Ac Calorimetry Research Articles

Phase separation in nematic microemulsions probed by one-dimensional spectroscopic deuteron magnetic resonance microimaging

We present a study of a phase-transition-driven separation in microemulsions of nanosized lyotropic inverse micelles and thermotropic liquid crystal pentylcyanobiphenyl (5CB) with 5%, 8%, and 15% micelle concentration. Using deuteron nuclear magnetic resonance (DNMR) microimaging in combination with conventional microscopy as well as ac calorimetry, we demonstrate a phase separation scenario in which micelles are expelled from the nematic phase during the I-N conversion. Due to a difference in density the micelle-rich isotropiclike phase spatially separates from the micelle-free nematic phase. A relatively sharp interface, formed between the two phases, can be controllably shifted by temperature-induced conversion between the phases. Once expelled, micelles do not remix into the nematic phase, whereas in the isotropic state their remixing takes place over several days. Temperature dependence of the linewidth of isotropic spectral component has been analyzed in terms of molecular reorientations mediated by translational displacements, assuming isotropically distributed directors of nanosized nematic domains. With our results, the existence of the proposed transparent nematic state cannot be completely ruled out. However, if present, the nematic order in such a phase must be extremely low.

Heat capacity and latent heat measurements of CoMnSi using a microcalorimeter

A new method of utilizing a commercial silicon nitride membrane calorimeter to measure the latent heat at a first order phase transition is presented. The method is a direct measurement of the thermoelectric voltage jump induced by the latent heat, in a thermally isolated system ideally suited for single crystal and small microgram samples. We show that when combined with the ac calorimetry technique previously developed, the resultant thermal measurement capabilities are extremely powerful. We demonstrate the applicability of the combined method with measurements on a 100 microm size fragment of CoMnSi exhibiting a sizable magnetocaloric effect near room temperature, and obtain good agreement with previously reported values on bulk samples.

Properties of (CH3)2NH2H2PO4 crystal near the ferroelectric phase transition

The results of dielectric, thermal (AC calorimetry) and dilatometric measurements on DMAPcrystal in the ferroelectric phase transition region are presented. The experimental data arediscussed in the framework of the Landau theory, and expansion coefficients of the Landau potentialA0,B,C are estimated. A temperature dependence of the effective nonlinearity coefficientB has been revealed. Thepressure coefficient dTc/dp was obtained. The phase transition can be classified as a continuous oneand rather far from the tricritical point. The specific heat data show aλ-type anomaly and the value of the excess entropy is smaller than in typical order–disorderphase transitions.

Temperature-pressure phase diagram ofURu2Si2from resistivity measurements and ac calorimetry: Hidden order and Fermi-surface nesting

By performing combined resistivity and calorimetric experiments under pressure, we have determined a precise temperature-pressure $(T,P)$ phase diagram of the heavy fermion compound $\mathrm{U}{\mathrm{Ru}}_{2}{\mathrm{Si}}_{2}$. It will be compared with previous diagrams determined by elastic neutron diffraction and strain gauge techniques. At first glance, the low-pressure ordered phase referred to as hidden order is dominated by Fermi-surface nesting, which has strong consequences on the localized spin dynamics. The high-pressure phase is dominated by large moment antiferromagnetism (LMAF) coexisting with at least dynamical nesting needed to restore on cooling a local moment behavior. ac calorimetry confirms unambiguously that bulk superconductivity does not coexist with LMAF. $\mathrm{U}{\mathrm{Ru}}_{2}{\mathrm{Si}}_{2}$ is one of the most spectacular examples of the dual itinerant and local character of uranium-based heavy fermion compounds.

Host-Guest Interactions in Single-Ion Lithium Polymer Electrolyte

The complex interplay between the ionic conductivity and structure of the polymer electrolyte, induced by the calix[6]pyrrole anion receptor, has been investigated by different experimental methods, including ac impedance, calorimetry, X-ray diffraction, and Fourier-transform infrared spectroscopy. It was found that calix[6]pyrrole, even at low concentrations, can form stable bidentate complexes with triflate anions, thus making cation transport dominating, which results in close to unity. It should be noted that the incorporation of an anion trap does not suppress the bulk ionic conductivity of polymer electrolytes at above 60°C. We attribute this effect to the dissociation of ion aggregates and structural changes imposed by the additive. Stable solid electrolyte interface resistance was achieved in the polymer electrolytes containing calix[6]pyrrole.

Temperature dependence of thermal property for lead nitrate crystal

Thermal property was measured in a lead nitrate crystal, Pb(NO 3) 2, at temperatures from 90 to 340 K by use of ac calorimetry technique. The heat capacity derived from the measurements showed temperature dependence with thermal hysteresis, in the temperature region from 240 to 300 K. The anomaly of the heat capacity was found in the vicinity of 275.22 K. The broad temperature variation in the heat capacity was observed in the region from 235 to 260 K.

