Universality of sol--gel phase transition of κ-carrageenan in various salts: a steady state fluorescence study

Thermal phase transitions of κ-carrageenan in NaCl, KCl and CaCl₂ solutions were studied using steady state fluorescence (SSF) technique. Pyranine was introduced as a fluorescence probe for studying sol--gel phase transitions. Scattered light, I _sc and fluorescence intensity, I was monitored against temperature to determine the sol--gel (T _sg) phase transition temperatures. It was observed that T _sg values are strongly correlated to NaCl, KCl and CaCl₂ contents. The weight average degree of polymerization, DP _w and gel fraction G, exponents (γ and β) were measured and found to be in accord with the classical Flory--Stockmayer model, i.e., γ and β were found to be close to 1.0, independent of salt content.     

The scaling of the current voltage characteristics of the YBa2Cu3O7:Y2BaCuO5 composites

We have investigated features in the current voltage (I—V) characteristics of YBa₂Cu₃O₇:Y₂BaCuO₅ composite sinters which indicate the occurrence of a second-order phase transition. For each sinter composition similar families of characteristics are found for a wide range of magnetic fields. The relation between V and I is seen to be of the form V = f(I, ζ(B, T)) where a single parameter ξ contains all of the field and temperature dependence. The locus of points in the B, T plane for which ζ(B, T) is a constant have been examined, and a low field and a high field régime have been identified. Samples of different composition show families of I—V characteristics which scale onto one another in a simple way, hence we are led to suppose the V = f(I, ζ(B, T, p)) where p is the dilution fraction. Data taken at temperatures below the phase transition agrees well with a simple scaling picture of dissipation close to a second order phase transition, with exponents which are independent of magnetic field and composition. Above the transition, however, the behaviour is more complex.     

Electronic phase separation in TmBa2Cu4O8

The NQR spectra of Cu(2) in the superconductor TmBa₂Cu₄O₈ are studied at temperatures from 300 to 4.2 K. In analyzing the spectra it is assumed that the NQR line of each isotope contains two Gaussian components — narrow (n) and broad (b). It is discovered that the NQR frequencies have a minimum at the temperature T*=150 K. The frequencies of the components of the spectrum are close at temperatures from T* to 4.2 K and differ substantially at temperatures T>T*. Both components are broadened as the temperature decreases, but this broadening occurs especially rapidly at temperatures T<T*. The relative intensity of the narrow component I _n/(I _n+I _b) equals 1/6 for T=225−160 K, increases abruptly at T=T*, and remains constant (1/3) at temperatures T from 125 to 4.2 K. Analysis of the experimental data showed that the anomalous temperature dependences of the Cu(2) NQR spectra could be due to electronic phase separation (stratification) in the CuO₂ planes at temperatures T⩽T*. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 214–219 (10 February 1996)     

Critical currents in polycrystalline thin films of high-T c superconductors

Summary It is shown that the behaviour of the temperature dependence of the critical current in polycrystalline thin films of high-T _c superconductors depends crucially on the assumption made concerning the nature of the intergranular material. The usual assumption of a superconductor-insulator-superconductor (=SIS) ?sandwich? between each grain leads to a crossover fromI _c∼(1−T/T _c) toI _c∼(1−T/T _c)^3/2, for temperatures nearT _c (whereI _c is the critical current,T the absolute temperature, andT _c the superconducting transition temperature). Instead, for a superconductor-normal metal-superconductor (=SNS) sandwich the dependenceI _c∼(1−T/T _c)² is found for all temperatures. Consideration is given to the effect of self-magnetic field on the analysis. The comparison between expressions for continuous and granular systems is extended. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

Low-temperature properties of CePd2In

We report measurements of the specific heatC _p(T), electrical resistivity ϱ(T) and magnetic susceptibility ξ(T) of hexagonal CePd₂In, at low temperatures. Anomalies inC _p(T), χ(T) and ϱ(T) atT=1.23 K, indicate a phase transition, most likely to an antiferromagnetically-ordered phase. The electronic entropy reachesR ln2 per mole Ce at 9.2K, suggesting that the phase transition involves a doublet state. The ordered phase coexists with moderately correlated itinerant electrons.     

Thermal diffusion near glass transition in Ge-Se glasses measured by photoacoustics

The glass transition in Ge _x Se_1−x ) (0·1 ⩽x ⩽ 0·25) glasses has been investigated using the photoacoustic (PA) technique. It is found that the PA amplitude and phase undergo anomalous changes at the glass transition temperatureT _g. The amplitude has critical minimum and phase has maximum values atT _g. The variation of the thermal diffusivity, determined by measuring the frequency dependence of the PA amplitude and phase, with temperature shows sharp decrease near the glass transition temperature. The temperature dependence of the optical energy gap also has been measured and it shows a decrease with temperature for all compositions, the rate of decrease being higher for temperatures greater thanT _g.     

