Derivation of the heat capacity anomaly at a first-order transition by using a semi-adiabatic relaxation technique

International audienceThis paper deals with the problem of determining the heat capacity anomaly associated with a first-order transition when using relaxation calorimetry. A method of data recording and analysis is proposed, which is shown to be well suited to investigate such a feature, including its hysteretical character. This technique is applied to spinel vanadates, allowing us to shed light on a recent controversy about the double-transition which takes place in these oxides

Phase diagram of the spinel oxide MnV 2 O 4

International audienceMagnetization, susceptibility, and heat-capacity measurements were carried out on MnV2O4 ceramic samples. The two main results are as follows: (i) a consistent set of experimental features provides strong support to the existence of two separate transitions in zero-field and (ii) beside the ferrimagnetic transition, the phase diagram in the H−T plane contains two first-order transition (FOT) lines, both connected to the magnetostructural transition temperature in zero field. The new FOT line found at low-H/low-T is discussed in terms of field-induced alignment within the structure of tetragonal domains

Spin-Glass-like State and Reversible Room-Temperature Magnetocaloric Effect in Double Distorted Perovskites Nd(Cu 3– x Mn x )Mn 4 O 12

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Spin-Glass-like State and Reversible Room-Temperature Magnetocaloric Effect in Double Distorted Perovskites Nd(Cu 3– x Mn x )Mn 4 O 12

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Phase diagram of the spinel oxide MnV 2 O 4

International audienceMagnetization, susceptibility, and heat-capacity measurements were carried out on MnV2O4 ceramic samples. The two main results are as follows: (i) a consistent set of experimental features provides strong support to the existence of two separate transitions in zero-field and (ii) beside the ferrimagnetic transition, the phase diagram in the H−T plane contains two first-order transition (FOT) lines, both connected to the magnetostructural transition temperature in zero field. The new FOT line found at low-H/low-T is discussed in terms of field-induced alignment within the structure of tetragonal domains

Derivation of the heat capacity anomaly at a first-order transition by using a semi-adiabatic relaxation technique

International audienceThis paper deals with the problem of determining the heat capacity anomaly associated with a first-order transition when using relaxation calorimetry. A method of data recording and analysis is proposed, which is shown to be well suited to investigate such a feature, including its hysteretical character. This technique is applied to spinel vanadates, allowing us to shed light on a recent controversy about the double-transition which takes place in these oxides

CdI2 structure type as potential thermoelectric materials: Synthesis and high temperature thermoelectric properties of the solid solution TiSxSe2−x

peer reviewedPolycrystalline samples of the solid solution TiSxSe2−x with x varying from 0 to 2 were prepared using direct high temperature reaction of stoichiometric amounts of the elements. Rietveld refinements of powder X-ray diffraction data are consistent with the existence of a full solid solution. High temperature Seebeck coefficient, electrical resistivity and thermal diffusivity measurements were performed on pellets densified by spark plasma sintering. These measurements reveal that along the solid solution the transport properties vary from the rather metallic and p-type character of TiSe2 to the semiconducting and n-type of TiS2. This change of conduction regime is responsible for the peculiar evolutions of transport properties of TiS0.5Se1.5 with increasing temperature that vary somewhat differently than that of the other members of the solid solution. As expected, the disorder generated by the mixed occupancy of the S and Se on the anionic site is responsible for the diminution of the lattice thermal conductivity. A maximum zT above 0.4 at 400 °C is reached for TiS1.5Se0.5

Investigation of the magnetocaloric effect in double distorted perovskites Ca(Cu3-xMnx)Mn4O12 (1 ≤ x ≤ 2): From standard ferrimagnetism to glassy ferrimagnetism

International audienceThe double distorted perovskites Ca(Cu3-xMnx)Mn4O12 (x = 1, 1.25, 1.5 and 2) exhibit a paramagnetic-to-ferrimagnetic transition at a temperature decreasing from TC = 280 K for x = 1 down to TC = 163 K for x = 2. In the latter compound, a reentrant-like spin-glass state emerges around Tg = 70 K. For each composition, the isothermal entropy change /DeltaSM was derived from magnetic measurements recorded up to 9 T. The obtained /DeltaSM(T) curves exhibit a broad and dissymmetric distribution, leading to high values of refrigerant capacity. The impact of the x value on the magnetic and magnetocaloric properties is addressed in detail, taking into account the various spin interactions present in these compounds

Investigation of the magnetocaloric effect in double distorted perovskites Ca(Cu3-xMnx)Mn4O12 (1 ≤ x ≤ 2): From standard ferrimagnetism to glassy ferrimagnetism

International audienceThe double distorted perovskites Ca(Cu3-xMnx)Mn4O12 (x = 1, 1.25, 1.5 and 2) exhibit a paramagnetic-to-ferrimagnetic transition at a temperature decreasing from TC = 280 K for x = 1 down to TC = 163 K for x = 2. In the latter compound, a reentrant-like spin-glass state emerges around Tg = 70 K. For each composition, the isothermal entropy change /DeltaSM was derived from magnetic measurements recorded up to 9 T. The obtained /DeltaSM(T) curves exhibit a broad and dissymmetric distribution, leading to high values of refrigerant capacity. The impact of the x value on the magnetic and magnetocaloric properties is addressed in detail, taking into account the various spin interactions present in these compounds

Revisiting some chalcogenides for thermoelectricity

International audienceThermoelectric materials that are efficient well above ambient temperature are needed to convert waste-heat into electricity. Many thermoelectric oxides were investigated for this purpose, but their power factor (PF) values were too small (∼10−4 W m−1 K−2) to yield a satisfactory figure of merit zT. Changing the anions from O2− to S2− and then to Se2− is a way to increase the covalency. In this review, some examples of sulfides (binary Cr–S or derived from layered TiS2) and an example of selenides, AgCrSe2, have been selected to illustrate the characteristic features of their physical properties. The comparison of the only two semiconducting binary chromium sulfides and of a layered AgCrSe2 selenide shows that the PF values are also in the same order of magnitude as those of transition metal oxides. In contrast, the PF values of the layered sulfides TiS2 and Cu0.1TiS2 are higher, reaching ∼10−3 W m−1 K−2. Apparently the magnetism related to the Cr–S network is detrimental for the PF when compared to the d0 character of the Ti4+ based sulfides. Finally, the very low PF in AgCrSe2 (PF = 2.25 × 10−4 W m1 K−2 at 700 K) is compensated by a very low thermal conductivity (κ = 0.2 W m−1 K−1 from the measured Cp) leading to the highest zT value among the reviewed compounds (zT700K = 0.8). The existence of a glassy-like state for the Ag+ cations above 475 K is believed to be responsible for this result. This result demonstrates that the phonon engineering in open frameworks is a very interesting way to generate efficient thermoelectric materials