Cheng Tien

Measurement of heat capacity by fitting the whole temperature response of a heat-pulse calorimeter

A new method that fits the whole temperature response of a heat-pulse calorimeter for heat capacity is developed. Analyzing the thermal response of a heat-pulse calorimeter on a model that was used by the relaxation method, the authors derived some useful relations and further utilized the numeric method of the general linear least squares to determine the heat capacity of a sample. Absolute accuracy of the proposed method was verified by determining the heat capacity of a 0.249 76 g copper sample (purity 99.999%) on a self-designed and fully automated calorimetric system from 4.5 to 80 K. Comparing the result with the literature, the deviation in average was 1.2% from 4.5 to 20 K and 2.0% from 20 to 80 K. It is found that the proposed method is capable of measuring heat capacity regardless if the sample is adiabatically or nonadiabatically isolated. The size of the specimen is not critical for the application of the method and the temperature range of measurement can be expanded. It also deals with the τ...

Ferroelectricity in an Array of Electrically Coupled Confined Small Particles

The influence of the pore network geometry and interparticle long-distance electric coupling on the ferroelectric phase transition in small particles embedded into mesoporous matrices is considered. It was shown that the temperature of the ferroelectric phase transition in a system of electrically linked particles can be well different from that in isolated small particles. In particular, the model suggests an explanation for the weakening of size-effects on the ferroelectric phase transition in confined geometry.

Dielectric and NMR Studies of Nanoporous Matrices Loaded with Sodium Nitrite

NMR and dielectric studies have been performed on NaNO2 loaded in mesoporous matrices of MCM-41 and SBA-15 with pore sizes of 20, 37, and 52 Å. The spin-lattice relaxation rate and 23Na NMR line shape, as well as the complex impedance, were measured within a broad temperature interval including the ferroelectric phase transition in bulk NaNO2. Two different phases of sodium nitrite, the crystalline and melt phases, are shown to coexist under conditions of a restricted geometry. The crystalline phase undergoes a ferroelectric phase transition. The melt fraction increases with temperature. The existence of two phases accounts for all experimental data on NaNO2 under conditions of a restricted geometry.

Phenomenological model for the antiferroelectric phase transition in thin films and small particles

A phenomenological Landau model for the antiferroelectric phase transition in thin films and small cylindrical and spherical particles is developed. The boundary value problem was solved numerically for polarizations of polar sublattices and components of the susceptibility tensor. The spatial distribution of polarization and susceptibility and their temperature dependences were calculated. The enhancement of diagonal components of the susceptibility tensor compared to bulk was found. The size-dependent shift of the antiferroelectric phase transition temperature was obtained.

NMR studies of structure and ferroelectricity for Rochelle salt nanoparticles embedded in mesoporous sieves

NMR studies were carried out for Rochelle salt embedded in molecular sieves. 23 Na magic angle spinning (MAS) and multiple quantum (MQ) MAS NMR spectra revealed a complex structure of the confined crystalline material. The major part of particles within nanopores had a structure similar to that of bulk Rochelle salt. The 23 Na spin‐lattice relaxation times at various temperatures associated with this modification were also similar to those for bulk Rochelle salt and showed broad minima that corresponded to the ferroelectric and re-entrant phase transitions under nanoconfinement at temperatures just below the relevant transitions in bulk. This result suggests that the bulk-like modification within pores is ferroelectric in between. Fast spin relaxation in the rest of the confined material reflected high molecular mobility.

Effect of confined geometry on linear and nonlinear dielectric properties of triglycine sulfate near the phase transition

The temperature dependence of the linear permittivity and the third harmonic generation amplitude of nanocomposites representing nanoporous silica matrices (opal matrix and SBA-15) with triglycine sulfate embedded in pores has been studied in the region of the ferroelectric phase transition. A broadening of the phase transition and an increase its temperature in comparison with bulk triglycine sulfate have been revealed. The latter becomes more significant as the pore size decreases. It has been shown that the nonlinear dielectric properties of nanocomposites near the phase transition differ significantly from the properties of bulk triglycine sulfate.

Antiferromagnetism and mass-enhanced behavior in Ce2CuSi3

Temperature dependences of the specific heat, electrical resistivity and magnetic susceptibility of single-phased AlB2-type compound Ce2CuSi3 as well as its nonmagnetic counterpart La2CuSi3 are reported. La2CuSi3 is Pauli paramagnetic whereas Ce2CuSi3 shows complex magnetic properties. An antiferromagnetic ordering at 2.1 k is observed by low field (50 Oe) susceptibility measurement on Ce2CuSi3. Magnetic susceptibility measured under 2000 Oe, however, appears to be saturated below 2.1 K. Heat capacity measurement down to 4.5 K reveals an upturn in C(T) of Ce2CuSi3 which could be correlated with magnetic transition at 2.1 K. The electronic specific heat in Ce2CuSi3 is obtained, indicating an enhanced temperature-independent γ of 76mJK−2 mol−1 Ce. A negative contribution to the enhanced γ that could be due to spin fluctuation, is observed. The resistivity data of Ce2CuSi3 shows the typical behavior of a Kondo compound, which might be responsible for the enhanced effective mass.

Nuclear magnetic resonance, resistance and acoustic studies of the melting-freezing phase transition of gallium in Vycor glass

The melting-freezing phase transition of gallium confined within Vycor glass was studied by NMR, resistance and acoustic techniques. A single although broadened 71Ga NMR line corresponding to melted gallium was observed in contrast to lineshapes found until now for liquid gallium in porous matrices. A difference between results obtained using the three methods was explained by formation of various confined solid gallium modifications. A depression of the freezing and melting phase transition temperatures and a pronounced hysteresis in the melting-freezing processes were found and are discussed. Heterogeneous nucleation was suggested to explain the dependence of crystallization on temperatures of pre-warming. Irreversible melting was observed for the second gallium modification.

Phase transitions in K1−xNaxNO3 embedded into molecular sieves

Dielectric studies of molecular sieves MCM-41 filled with K(1-x)Na(x)NO(3) salts (x = 0, 0.05 and 0.10) were carried out and compared with results obtained for their bulk counterparts. The regular increase upon warming of the temperature of the reconstructive phase transition from phase II to phase I with decreasing pore sizes was observed for all x. A thermodynamic model for shifts of reconstructive phase transitions was suggested. Ferroelectric phase III was shown to occur upon cooling in bulk binary salts and for all x in confined geometry under the thermal conditions when phase III does not arise in bulk KNO(3). Ferroelectricity persisted below room temperature in the bulk binary salt with x = 0.10.

X-ray and resistance studies of the melting and freezing phase transitions for gallium in an opal

The melting and freezing processes and the structure of gallium in an opal were studied using x-ray powder diffraction within the temperature range 10 to 320 K. Four different modifications of solid confined gallium were found. Two of these modifications did not coincide with any known gallium structures; another two coincided with and disordered . The broadening of the total melting and freezing processes and reduction of the phase transition temperatures compared to bulk phase were obtained. The size of confined gallium crystallites corresponding to the different modifications was estimated for cooling and warming. The reproducible freezing was treated as a result of the steep temperature dependence of the nucleation rate in supercooled melts. Additional measurements of resistance for the opal filled with gallium revealed that it was sensitive mainly to the melting and freezing within the gallium modification coinciding with disordered . Melting-freezing hysteresis for this modification was found to depend on temperatures of pre-warming. It was suggested that the freezing of this modification is driven by the amount of frozen crystallites of the phase with the highest onset of freezing. The melting broadening and the lowering of the phase transition temperatures for the confined gallium modifications were discussed.