The effect of UV irradiation on temperature dependence of photoluminescence and photoacoustic response in porous silicon

The influence of UV irradiation on temperature dependence of the integral intensity of luminescence I _lum(T) in porous silicon was studied. It was found that, if luminescence decays with temperature nonmonotonically, the peak of I _lum(T) shifts toward higher temperatures as the exciting radiation density increases. Under UV irradiation, the function I _lum(T) becomes monotonically decreasing. If the function I _lum(T) is initially monotonically decreasing, UV preirradiation changes the emission spectrum and accelerates temperature quenching of the red-orange emission band. The variation of the amplitude of the pulsed photoacoustic response with UV irradiation dose was studied. The dependence found is explained by the removal of foreign inclusions from the developed surface of porous silicon. An energy level diagram that makes it possible to explain the behavior of I _lum(T) is suggested. It is noted that the shape of the function I _lum(T) can be used as a test whereby the contributions from dissimilar oscillators to the red-orange emission band are estimated.     

Fractal Dimensions of κ-Carrageenan Gels during Gelation and Swelling

A photon scattering technique for research on the sol-gel and gel-sol transitions in κ-carrageenan-water systems with various carrageenan contents (CC) was utilized for characterizing the fractal dimensions during gelation and swelling. It was observed that the scattered photon intensity, I_sc, increased at all temperatures with an increase in the CC when I_sc was monitored against temperature. Additionally, the sol-gel transition temperatures were found to be much lower than the gel-sol transition temperatures, causing hysteresis of the phase transition loops. I_sc increased with an increase in CC at all test temperatures, which is attributed to the formation of a fractal-like carrageenan gel. After drying, the gels were used in swelling experiments where the gels were immersed in water at room temperature, reswelling to the original structure. It was observed that I_sc from the carrageenan gels increased as the CC was increased. The fractal dimension, d, during gelation was found to increase as the gelation temperature was increased. On the other hand, the d values during swelling decreased as the swelling time was increased.     

Ferroelectric phase transitions of the 0.32PIN-0.345PMN-0.335PT single crystals studied by temperature-dependent Raman spectroscopy,dielectric and ferroelectric performance

The ferroelectric phase transition characteristics of the 0.32Pb(In_1/2Nb_1/2)O₃-0.345Pb(Mg_1/3Nb_2/3)O₃-0.335PbTiO₃ (0.32PIN-0.345PMN-0.335PT) single crystals were studied by the temperature-dependent Raman spectroscopy and some electrical properties. Ferroelectric monoclinic phase was confirmed at room temperature by the numbers of the Raman modes. Successive ferroelectric phase transitions, i.e. ferroelectric monoclinic phase to ferroelectric tetragonal phase transition (FE_M-FE_T) and ferroelectric tetragonal phase to paraelectric cubic phase transition (FE_T-P_C), are evidenced by the anomalies of Raman modes line width, peaks intensity and their ratios around T_M-T and T_C/T_m temperatures. The temperature dependent permittivity derivative ξ = d?/dT not only provides further evidence of the successive ferroelectric phase transitions, but also demonstrates the second-order transition characteristic of the FE_M-FE_T phase transition and the first-order transition feature of the FE_T-P_C phase transition. The FE_M-FE_T phase transition is also confirmed by the abnormal narrowing of the P-E loops, decrease of the P_r and E_c values, and extremums of the pyroelectric performance.     

Molecular Weight Dependence of Primary Nucleation Rate of Poly(Ethylene Succinate)

The molecular weight (MW) dependence of the primary nucleation rate (I) was studied for poly(ethylene succinate). The primary nucleation rate was measured in a wide range of temperatures. It has a bell shape with a maximum nucleation rate (I _max ) that showed a remarkable MW dependence. I _max decreased first with MW to about 3000 and then increased. The MW of about 3000 was attributed to the transition from intermolecular nucleation to intramolecular nucleation. The MW dependence of I _max was expressed as I _max ∝MW^α. The value of α was negative unity for intermolecular nucleation and positive unity for intramolecular nucleation.     

