Papers by Keyword: Clathrate

Abstract: This paper is devoted to the history of exploration of sintezed and natural gas hydrate. Academic, engineering and energy periods of the history of gas hydrates studies are described. The most significant researches in this area are described. The main practical projects in the world for the study and production of gas hydrates are reviewed.

Abstract: The influence of heat treatments in air on the surface and thermoelectric properties of polycrystalline silicon clathrate of nominal composition Ba₈Al₁₅Si₃₁ was investigated. The samples were prepared by combining arc melting and spark plasma sintering methods. Heat treatments were conducted in air at 873 K for 0-480 h. The surface oxidation was investigated by bulk surface X-ray diffraction (XRD) measurements, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The Seebeck coefficient and the electrical conductivity of samples were measured before and after heat treatments. XRD, SEM, EDS, and XPS results revealed that the clathrate phase near the surface was changed to Al-poor clathrate phases and an oxide phase (probably barium aluminate: BaAl₂O₄). However, the thermoelectric properties of samples were almost unchanged by the heat treatment in air at 873 K for up to 480 h because the chemical composition of the inner part of sample was almost stable.

Abstract: Poly-crystalline Ba₈Cu₆Ag₂Si₃₈ clathrates were synthesized by the arc-melting method and a subsequent annealing treatment. The main phase of all the samples was type-I clathrate. However, there were some precipitates which rich in Cu, Ag or Si in the grain boundary of the as-synthesized sample and the precipitates gradually reduced with the increase of annealing time. The transition metal content in the clathrate increased with the annealing time because of the diffusion of the Cu and Ag atoms from precipitates phase into clathrates phase. The as-synthesized sample was n-type material. For the 576 hours annealed sample, the carrier type changed from n-type to p-type because of the increase of Cu and Ag content in the clathrate.

Abstract: In recent years, solid clathrate gas hydrates are considered to be promising materials for hydrogen storage because they can trap molecular hydrogen within their cages formed by a hydrogen-bond water network. In this paper, we firstly synthesized the nitrogen hydrates, and then used these hydrates for hydrogen storage. The H₂ storage potential in these hydrates is investigated by Raman spectrometry technique. The spectral properties show that the multiple H₂ occupancies of large cages of N₂ hydrates have been realized under mild condition (16 MPa and 255 K) when exposing N₂ hydrates in pressurized H2 gas. The results suggest that nitrogen clathrate hydrates are prospective media for H₂ storage and may help to design and produce new hydrogen storage materials.

Abstract: We Review Our Dynamical Property Measurements of Rattling Phonons in Type-I Clathrate Compounds, the Quasi-on-Center Ba₈Ga₁₆Ge₃₀ (BGG) and Off-Center Ba₈Ga₁₆Sn₃₀ (BGS), Using a Terahertz Time-Domain Spectrometer. The Lowest-Lying Vibrational Modes of Rattling Ba Ions in the Oversized Cage Show Anomalous Temperature Dependence in their Spectra. For BGG, the Temperature Dependence Is Mostly Consistent with a Local Anharmonic Potential Approximation that Predicts Softening towards Low Temperature. On the other Hand, for BGS, a Single Broad Peak of Off-Center Rattling Phonons Splits into Two Subpeaks below 120 K, and, with Further Lowering Temperature, the Spectra Show Anomalous Broadening. While the Splitting Can Be Understood by the Double Well Potential, the Linewidth Broadening Must Be Attributed to some Relevant Interactions with such Excitations as Acoustic Phonons and Doped Carriers.

Abstract: Ba8Ga16Ge30 clathrate material has being intensely investigated as a candidate of promising thermoelectric materials. In this work, Ba8Ga16+xSbxGe30-2x (x=0,1) single crystals have been synthesized by the Ga flux method, using high purity elemental Ba, Sb, Ga, and Ge as starting materials. Powder X-ray diffraction, Scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy and Raman spectroscopy were used to characterize the single crystals. Seebeck coefficient of the crystals was measured from 300 to 800 K. The result indicates that the crystals are p-type semiconductor, which is different from the crystals synthesized by the Czochralski method. The Seebeck coefficient almost does not change after doping Sb in the whole temperature measured and it is in the range of 200 to 300μV/K.

Abstract: The Arc melting technique was employed to synthesize the type I clathrate of Ba8Al16Si30 compound. Phase transformations during synthesis and homogenization treatment were investigated using X-ray diffraction (XRD) and thermal analysis (TG/DSC). Thermoelectric properties as functions of temperature and homogenization treatment time were evaluated in this study. Maximum ZT was 0.14 at 590K for homogenized at 1173 K for 168hrs specimen and it is strongly expected to show higher value above 600K.

Abstract: Ba8Al16Si30 type I clathrate was produced by arc melting and thermoelectric properties were investigated. The phase transformation behavior of arc-melted type I Ba8Al16Si30 was examined by thermogravimetric analysis, differential scanning calorimetry, hardness test, density measurement, X-ray diffraction and scanning electron microscope analyses. Homogenization was carried out to induce the transformation to a thermoelectric phase at 773K to 973K for 5 hours and 24 hours in the vacuum furnace. Thermoelectric properties in the temperature range between 300K and 600K were measured and evaluated. Electrical conductivity was decreased and Seebeck coefficient was increased with increasing homogenization temperature and time. The arc-melted and the homogenized specimens represented n-type conduction at temperatures examined, and they showed reliable thermoelectric behaviors with increasing homogenization temperature and time.

Abstract: Clathrate Ba8Al16Si30 was produced by arc melting and annealing effects on the microstructure and thermoelectric properties were investigated. The phase transformation behavior of arc-melted Ba8Al16Si30 was examined by thermogravimetric analysis, differential scanning calorimetry, hardness test, X-ray diffraction and scanning electron microscope analyses. Isothermal annealing was carried out to induce the transformation to a thermoelectric phase at 500°C to 700°C for 5 hrs. Thermoelectric properties in the temperature range between 300K and 600K were measured and evaluated. Electrical conductivity was decreased and Seebeck coefficient was increased with increasing isothermal annealing temperature. The arc-melted and the isothermal annealed specimens represented n-type conduction at temperatures examined, and they showed reliable thermoelectric behaviors.