Several processing methods were developed and evaluated for synthesizing empty silicon clathrates. A solution synthesis method based on the Hofmann-elimination oxidation reaction was successfully utilized to produce 20 mg of empty Si46. Half-cells using the Si46 electrodes were successfully cycled for 1000 cycles at rate of 5.3C. The capacity of the Si46 electrode in long-term tests was 675 mAh/g at the 4th cycle, but increased to 809 mAh/g at 50 cycles. The corresponding Coulombic efficiency was better than 99%. The capacity dropped from 809 to 553 mAh/g after 1000 cycles while maintaining a 99% Coulombic efficiency. In comparison, a Ba8Al8Si38 electrode could be cycled for about 200 cycles with a lower capacity and Coulombic efficiency. Potential applications of empty silicon clathrates as anode materials in Li-ion batteries are discussed.

Seebeck effect is widely used for the energy harvesting from wasted heat. In the Seebeck effect, the electric power can be generated by the temperature difference between both ends of the thermoelectric materials. However, low conversion efficiency is caused by heat flux from hot side to cold side of sample. In this paper, we have proposed a new thermal power generation mechanism with no temperature difference. We investigated the band structure of Ba8AuxSi46-x clathrate single crystal synthesized by Czochralski method. The single crystal has a gradient of the gold contents along the growth direction. According to the results of Seebeck coefficient, the electrical properties of the Ba8AuxSi46-x clathrate dramatically changed depending on the gold contents. In the case of gold content of lower than 5.33, the Ba8AuxSi46-x clathrate showed a n-type semiconductor. In the case of gold content of higher than 5.33, the Ba8AuxSi46-x clathrate showed a p-type semiconductor. The band gap of the n-type and p-type Ba8AuxSi46-x clathrate were wider than the intrinsic semiconductor. We can successfully synthesize a n-p junction single crystal, which obtaining energy band curve generated from the difference of Fermi level between p- and n- type semiconductors. The single crystal was heated under the uniform temperature and able to obtain generated electric voltage of around 0.6 mV at 400°C. These results suggested that the obtained electric voltage can be generated from the separation of hole-electron pair excited by heating at the intrinsic part with a narrow band gap along to the energy band curve formed by p-n junction.

Concerning a materials ability to convert heat to electrical energy, the electrical power factor S2/ρ as well as the thermal conductivity at elevated temperatures are of special interest. Since Flash experiments measure the thermal diffusivity and standard steady-state heat-flow experiments are inaccurate at elevated temperatures due to radiation errors inherent to this technique, direct and accurate thermal conductivity data on type-I clathrate single crystals at elevated temperatures are scarce in literature. Here we report 3ω thermal conductivity data on single crystalline Ba8Cu5.09Ge40.91 (BCG), La1.23Ba6.99Au5.91Si39.87, and Ce1.06Ba6.91Au5.56Si40.47 in the temperature range between 80 and 330 K, and specific heat data on BCG between 2 and 300 K. The comparison of our room temperature phonon thermal conductivity data (κph) to results on transition metal (TM) free type-I clathrates in terms of the guest free space (Rfree) suggests a stronger dependence of κph on Rfree for the clathrates containing TM elements.

This review article evaluates the structure–property relations of inorganic clathrates and clathrate hydrates and their potential role in energy harvesting. There is potential cross-fertilization between the two research areas.

Guest–host clathrate compounds exhibit unique structural and physical properties, which lead to their versatile roles in energy applications. Prominent classes of clathrate compounds are gas hydrates and inorganic clathrates. That said, there is limited cross-fertilization between the clathrate hydrate and inorganic clathrate communities, with researchers in the respective fields being less informed on the other field. Yet the structures and unique guest–host interactions in both these compounds are common important features of these clathrates. Common features and procedures can inspire and inform development between the compound classes, which may be important to the technological advancements for the different clathrate materials, e.g., structure characterization techniques and guest–host dynamics in which the “guest” tends to be imprisoned in the host structure, until external forces are applied. Conversely, the diversity in chemical compositions of these two classes of materials leads to the different applications from methane capture and storage to converting waste heat to electricity (thermoelectrics). This article highlights the structural and physical similarities and differences of inorganic and methane clathrates. The most promising state-of-the-art applications of the clathrates are highlighted for harvesting energy from methane (clathrate) hydrate deposits under the ocean and for inorganic clathrates as promising thermoelectric materials.

On the search for cost-competitive thermoelectric clathrates we have investigated the influence of Sn substitutions for Ge on the structural and thermoelectric properties of the type-I clathrate Ba8Cu5Si6Ge35. The solid solubility of Sn was found to be limited to 0.6 atoms per unit cell. A series of compounds with the nominal compositions Ba8Cu5Si6Ge35-xSnx (x = 0.2, 0.4, 0.6) was synthesized in a high-frequency furnace. The samples were annealed, and subsequently ball milled and hot pressed. The hot pressed samples were characterized by X-ray powder diffraction, energy-dispersive X-ray spectroscopy and transport property measurements. Our results show that the substitution of Ge by Sn introduces vacancies at the 6d site of the type-I clathrate structure and shifts the highest dimensionless thermoelectric figure of merit ZT from 570 °C for the Sn free sample to lower temperatures. The highest figure of merit ZT = 0.42 is reached at about 320 °C for the Sn-substituted sample Ba8Cu5Si6Ge35Sn0.6.

