly(methyl methacrylate), low density polyethylene, and indium tin alloy. The PCM stiffness was analyzed using dynamic mechanical analysis and the results showed that all four PCMs exhibited two thermal transitions: a low temperature transition at ?58?°C due to the glass transition of the natural rubber matrix and a high temperature transition at ～100-120?°C due to the thermal transitions in the four filler materials. The degree of stiffness change of all PCMs at 100-120?°C was found to be strongly influenced by the type of the phase changing filler as well as the type of thermal transition (glass transition versus melt transition). The compatibilities of the phase changing materials were analyzed using the Flory-Huggins interaction parameter and it was shown that natural rubber and low density polyethylene had the best compatibility. All systems exhibited effective stiffness modulation with the change in temperature, enabling them to be used for applications requiring variable stiffness control.     

Study on High-Temperature Spectra of Asymptotic Asymmetric-Top Molecule O3

The line intensities of 001-000 transition of the asymptotic asymmetric-top O₃ molecule at several temperatures are calculated by directly calculating the partition functions and regarding the rotationless transition dipole moment squared as a constant. The calculated values of the total internal partition sums (TIPS) are consistent with the data of HITRAN database with -0.61% at 296 K. The calculated line intensity data at 500 K and 3000 K are also in excellent agreement with the data in HITRAN database with less than 0.659% and 5.458% at 500 K and 3000 K, which provide a strong support for the calculations of partition function and line intensity at high temperature. Then we extend the calculation to higher temperatures. The line intensities and simulated spectra of ν₃ band of the asymptotic asymmetric-top O₃ molecule at 4000 and 5000 K are reported. The results are of significance for the studying of the molecular high-temperature spectrum including experimental measurements and theoretical calculations.     

Photon echo experiments on 3–3′ diethylthiacarbocyanine in glass-forming liquids with ultrafast laser pulses

Summary  Photon echo experiments on diethylthiacarbocyanine iodide (DTCI) in dibuthylphthalate (DBP) and glycerol in the glass phase with femtosecond pulses are presented. The PE signal shows nonexponential decays even at low temperatures. This behavior is attributed to the many vibronic resonances of the chromophore/ glass system which can be simultaneously excited by the spectral bandwith of the laser pulses. We are able to extract from the PE decays the contribution of the homogeneous dephasing rate of the 0-0 transition of DTCI up to 20 K in dibutylphthalate and 40 K in glycerol. A beating structure in the echo decay for the system DTCI/DBP is also observed from 8 K up to 180 K (glass transition temperature for DBP). This feature is attributed to an intereference process involving excitation of a vibronic transition of DTCI characterized by a vibrational frequency of 300 cm^-1.     

Investigation of Pendant Group and Chain Segment Motions in P(MMA/BA) Copolymers by Thermally Stimulated Depolarization Current

Poly(methyl methacrylate/butyl acrylate) [P(MMA/BA)] copolymers (M_η ～2×10⁵) with different mass percentages of MMA were synthesized by the method of solution polymerization. Thermally stimulated depolarization current (TSDC) technique was used to investigate the effect of copolymerization on pendant group and chain segment motions. Three TSDC peaks were observed over the temperature range from 310 to 400 K. The highest temperature, ρ peak originates from the detrapping of trapped charge carriers. The lower temperature, α peak corresponds to the glass transition. The activation energy of the α relaxation decreases from 1.2 eV for PMMA to 0.98 eV for MMA(75)/BA(25). In the fitting process, another peak, β′, is separated on the low temperature side. The apparent energy barrier of the β′ for PMMA is 0.80 eV. The β′ relaxation is thought to correspond to the motion of pendant groups including intra‐ and inter‐molecular interactions. All three peaks move to lower temperatures with an increase in BA component, and the activation energy for the α and β′ relaxations also decreases with the increase of BA component in copolymers, indicating that the flexible side groups of BA have an effect of plasticization on the glass transition and motion of pendent groups. The temperatures of the α and β′ peaks of P(MMA/BA) copolymers follow the Fox equation. Fitting results gives the α peak at 238 K and β′ peak at 225 K for polybutyl acrylate (PBA).     

Sticking of carbon nanotube Y junction branches

The coalescence of branches in the Y junctions of single-wall carbon nanotubes (10 nm long) is predicted to occur when the branches approach each other under the action of a load (～10 nN) applied to their ends. A transition to the new state with parallel branches bound by molecular interactions was simulated and the energy characteristics were calculated by the molecular dynamics method. The Y junctions with parallel branches are stable at temperatures up to 2000 K. It is established that there is a threshold distance between the branch ends, below which the branches exhibit spontaneous sticking under the action of molecular attraction forces. If the branches are unloaded before this threshold distance is reached, they oscillate (acting as a nanodimensional “tuning fork”) at a frequency of ～100 GHz.     

