Stevce Stefanoski

Table-like magnetocaloric effect and enhanced refrigerant capacity in Eu8Ga16Ge30-EuO composite materials

A large reversible magnetocaloric effect (MCE) and enhanced refrigerant capacity (RC) were observed in multiphase composite materials composed of type-I clathrate Eu8Ga16Ge30 and EuO. Eu8Ga16Ge30 undergoes two successive ferromagnetic transitions at 10 K and 35 K, and EuO exhibits a ferromagnetic transition at 75 K. A large RC of 794 J/kg for a field change of 5 T over a temperature interval of 70 K was achieved in the Eu8Ga16Ge30–EuO composite with a 40%-60% weight ratio. This is the largest value ever achieved among existing magnetocaloric materials for magnetic refrigeration in the temperature range 10 K-100 K. Adjusting the Eu8Ga16Ge30 to EuO ratio is shown to produce composites with table-like MCE, desirable for ideal Ericsson-cycle magnetic refrigeration. The excellent magnetocaloric properties of these Eu8Ga16Ge30–EuO composites make them attractive for active magnetic refrigeration in the liquid nitrogen temperature range.

Long-range ferromagnetism and giant magnetocaloric effect in type VIII Eu8Ga16Ge30 clathrates

Long-range ferromagnetism and low-field giant magnetocaloric effect are observed in Eu8Ga16Ge30 with the type VIII clathrate crystal structure, a material that is better known for its thermoelectric properties. Magnetization and modified Arrott plots indicate that the system undergoes a second-order ferromagnetic-paramagnetic phase transition at ∼13K. The low-field giant magnetic entropy change (−ΔSM∼11.4J∕kgK for Δμ0H=3T) coupled with the absence of thermal hysteresis and field hysteresis makes the system very attractive for low temperature magnetic refrigeration. The giant magnetic entropy change originates from the large magnetization (7.97μB per Eu ion) and the sharp change with temperature at the paramagnetic-ferromagnetic transition.

Low temperature transport properties and heat capacity of single-crystal Na8Si46.

The low temperature thermal conductivity, resistivity, and Seebeck coefficient of single-crystal Na(8)Si(46) are investigated revealing the intrinsic low temperature transport properties of this material. Metallic conduction is observed, with a higher residual resistance ratio than any other known type I clathrate. Heat capacity together with thermal conductivity provide insight into the Na disorder inside the polyhedra formed by the Si framework. Single-crystal structural refinement and thermal property analyses reveal anisotropic disorder for Na inside the tetrakaidecahedra due to a reduction in the symmetry inside these polyhedra, unlike that for Na inside the dodecahedra.

Synthesis and structural characterization of Na xSi 136 (0 <x ≤ 24) single crystals and low-temperature transport of polycrystalline specimens

Na(x)Si(136) clathrate-II single crystals with x = 2.9, 5.1, 8.2, and 14.7 were prepared by a two-step process. In the first step, Na(24)Si(136) single crystals were grown from the precursor Na(4)Si(4) by reaction of the vapor phase with spatially separated graphite in a closed volume. In the second step, the Na(24)Si(136) single crystals were subjected to thermal decomposition in a nitrogen atmosphere at 10 Torr and 405 °C. The Na content was controlled by the duration of thermal decomposition. The structural properties were investigated using single-crystal X-ray diffraction and compared with those of single-crystal Na(24)Si(136). The quality of the obtained products also allowed for low-temperature transport property measurements on agglomerates of crystals allowing for an investigation into the low-temperature electrical and thermal properties as a function of Na content.

Magnetocaloric effect and refrigerant capacity in Sr-doped Eu8Ga16Ge30 type-I clathrates

Magnetic properties, the magnetocaloric effect (MCE) and refrigerant capacity (RC) were investigated in Eu8Sr8−xGa16Ge30 (x=0,4) type-I clathrates. The substitution of Sr for Eu decreases the Curie temperature (TC) and saturation magnetization (MS) from 35 K and 65 emu/g for the x=0 composition to 15 K and 35 emu/g for the x=4 composition. This is attributed to the increase in the Eu–Eu distance with Sr substitution. The large MCE and RC are achieved in both specimens. For a field change of 3 T, the MCE and RC reach the largest values of 5.8 J/kg K and 127.6 J/kg for x=0 composition and 4.3 J/kg K and 72.1 J/kg for x=4 composition. The broadening of the MCE curves is likely associated with the ordering of the magnetic moments of Eu that occurs below 10 K. The large values of MCE and RC, in addition to the absence of thermal and field hysteresis indicate that these clathrate materials are very interesting for cryogenic magnetic refrigeration applications.

Enhanced cryogenic magnetocaloric effect in Eu8Ga16Ge30 clathrate nanocrystals

We observe an enhanced magnetic entropy change (−ΔSM) at cryogenic temperatures (T < 20 K) in Eu8Ga16Ge30 clathrate (type-I) nanocrystals prepared by a ball milling method. With reduction in the crystal size to 15 nm, −ΔSM is enhanced at low temperatures, reaching the highest value (∼10 J/kg K) at 5 K for a field change of 5 T. For all samples investigated, there is a cross-over temperature (∼25 K) in −ΔSM (T) above which −ΔSM decreases with crystal size, opposite to that observed at low temperatures. A careful analysis of the magnetic and magnetocaloric data reveals that as the crystal size decreases the magnetic interaction between Eu2+ ions on the Eu2 site governing the primary ferromagnetic transition at ∼35 K becomes gradually weaker, in effect, altering the interaction between Eu2+ ions occupying the Eu1 and Eu2 sites responsible for the secondary ferromagnetic transition at 15 K. As a result, we have observed a strong change in magnetization and the enhancement of −ΔSM at low temperature.

Ambient-Pressure Polymerization of Carbon Anions in the High-Pressure Phase Mg2C.

Experimental and theoretical methods were employed to investigate the ambient-pressure, metastable phase transition pathways for Mg2C, which was recovered after high-pressure synthesis. We demonstrate that at temperatures above 600 K isolated C(4-) anions within the Mg2C structure polymerize into longer-chain carbon polyanions, resulting in the formation of the α-Mg2C3 (Pnnm) structure, which is another local energy minimum for the carbon-magnesium system. Access to the thermodynamic ground state (decomposition into graphite) was achieved at temperatures above ∼1000 K. These results indicate that recoverable high-pressure materials can serve as useful high-energy precursors for ambient-pressure materials synthesis, and they show a novel mechanism for the formation of carbon chains from methanide structures.

Synthesis and Thermoelectric Properties of Lead Chalcogenide Nanocomposites

Doped lead telluride dimensional nanocomposites were prepared by densifying nanocrystals synthesized employing an alkaline aqueous solution-phase reaction. The nanocrystal synthesis procedure resulted in high product yields of over 2 g per batch. These nanocrystals were then subjected to Spark Plasma Sintering (SPS) for densification. Transport properties were evaluated through temperature dependent resistivity, Hall, Seebeck coefficient, and thermal conductivity measurements. The results for these lead telluride nanocomposites were compared to bulk polycrystalline lead tellurides with similar carrier concentrations.