Calorimetric measurements of the light-induced metastable high-spin state on the [Fe(PM-BiA) 2(NCS) 2] complex

Heat capacity measurements have been obtained on a pellet sample of [Fe(PM-BiA) 2(NCS) 2] formed by applying uniaxial pressure on the powder compound. They show a broad heat capacity peak near 170 K corresponding to a thermal spin crossover transition with enthalpy and entropy values around 4 kJ mol −1 and 24 J mol −1 K −1, respectively, and a hysteresis of a few kelvins. Magnetic data of the pellet sample show a gradual and incomplete thermal transition. Light irradiation ( λ = 750–850 nm) at 5 K promotes the high-spin (HS) state and photomagnetic measurements give T(LIESST) = 68 ± 1 K. The heat capacity of this excited state has been measured and the relaxation from the HS to the LS state has been followed with AC calorimetry. An extended anomaly is observed centred at 66.5 K. This is the first calorimetric report of a sample on a LIESST state.

Thermodynamic Studies on Non Centrosymmetric Superconductors by AC Calorimetry under High Pressures

We investigated the non centrosymmetric superconductors CePt 3 Si and UIr by the ac heat capacity measurement under pressures. We determined the pressure phase diagrams of these compounds. In CePt 3 Si, the Néel temperature T N = 2.2 K decreases with increasing pressure and becomes zero at the critical pressure P AF ≃0.6 GPa. On the other hand, the superconducting phase exists in a wider pressure region from ambient pressure to P AF ≃1.5 GPa. The phase diagram of CePt 3 Si is very unique and has never been reported before for other heavy fermion superconductors. In UIr, the heat capacity shows an anomaly at the Curie temperature T C1 = 46 K at ambient pressure, and the heat capacity anomaly shifts to lower temperatures with increasing pressure. The present pressure dependence of T C1 was consistent with the previous studies by the resistivity and magnetization measurements. Previous ac magnetic susceptibility and resistivity measurements suggested the existence of three ferromagnetic phases, FM1-3. C ac shows a bending structure at 1.98, 2.21, and 2.40 GPa. The temperatures where these anomalies are observed are close to the phase boundary of the FM3 phase.

Competition of Magnetic Order and Superconductivity in CeRhIn5

We report on ac calorimetry of CeRhIn 5 at low temperature under high pressure and high magnetic field. In zero field the antiferromagnetic order vanishes near p c \star = 2 GPa presumably due to a first order transition. The application of a magnetic field H ∥( a , b ) plane above p c \star induces a second transition, which suggests the coexistence of long range antiferromagnetic order and gapped superconducting state. The field-induced state vanishes close to the expected quantum critical point in absence of superconductivity. Resistivity measurements above the critical pressure p c indicate the unusual metallic state in the normal phase. A schematic field-pressure phase diagram at low temperature is presented.

Thermodynamic study on non-centrosymmetric superconductor UIr by ac calorimetry under high pressure

We investigated pressure-induced superconductor UIr by the ac heat capacity measurement under high pressure. The heat capacity shows an anomaly at the Curie temperature T C1 = 46 K at ambient pressure and the heat capacity anomaly shifts to the lower temperature side with increasing pressure. The present pressure dependence of T C1 was consistent with previous works by the resistivity and magnetization measurements. No heat capacity anomaly was found at T C2 and T C3 which were clarified by previous ac magnetic susceptibility measurement. This may be due to very small magnetic ordered moments (∼0.05μB/U) of the FM2 and FM3 phases.

AC calorimetry study on the pressure-induced superconductor UIr

We have measured the heat capacity of the pressure-induced superconductor UIr by AC calorimetry under pressures up to 1.6 GPa . The heat capacity shows an anomaly at the Curie temperature T C 1 , and this anomaly shifts to lower temperatures with increasing pressure. The present pressure dependence of T C 1 was consistent with the results obtained by the previous resistivity and magnetization measurements.

Resistivity and AC calorimetry measurements on [formula omitted] under pressure

We present resistivity and AC calorimetry measurements of single crystalline CeNiGe 2 under high pressure and low temperature in a diamond anvil cell. CeNiGe 2 is a highly anisotropic, antiferromagnetic Kondo system at ambient pressure, located close to the boundary with non-magnetic systems. It has two ordering temperatures, T N 1 = 3.9 K and T N 2 = 3.2 K , which are suppressed under pressure. Between 0.7 and 1.4 GPa only a single transition was identified, with T N reaching 1.9 K by 1.4 GPa. At 1.9 GPa two transitions could again be seen by AC calorimetry at 2.0 and 1.3 K, accompanied by a qualitative change in the behaviour of the resistivity. The Sommerfeld coefficient showed a maximum of 755 mJ mol - 1 K - 2 at 1.4 GPa, up from 448 mJ mol - 1 K - 2 at close to ambient pressure. The residual resistivity was found to decrease sharply above 1.4 GPa after an initial increase with pressure.

