December 22, 2017 02:04 PM

Thermoelectric materials allow the conversion of heat energy into electrical energy. The efficiency is governed by the dimensionless figure of merit (ZT), which is defined as ZT=(α2 σ T/κ), where α, σ, κ are the Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. A research team has recently developed a new capability at HPCAT. The thermoelectric properties of a polycrystalline SnTe have been measured up to 4.5 GPa at 330 K using the developed method. The SnTe shows an enormous enhancement in Seebeck coefficient, greater than 200 % after 3 GPa, which correlates to a known pressure-induced structural phase transition observed by X-ray diffraction measurement. Electrical resistance and relative changes to the thermal conductivity were also measured, enabling the determination of relative changes in ZT.

December 22, 2017 02:10 PM

The lead hybrid perovskite (e.g. MAPbI3, MA=CH3NH3+) solar cell has been under fast development, with the highest certified power conversion efficiency now exceeding 22 %. The type of perovskite has become the first solution-processable photovoltaic material to surpass the efficiency of dominant crystalline silicon panels. The recently discovered Cs2AgBiBr6 double perovskite exhibits attractive optical and electronic features, making it promising for various optoelectronic applications. However, its practical performance is hampered by the large band gap. A research team, using HPCAT facility, used high-pressure to regulate band gap in Cs2AgBiBr6 is, and observed a remarkable narrowing band gap through high pressure treatments. Moreover, the narrowed band gap is partially retainable after releasing pressure, promoting its optoelectronic applications.

November 2, 2017 12:23 PM

The behavior of the f-electrons in the lanthanides and actinides governs important macroscopic properties but their pressure and temperature dependence is not fully explored. For example, the one f-electron in Ce plays an important role in the volume dependence under pressure which contracts abruptly by ~15% at room temperature when the pressure reaches ~0.75 GPa. The crystallographic symmetry remains across the γ-α volume collapse, even though the two fcc phases are quite different. The iso-structural volume collapse from the γ- to the α-phase ends at high temperatures in a critical point (pC, VC, TC), unique among the elements.

October 6, 2017 07:03 PM

Lithium–air (Li–O2) batteries may possess energy densities close to that of gasoline. It is commonly known that the Li–O2 battery often forms insoluble discharge products, mainly oxides, which accelerate the degradation of the electrode and electrolyte, thus reducing the cycle stability and efficiency. Lithium superoxide has been considered as an alternative cathode based on reduced graphene oxide. The major problem is that the lithium superoxide is unstable at ambient condition. Investigation of the stability of oxygen-rich lithium oxides would greatly extend our knowledge on improving the capability and lifetime of lithium–air batteries. Based on the theoretical predictions, a research team has used the HPCAT facility in successfully synthesizing several new stable oxygen-rich lithium oxides under high pressure.

September 18, 2017 12:19 PM

Magnesium is the lightest structural metal, with 35% lighter than aluminum and 78% lighter than steel, thus possessing an exceptional lightweightness (strength-to-weight ratio). However, because of its hexagonal close packed (hcp) structure, the associated slips in hcp-Mg often causes Mg having limited ductility and formability. A research group has synthesized 5 nm/5 nm Mg/Nb nanocomposites that display exceptional strength, ductility, and thermal stability. Using the HPCAT facility, the group found that the Mg present in the 5 nm/5 nm Mg/Nb nanocomposite is of entirely body-centered cubic (bcc) structure, without any trace of hcp Mg.  The bcc nano-Mg leads to a 40% higher strength and 125% increase in strain to failure as compared to 50 nm/50 nm Mg/Nb nanocomposites (where Mg has an hcp structure) made by the same method.

August 29, 2017 01:26 PM

C, Si and Ge all have sp3-bonded crystalline phases with cubic diamond structure. Both Si and Ge display well-known sp3-bonded tetrahedral amorphous forms which have widespread applications. However, the counterpart in C, an amorphous form of diamond, has been conspicuously missing and remained a puzzle. By combining high pressure with in situ laser heating at a specific temperature window, a research group using HPCAT facilities successfully turned amorphous carbon to amorphous diamond. The amorphous diamond has a three dimensional sp3-bonded network structure similar to crystalline diamond, with ultrahigh incompressibility (bulk modulus) comparable to diamond, but lacking long-range periodic order. Moreover, this amorphous diamond is found to be recoverable at ambient conditions. More in Zeng et al., Nature Comm. 8, 322, (2017)

July 17, 2017 11:34 AM

The interplay of magnetism and superconductivity in iron-based superconductors remains a subject of extensive studies. Recently, pressure induced superconductivity was observed in Fe-ladder compound BaFe2S3, a quasi-one-dimensional prototype of iron-based superconductors. However, it is unclear whether the mechanism of superconductivity in BaFe2S3 is similar to two-dimensional iron-based superconductors. By combining the measurements of electric resistance, magnetic susceptibility, and Fe-Kb x-ray emission spectroscopy at HPCAT,  a research team observed superconductivity in a compressed BaFe2Se3 sample, with the results implying a new family of iron-based superconductors for such one dimensional Fe-ladder compounds .