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CrSi2 alloy presents a strong interest for thermoelectric applications; however, its thermal conductivity is still too high and limits strongly its figure of merit. By combining experiment and modeling, we show that the nanostructuring of CrSi2 leads to a strong decrease in the thermal conductivity without affecting much the electronic transport properties. The thermal conductivity of nanostructured CrSi2 (∼45 nm) has also been determined as a function of the density. We predict that it would be about 5.5 W/mK at RT for a 100% dense sample, i.e., twice smaller than in bulk samples. We also give for the first time, a prediction of the effect of alloying on the thermal conductivity using the Callaway-Klemens model. To consider final applications, it is necessary to also investigate the thermal stability of nanostructured compounds. We show that grain coarsening of nanostructured CrSi2, even if it remains limited, happens above 1073 K and will deserve further attention. [ABSTRACT FROM AUTHOR]

The efficiency of thermoelectric devices depends on phase stability of layers constituting the device. TaN and TaN-Ta-TaN thin films, 900 nm thick, were investigated as diffusion barriers for the CeFe4Sb12 thermoelectric substrate in contact with the Cu electrode. It is shown that Sb diffuses through the TaN layer when the sample is heated above 400 °C. Multilayer TaN-Ta-TaN acts as the diffusion barrier for Sb and is efficient up to 500 °C. When diffusion of Sb occurs above 400 °C for TaN and above 500 °C for TaN-Ta-TaN, the formation of FeSb2 and Cu2Sb precipitates was identified by XRD and TEM/energy dispersive X-ray spectroscopy. [ABSTRACT FROM AUTHOR]

CrSi2 alloy presents a strong interest for thermoelectric applications; however, its thermal conductivity is still too high and limits strongly its figure of merit. By combining experiment and modeling, we show that the nanostructuring of CrSi2 leads to a strong decrease in the thermal conductivity without affecting much the electronic transport properties. The thermal conductivity of nanostructured CrSi2 (∼45 nm) has also been determined as a function of the density. We predict that it would be about 5.5 W/mK at RT for a 100% dense sample, i.e., twice smaller than in bulk samples. We also give for the first time, a prediction of the effect of alloying on the thermal conductivity using the Callaway-Klemens model. To consider final applications, it is necessary to also investigate the thermal stability of nanostructured compounds. We show that grain coarsening of nanostructured CrSi2, even if it remains limited, happens above 1073 K and will deserve further attention. [ABSTRACT FROM AUTHOR]

We investigated the site-disorder and thermal expansion of LnCa 4 O(BO 3 ) 3 ( Ln = Pr, Nd, Gd, Er, Tm) through a single crystal structural study conducted between 100 and 300 K. Additionally, a high-pressure synchrotron X-ray diffraction study at low temperatures was conducted to determine the compressibility of both ordered and disordered prototype oxyborate lanthanides, specifically NdCa 4 O(BO 3 ) 3 and ErCa 4 O(BO 3 ) 3 . The study revealed distinct behaviors consistent with their respective ionic radii. Furthermore, satellite reflections at 50 K, along with the onset of a triple a -parameter in the Nd crystal, indicate that the ground state of this extensively studied family may be different from what was previously reported., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Barium-cobaltate-based perovskite (BaCoO 3-δ ) and barium-cobaltate-based nanocomposites have been intensively studied in energy storage and conversion devices mainly due to flexible oxygen stoichiometry and tunable nonprecious transition metal oxidation states. Although a rich and complex family of structural polymorphs has already been reported for these perovskites in the literature, the potential structural evolution that may occur during the oxygen reduction reaction and the oxygen evolution reaction has not been investigated so far. In this study, we synthesized and characterized the lowest Co-oxidation state possible in the compound, BaCoO 2 , which exhibits a quartz-derived, trigonal structure with a helicoidally corner-sharing, CoO 4 -tetrahedral-framework as already proposed by Spitsbergen et al. Oxygen can reversibly be inserted in such a crystal structure to form BaCoO 3-δ , i.e., with 0 ≤ δ ≤ 1, based on the results of an in situ coupled thermogravimetric - neutron diffraction study and which presents therefore giant oxygen capacity storage due to the extreme tunability of the electronic configuration of the cobalt cations which defines the fundamental origins of the materials performance. The reversible conversion of BaCoO 2 to BaCoO 3-δ associated with a similar electronic conductivity above 900 K permits to clarify the high potential of BaCoO 3-δ -based energy storage and conversion devices.

In the present study, we have investigated the stability of different Ba-Si clathrates with pressure and temperature using DFT calculations and studied the stability of type I Ba 8 Si 46 and type IX Ba 24 Si 100 clathrates using high pressure─high temperature synthesis technique, calorimetry, and diffraction experiments. When increasing pressure, the type I Ba 8 Si 46 clathrate and BaSi 6 become more stable. In good qualitative agreement with experiments, the type IX Ba 24 Si 100 clathrate becomes stable at a pressure of 1-2 GPa thanks to the pressure and thermal effect of both electronic and vibrational contributions. One can notice that the presence of Ba in the cages of type IX clathrate increases significantly the stability and the mechanical properties of type IX clathrate. We have determined the P-T existence domain of type IX Ba 24 Si 100 clathrate from ex situ experiments, which was confirmed by in situ synchrotron X-ray experiments. At room pressure and under an oxidizing atmosphere, the type I Ba 8 Si 46 and the type IX Ba 24 Si 100 clathrates are stable up to about 560 °C and up to about 600 °C, respectively. The thermoelectric properties of type IX Ba 24 Si 100 are also reported.