Thermopower of beech wood biocarbon

This paper reports on measurements of the thermopower S of high-porosity samples of beech wood biocarbon with micron-sized sap pores aligned with the tree growth direction. The measurements have been performed in the temperature range 5–300 K. The samples have been fabricated by pyrolysis of beech wood in an argon flow at different carbonization temperatures (T _carb). The thermopower S has been measured both along and across the sap pores, thus offering a possibility of assessing its anisotropy. The curves S(T _carb) have revealed a noticeable increase of S for T _carb < 1000°C for all the measurement temperatures. This finding fits to the published data obtained for other physical parameters, including the electrical conductivity of these biocarbons, which suggests that for T _carb ∼ 1000°C they undergo a phase transition of the insulator-(at T _carb < 1000°C)-metal-(at T _carb > 1000°C) type. The existence of this transition is attested also by the character of the temperature dependences S(T) of beech wood biocarbon samples prepared at T _carb above and below 1000°C.     

Magnetic field and temperature-induced first-order transition in Gd5(Si1.5Ge2.5): a study of the electrical resistance behavior

Magnetic field (0–4 T) and temperature dependencies (4.2–320 K) of the electrical resistance of Gd₅(Si_1.5Ge_2.5), which undergoes a reversible first-order ferromagnetic↔paramagnetic phase transition, have been measured. The electrical resistance of Gd₅(Si_1.5Ge_2.5) indicates that the magnetic phase transition can be induced by both temperature and magnetic field. The temperature dependence of the electrical resistance, R(T), for heating at low temperatures in the zero magnetic field has the usual metallic character, but at a critical temperature of T_cr=216 K the resistance shows a 20% negative discontinuity due to the transition from the low-temperature high-resistance state to the high-temperature low-resistance state. The R(T) dependence for cooling shows a similar but positive 25% discontinuity at 198 K. The isothermal magnetic field dependence of the electrical resistance from 212T224 K indicates the presence of temperature-dependent critical magnetic fields which can reversibly transform the paramagnetic phase into the ferromagnetic phase and vice versa. The critical magnetic fields diagram determined from the isothermal magnetic field dependencies of the electrical resistance of Gd₅(Si_1.5Ge_2.5) shows that the FM↔PM transition in zero magnetic field on cooling and heating occurs at 206 and 213 K, respectively. The full isothermal magnetic filed hysteresis for the FM↔PM transition is 2 T, and the isofield temperature gap between critical magnetic fields is 7 K.     

Er3+/Yb3+共掺碲硼硅酸盐玻璃的光谱性质和热稳定性研究

制备了系列Er³⁺/Yb³⁺共掺碲硼硅酸盐玻璃样品(85-x)TeO₂-15B₂O₃-xSiO₂ (TBS x=0,5,10,15,20 mol%).测试和分析了样品的吸收光谱、荧光光谱、能级寿命、红外透射光谱及差热特性.并通过对Er³⁺离子⁴I_13/2→⁴I_15/2跃迁发射谱线的高斯拟合，设计了一个简单的四能级结构估算了Er³⁺离子⁴I_13/2和⁴I_15/2能级在碲硼硅酸盐中的Stark分裂情况.研究表明SiO₂的引入能有效地改善玻璃的热稳定性和光谱性能，玻璃析晶温度T_x与玻璃转变温度T_g之差(ΔT=T_x-T_g)可达178℃，说明碲硼硅酸盐是一种适合于光纤拉制的玻璃基质材料.比较了不同基质玻璃中Er³⁺离子的荧光半高宽和受激发射截面，结果表明TBS玻璃系统具有较好的带宽性能，是一种优良的宽带光纤放大器候选基质材料. 关键词： 碲硼硅酸盐 热稳定性 高斯拟合 -基')" href="#">OH^-基     

Dielectric and polar order behaviors of BaTiO<Subscript>3</Subscript>-Bi(Mg<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramics

New perovskite solid solution ceramics of (1−x)BaTiO₃-xBi(Mg_1/2Ti_1/2)O₃ ((1−x)BT-xBMT, x≤0.09) were synthesized by the solid-state reaction technique. X-ray diffraction studies have revealed a stable single perovskite structure for all samples. Dielectric measurements were carried out at different frequencies and temperatures. The polarization evolutions with temperatures were measured to investigate the ferroelectric properties. All the compositions show features of ferroelectrics with diffuse phase transition, though the temperature T _m of their dielectric constant maximum ε _m is frequency dependent. The dielectric constant peak ε(T) of (1−x)BT-xBMT ceramics become broad with increasing BMT content. During the temperature range of ε(T) peak summit, (1−x)BT-xBMT ceramics present quasi-linear dielectric phenomenon under high electric field with very high dielectric constant.     