Phase behaviour of the solid proton conductorCsHSO4

The sequence of previously found phase transitions inCsHSO4 at around60, 122, and 141 °C was carefully examined by using thermo-gravimetric analysis (TGA), massspectroscopy (MS), differential thermal analysis, ac calorimetry, impedancespectroscopy, and x-ray diffraction (XRD). Our results show evidence that at all thesetransition temperatures, the dehydration processes take place at or very nearthe surface of the crystal. As a consequence, our results support the phase ofCsHSO4 above141 °C being not a superprotonic-conducting phase but rather a mixture ofCsHSO4 andCs2S2O7 causedby a partial thermal decomposition that includes the break-up of hydrogen bonds and the formationof H2O molecules.

Critical behaviour of the specific heat of La 0.9Ag 0.1MnO 3 manganite

For the first time the specific heat of polycrystalline La 0.9Ag 0.1MnO 3 manganite near the Curie temperature was precisely measured by AC calorimetry and its critical behaviour was studied. The critical behaviour of the specific heat was approximated by the relationship taking into account corrections to the scaling. The regularities of variations in the universal critical parameters near the critical point are determined, and their values are calculated. Detailed analysis and comparison of the received results with the theoretical predictions and other experimental results was carried out.

Two-Dimensional Ising Model Criticality in a Three-Dimensional Uniaxial Relaxor Ferroelectric with Frozen Polar Nanoregions

The charge-disordered three-dimensional uniaxial relaxor ferroelectric Sr0.61Ba0.39Nb2O6 splits up into metastable polar nanoregions and paraelectric interfaces upon cooling from above Tc. The frozen polar nanoregions are verified by piezoresponse force microscopy, respond domainlike to dynamic light scattering and dielectric excitation, reveal nonergodicity at T>Tc via global aging, and coalesce into polar nanodomains below Tc. Contrastingly, the percolating system of unperturbed interfaces becomes ferroelectric with two-dimensional Ising-model-like critical exponents alpha=0, beta=1/8, and gamma=7/4, as corroborated by ac calorimetry, second harmonic generation, and susceptometry, respectively.

Coexistence of antiferromagnetism and superconductivity inCeRhIn5under high pressure and magnetic field

We report on detailed ac calorimetry measurements under high pressure and magnetic field of CeRhIn5. Under hydrostatic pressure the antiferromagnetic order vanishes near p_c*=2 GPa due to a first order transition. Superconductivity is found for pressures above 1.5 GPa inside the magnetic ordered phase. However, the superconductivity differ from the pure homogeneous superconducting ground state above 2 GPa. The application of an external magnetic field H || ab induces a transition inside the superconducting state above pc* which is strongly related to the re-entrance of the antiferromagnetism with field. This field-induced supplementary state vanishes above the quantum critical point in this system. The analogy to CeCoIn5 is discussed.

High pressure study on the heavy fermion superconductor [formula omitted] by AC calorimetry

We have determined the pressure-temperature phase diagram of heavy-fermion superconductor CePt 3 Si by the AC heat capacity measurement. The Néel temperature T N = 2.2 K decreases with increasing pressure and becomes zero at the critical pressure P AF ≃ 0.6 GPa . On the other hand, the superconducting phase exists in a wider pressure region from ambient pressure to about 1.5 GPa. The phase diagram of CePt 3 Si is thus very unique and has never been reported before for other heavy fermion superconductors.

Thermal characterization of materials at high pressures

This review deals with the development of some novel techniques for the measurement of thermal properties of materials at ambient and high pressures. Thermal characterization techniques like Absolute thermoelectric—power, AC electrical Resistivity, Differential thermal analysis and AC Calorimetry with specific reference to their usage at high pressures will be discussed in some detail. Some typical data on materials like Chromium alloys, Chalcogenide and metallic glasses will be presented.

On the high-pressure phase diagram of [formula omitted

We studied the high-pressure phase diagram of YbRh 2 Si 2 by AC calorimetry under high pressure up to 12 GPa. Two magnetic transitions are observed in the pressure range up to 7 GPa. The initial linear increase of the magnetic ordering temperature T N with pressure is followed by a regime up to 8.5 GPa where valence fluctuations are important. Above 9 GPa T N is increasing as expected from the Doniach model. Further the influence of the local crystal electric field on the magnetic ordering is discussed.

Thermodynamics Investigation on Pressure-induced Superconductor CeNiGe3by ac Calorimetry

We measured the heat capacity of the pressure-induced superconductor CeNiGe 3 by the ac calorimetry under high pressure up to 2.4 GPa using a single crystal sample grown by the Bi-flux method. The heat capacity C ac shows the λ-type anomaly at Neel temperature T N . The pressure dependence of T N is consistent with those determined previous studies by the resistivity measurement under high pressure. At 2.1 and 2.4 GPa, the heat capacity anomaly was observed around 0.3 K. This temperature is close to the superconducting transition temperature T sc determined by the previous resistivity measurement. Therefore, the heat capacity anomaly may be due to the superconducting transition. The value of T sc by the present heat capacity measurement was larger than those by the resistivity measurement. This might reflect a sample dependence.