We report the lattice dynamics and thermoelectric properties of topological semimetal Ba 3 Si 4 . The lattice dynamics has been studied by Raman and inelastic neutron scattering experiments. Good agreement has been found with first-principles calculations. The presence of low-energy optical modes at about 7 meV mainly due to the heavy mass of the Ba atoms suggests a propensity to low thermal conductivity, which is favorable for thermoelectric applications. Our density functional theory calculations indicate that the semimetallic nature of Ba 3 Si 4 is the origin for the rather large thermopower. Ba 3 Si 4 shows high potential for a thermoelectric material with a Seebeck coefficient as large as -120 μV K -1 for 0.2 electrons/formula units through the substitution of Ba by appropriate cations, such as Y.

Nanostructured β-FeSi 2 and β-Fe 0.95 Co 0.05 Si 2 specimens with a relative density of up to 95% were synthesized by combining a top-down approach and spark plasma sintering. The thermoelectric properties of a 50 nm crystallite size β-FeSi 2 sample were compared to those of an annealed one, and for the former a strong decrease in lattice thermal conductivity and an upshift of the maximum Seebeck's coefficient were shown, resulting in an improvement of the figure of merit by a factor of 1.7 at 670 K. For β-Fe 0.95 Co 0.05 Si 2 , one observes that the figure of merit is increased by a factor of 1.2 at 723 K between long time annealed and nanostructured samples mainly due to an increase in the phonon scattering and an increase in the point defects. This results in both a decrease in the thermal conductivity to 3.95 W/mK at 330 K and an increase in the power factor to 0.63 mW/mK 2 at 723 K.

Chromium disilicide (CrSi2) particles were synthesized by using an arc melting furnace followed by mechanical milling. XRD and DLS analyses show that aggregates of around 3 μm containing about 10 nm sized crystallites were obtained. These aggregates were functionalized in solution by coupling agents with different anchoring groups (silane, phosphonic acid, alkene and thiol) in order to disperse them into an organic polymer. Dodecene was used to modify the CrSi2 surface during mechano-synthesis in a grinding bowl with quite little solvent quantity and the optimization step allowed the aggregate size to be reduced to 500 nm. A thermoelectric composite was then made of alkene CrSi2 grafted samples and poly(p-phénylène-2,6-benzobisoxazole). This study opens the route for new surface grafting of intermetallic silicides for applications linked to electronics and/or energy.

Recrystallization of amorphous compounds can lead to the stabilization of metastable crystalline phases, which offers an interesting way to unveil novel binary or ternary compounds and control the transport properties of the obtained glass ceramics. Here, we report on a systematic study of the Cu-As-Te glassy system and show that under specific synthesis conditions using the spark-plasma-sintering technique, the α-As 2 Te 3 and β-As 2 Te 3 binary phases and the previously unreported AsTe 3 phase can be selectively crystallized within an amorphous matrix. The microstructures and transport properties of three different glass ceramics, each of them containing one of these phases with roughly the same crystalline fraction (∼30% in volume), were investigated in detail by means of X-ray diffraction, scanning electron microscopy, neutron thermodiffraction, Raman scattering (experimental and lattice-dynamics calculations), and transport-property measurements. The physical properties of the glass ceramics are compared with those of both the parent glasses and the pure crystalline phases that could be successfully synthesized. SEM images coupled with Raman spectroscopy evidence a "coast-to-island" or dendriticlike microstructure with microsized crystallites. The presence of the crystallized phase results in a significant decrease in the electrical resistivity while maintaining the thermal conductivity to low values. This study demonstrates that new compounds with interesting transport properties can be obtained by recrystallization, which in turn provides a tuning parameter for the transport properties of the parent glasses.

We report for the first time the full crystal structure, the electronic structure, the lattice dynamics, and the elastic constants of metastable monoclinic AlGe. In addition to ultrarapid cooling techniques such as melt spinning, we show the possibility of obtaining monoclinic AlGe by water-quenching in a quartz tube. Monoclinic AlGe and rhombohedral Al 6 Ge 5 are competing phases with similar stability since they both begin to decompose above 230 °C. The crystal structure and electronic bonding of monoclinic AlGe are similar to those of ZnSb and comply with its 3.5 valence electrons per atom: besides classical two electron-two center Al-Ge and Ge-Ge covalent bonds, Al 2 Ge 2 parallelogram rings are formed by uncommon multicenter bonds. Monoclinic AlGe could be used in various applications since it is found theoretically to be an electron-poor semiconductor with a narrow indirect energy bandgap of about 0.5 eV. The lattice dynamics calculations show the presence of low energy optical phonons, which should lead to a low thermal conductivity.

First principles calculations have been carried out to study the native single-defects and multi-vacancies in TaN and TiN with a cubic rocksalt structure mainly used as diffusion barriers. Our results indicate that vacancies are the most stable single-defects in both compounds and that nitrogen interstitial defects in tetrahedral interstitial site are significantly more stable in TaN than in TiN. The interactions between vacancies are attractive in TaN in contrast to the case of TiN. The vacancies show a much larger tendency to cluster and to form bi- and tri- vacancies in TaN than in TiN. We suggest that the number of d electrons might explain this difference in the defect stability. These results will have impact on the use of these materials as diffusion barrier.