Film formation from TiO2-polystyrene latex composite: a fluorescence study

This work reports the use of the steady state fluorescence (SSF) technique for studying film formation from TiO₂ covered polystyrene (PS) latex particles. The composite films were prepared from pyrene (P)-labeled PS particles by covering them with TiO₂ at room temperature and then annealed at elevated temperatures in 10 min time interval above glass transition (T _g) temperature of polystyrene. Five different composite films were studied in various TiO₂ layer contents. Fluorescence intensities I _P from P were measured after each annealing step to monitor the stages of film formation. Films showed considerable increase in I _P above the certain onset temperature called minimum film forming temperature, T ₀. Void closure and interdiffusion stages were modeled and related activation energies were determined and found to be 23.12 and 92.80 kJ mol^?1, respectively.     

DSC study of superionic phase transition in (NH4)4H2(SeO4)3 single crystals

DSC and complex impedance studies of the protonic conductor (NH₄)₄H₂(SeO₄)₃, which undergoes a superionic phase transition of first order at T_s = 378 K show that the activation energy of ionic conductivity d(lg σ)/dt and the ordering enthalpy ΔC_p of the crystal are proportional: d(lg σ)/dT = XΔC_p/RT_s + const, as found for MAg₄I₅ crystals undergoing a second-order superionic phase transition. Thus the short-range order environment of the species involved in fast-ion transport plays the main role in the superionic phase transition. This is also supported by the value of the entropy change at T_s, ΔS = 43 J/mole·K. A new metastable phase was found to be induced on heating the (NH₄)₄H₂(SeO₄)₃ crystal above T_s.     

Percolation approach to film formation from surfactant-free polystyrene particles

In this study, a film formation process from surfactant-free polystyrene (PS) latex particles is reported. Steady state fluorescence (SSF) and photon transmission (UVV) techniques were used to study the evolution of film formation. The latex films were prepared from pyrene (P)-labeled PS particles at room temperature and annealed at time intervals of 2.5?min above the glass transition temperature (T _g) of PS. During the annealing processes, the transparency of the film changed considerably. Fluorescence intensity (I _0P) from P was measured after each annealing step to monitor the stages of film formation. Evolution of transparency of latex films were monitored by using photon transmission intensity, I _tr. A drastic increase in I _tr and I _0P above the critical annealing times, t _r and t _c were attributed, respectively to percolation behavior of PS material from one side to the other side of the latex film. Critical exponents, β of percolation clusters were measured and found to be around 0.35 and 0.25 for I _tr and I _0P measurements, respectively.     

Co(S1-xSex)2系统中的铁磁量子相变

针对Co(S_1-xSe_x)₂系统在x=0.11附近发生的铁磁金属到顺磁金属相变,制备了一系列不同Se替代浓度的多晶样品.通过对其结构和电阻率-温度ρ(T)关系的系统观测,结果发现,样品铁磁相变温度T_C随着Se替代浓度x值的增加,以(1-x)^1/2关系单调下降,其二级铁磁相变转变为一级相变 关键词： 量子相变 自旋量子涨落 1-xSe_x)₂')" href="#">Co(S_1-xSe_x)₂     

Raman measurements in the intermediate phase of [N(CH3)4]2ZnI4

Raman spectra and deviation angle of the indicatrix have been measured in the intermediate phase, between the two transition temperatures T _c1 = 255 K and T _c2 = 217 K. The results are consistent with the existence, in this phase, of large domains, piled up along the c-axis (Pnma notation).     

Calorimetric study of phase transitions in nanocomposites of quantum dots and a liquid crystal

The complex specific heat is measured over a wide temperature range for the liquid crystal (LC) 4-cyano-4-octylbiphenyl (8CB) and cadmium sulfate quantum dots (QDs) composites as a function of QD concentration. The thermal scans were performed under near-equilibrium conditions for all samples having QDs weight percent (φ_w) from 0 to 3wt% over a wide range of temperature well above and below the two transitions in pure 8CB. Isotropic (I) to nematic (N) and nematic to smectic-A (SmA) phase transitions evolve in character and their transition temperatures offset by (～2.3 to 2.6 K) lower for all composite samples as compared to that in pure 8CB. The enthalpy change associated with I–N phase transitions shows slightly different behavior on heating and cooling and it also shows crossover behavior at lower and higher QD content. The enthalpy change associated with N–SmA phase transitions is independent of QD loading and thermal treatment. Given the homogeneous and random distribution of QD in these nanocomposites, we interpret that these results as arising that the nematic phase imposes self-assembly on QDs to form one-dimensional arrays leading to QDs and induces net local disordering effect in LC media.