We prepared the type-I Ba8AuxSi46-x (x=5.5, 5.4 and 5.33) clathrates with p-type conduction by arc-melting and subsequent annealing. The Seebeck coefficients of all as-synthesized samples were negative. After annealing, in the case of x=5.4 and 5.5, the Seebeck coefficients changed to positive, and in the case of x=5.33, this value was negative for the annealed sample. The chemical composition revealed that Au content in the Ba8AuxSi46-x clathrates increased after the annealing process. The backscattered electron images showed that there were many Au-excess regions which were not clathrate structure in the as-synthesized samples, and these regions decreased and even disappeared after the annealing process. The disappearing of Au-excess regions can be explained by the diffusion of Au atoms from the Au-excess regions into the clathrates. Based on these results, annealing treatment can be used to tune the carrier conduction by controlling the content of Au for type-I Ba8AuxSi46-xclathrates.

Ba8Ga16−xGe30+x is the clathrate with the highest thermoelectric figure of merit known to date. However, no p-type material could be obtained by conventional synthesis from the melt. Here we show that the time and cost-effective melt spinning technique can produce Ba8Ga16−xGe30+x in a metastable state, where x can be varied continuously from negative to positive values, resulting in both p- and n-type materials. The quenched phases were characterized by x-ray powder diffraction and transmission electron microscopy. It was surprising that they were perfectly crystalline, with large grain sizes of the order of a micrometer. Temperature dependent measurements of the electrical resistivity, Hall effect, thermopower, and thermal conductivity are presented and discussed in terms of a two-band model.

We have synthesized single crystalline Ba8AlxSix-46 clathrates by using the flux Czochralski (CZ) method with Al-rich melt. The specific electric resistivity, the Seebeck coefficient and the power factor of single crystalline Ba8Al14Si32 were 0.73 mΩcm,70.0μV/K and 6.8×10-4 V2/K2Ωm, respectively. These values are higher than that of single crystalline Ba8Al12Si34 clathrate because of the reduced carrier concentration. It is indicated that Al contents and the carrier concentration of single crystalline Ba8AlxSi46-x can be controlled by using the flux Czochralski method.

Understanding the high-pressure kinetics associated with the formation of methane hydrates is critical to the practical use of the most abundant energy resource on earth. In this study, we have studied, for the first time, the compression rate dependence on the formation of methane hydrates under pressures, using dynamic-Diamond Anvil Cell (d-DAC) coupled with a high-speed microphotography and a confocal micro-Raman spectroscopy. The time-resolved optical images and Raman spectra indicate that the pressure-induced formation of methane hydrate depends on the compression rate and the peak pressure. At the compression rate of around 5 to 10 GPa/s, methane hydrate phase II (MH-II) forms from super-compressed water within the stability field of ice VI between 0.9 GPa and 2.0 GPa. This is due to a relatively slow rate of the hydrate formation below 0.9 GPa and a relatively fast rate of the water solidification above 2.0 GPa. The fact that methane hydrate forms from super-compressed water underscores a diffusion-controlled growth, which accelerates with pressure because of the enhanced miscibility between methane and super-compressed water.

We have synthesized type-VIII and type-I Ba8Ga16Sn30 clathrates by using different annealing treatments, confirmed with XRD and electron microprobe measurements. NMR lineshape measurements identified a broad resonance corresponding to first-order-shifted satellites. Simulations for the type I structure based on first principles calculations provided an excellent fit to the data, with the best agreement provided by the calculated lowest-energy configuration, having no Ga-Ga bonds. These results allow us to address local configurations within the random type-I alloy, as well as atomic displacements and bond-length distributions, which we compare to experiment.

Structural rules have been formulated for the molecular crystal of ammonia borane (AB, NH3BH3) and clathrates thereof. These rules are similar to the “ice rules” in water. Periodic structures of possible clathrates of AB have been predicted. A rigorous analysis of the uniform space filling tessellations of 3D space has been performed resulting in the identification of two promising structures for AB clathrates. Additional five structures have been identified exploiting features and properties of AB molecules. The screening, or evaluation, of proposed periodic structures has been performed on the basis of their stability determined at the density functional level of theory. Hydrogen capacity of the most stable periodic structure (cantitruncated cubic honeycomb) is estimated to be 21 wt%, 19% chemically bound in AB and 2 wt% of H2 physisorbed in the cages of AB.

We have developed a new thermoelectric power-generating module composed of 72 pieces of n-type Ba8Al18Si28 clathrate elements made by arc melting. The Seebeck coefficient, specific electric resistance and thermal conductivity of Ba8Al18Si28 clathrate were 250 μV/K, 1.9 mΩcm and 3.1 W/mK at 500 °C, respectively, and the thermoelectric figure of merit (ZT) was 0.8. The new thermoelectric module was constructed using only n-type thermoelectric elements connected in series with hook-shaped electrodes. The open-circuit voltage of the module increased with hot-side temperature up to 1.8 V at 500 °C and generated 0.24 W. The module was successfully used to charge lithium-ion batteries for mobile phones.