Initial nucleation stages and properties of CuCl nanoparticles in glasses

The time variation and temperature dependence of a CuCl phase nucleation in a glass was studied by exciton spectroscopy. The phase formation kinetics at three temperatures was measured. A time delay in attaining a stationary rate of the new phase growth was observed at all temperatures, in agreement with the Zeldovich theory. The kinetic parameters of the CuCl phase formation were determined in the initial stage, when the critical nuclei possessing a zero surface energy (and an effective radius below 1.3 nm) appear in the glass matrix. The first-order phase transition in the new phase is 200 K below the melting temperature of CuCl single crystals. The temperature dependence of the CuCl phase nucleation rate reveals the second and third stages of the new phase formation. The activation energies for diffusion of the CuCl phase components in the glass matrix are determined.     

金红石→萤石TiO2相变的第一性原理计算

利用第一性原理平面波密度泛函理论并结合准谐德拜模型,对TiO₂从金红石结构到萤石结构的相变进行理论研究.依据焓相等原理,得到TiO₂从金红石结构到萤石结构的相变压强为45.32 GPa,并通过吉布斯能的计算发现相变压强随温度的增加而增加,运用拟合的方法估计实验条件下的相变压强(温度为1900-2100K时,P_t=47.604~47.756 GPa).运用等吉布斯能原理,得到零压下从金红石到萤石结构TiO₂的相变温度为2029 K.计算结果与实验及其他理论值相符,并通过准谐德拜模型成功获得相变前后的结构和热力学性质,如相对体积、热膨胀系数、热容和德拜温度等.     

Electron spin-lattice relaxation of nitroxyl radicals in temperature ranges that span glassy solutions to low-viscosity liquids

Electron spin-lattice relaxation rates, 1/T1, at X-band of nitroxyl radicals (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, 4-oxo-2,2,6,6-tetramethylpiperidin-1-oxyl, 3-carbamoyl-2,2,5,5-tetramethylpyrrolidin-1-oxyl and 3-carbamoyl-2,2,5,5-tetramethylpyrrolin-1-oxyl) in glass-forming solvents (decalin, glycerol, 3-methylpentane, o-terphenyl, 1-propanol, sorbitol, sucrose octaacetate, and 1:1 water:glycerol) at temperatures between 100 and 300K were measured by long-pulse saturation recovery to investigate the relaxation processes in slow-to-fast tumbling regimes. A subset of samples was also studied at lower temperatures or at Q-band. Tumbling correlation times were calculated from continuous wave lineshapes. Temperature dependence and isotope substitution (2H and 15N) were used to distinguish the contributions of various processes. Below about 100K relaxation is dominated by the Raman process. At higher temperatures, but below the glass transition temperature, a local mode process makes significant contributions. Above the glass transition temperature, increased rates of molecular tumbling modulate nuclear hyperfine and g anisotropy. The contribution from spin rotation is very small. Relaxation rates at X-band and Q-band are similar. The dependence of 1/T1 on tumbling correlation times fits better with the Cole-Davidson spectral density function than with the Bloembergen-Purcell-Pound model.     

The lithium ion conductor β-spodumene: an orientational glass

Polycrystals of β-spodumene were investigated using impadence spectroscopy for frequencies 0.1 Hz < ν < 235 GHz and for temperatures 5 K < T < 500 K. At high temperatures the Li ions are mobile and lead to dc and ac conductivity phenomena. These are analyzed in terms of the universal dielectric response and using the modulus formalism. The results obtained by both procedures are critically compared. At low temperatures or very high frequencies dielectric loss due to a dipolar freezing process is observed which bears a close resemblance to an orientational glass transition. To further confirm the glassy character of β--spodumene, we report measurements of the low temperature specific heat (3 K < T < 50 K) which provide evidence for the existence of excess contributions that are usually observed in amorphous materials.     

Evidence for a phase transition in silicate melt at extreme pressure and temperature conditions

Laser-driven shock compression experiments reveal the presence of a phase transition in MgSiO(3) over the pressure-temperature range 300-400 GPa and 10?000-16?000 K, with a positive Clapeyron slope and a volume change of ～6.3 (±2.0) percent. The observations are most readily interpreted as an abrupt liquid-liquid transition in a silicate composition representative of terrestrial planetary mantles, implying potentially significant consequences for the thermal-chemical evolution of extrasolar planetary interiors. In addition, the present results extend the Hugoniot equation of state of MgSiO(3) single crystal and glass to 950 GPa.     

Langevin description of Markov master equations II: Noise correlations

We present experimental results of dielectric and birefringence measurements on 3.5% and 8% Li∶KTaO₃ as a function of temperature. These results indicate that the 8% Li∶KTaO₃ sample undergoes a ferroelectric phase transition at 97 K. The 3.5% sample seems to undergo a distortive transition at ～75 K. Raman experiments show that the splitting of the TO₁ mode of the 8% Li sample extrapolates to zero at ～97 K supporting the conclusion that this sample undergoes a phase transition at this temperature. Raman spectra of both samples at temperatures above the phase transition or freezing temperatures exhibit in addition to second order Raman features first order Raman lines. These lines have the properties of fluctuation induced first order Raman scattering. Measurement of the integrated intensity of these lines is used to obtain the temperature dependence of the fluctuation intensity. The results are compared to the Halperin and Varma theory. The data obtained for the 3.5% Li sample partialy fits the theory. However the results for the 8% Li sample do not fit the theory. The misfit is attributed to the overlap of defect zones.