Acoustic Emission Detection of Micro-cracks under High Pressure and High Temperature in a Deformation Large-Volume Apparatus at the Endstation P61B, PETRA III

We successfully developed an in situ acoustic emission (AE) detection setup that allows recording of AE waveforms (triggered and streaming) and simultaneous x-ray diffraction and imaging on samples deformed at high pressure and high temperature (HPHT) conditions in the Aster-15 Large Volume Press at the synchrotron beamline station P61B. This high pressure AE detection system is a powerful tool to investigate AE phenomena from the HPHT chamber. Six commercial acoustic sensors, protected by a tungsten carbide support ring on each anvil of the same material, have excellent survivability throughout each successive experiment. By pulsing each sensor in succession, the average wave velocity through the anvils and cell assembly can be determined at any press load. The distance between the sensors is obtained by x-ray radiography and by logging the positions of each hydraulic ram. This provides a basis for accurately locating AE events in the sample. The feasibility of this AE detection setup was confirmed by compression and deformation test runs using several different self-designed AE sources in specialized assemblies. The present setup proves to be extremely efficient and accurate in measuring brittle processes in samples under HPHT. It is now available for applications for beam time and experiments without x rays at P61B. Combined with synchrotron x rays, in situ pressure, temperature, strain rate and stress, and phase changes can be monitored while recording AE activity. We provide a powerful tool to investigate the origin of earthquakes, for example, causing AE emissions due to brittle dehydration reactions or phase transformations in the Earth

On the Eventually Periodic Continued <i>β</i>-Fractions and Their Lévy Constants

In this paper, we consider continued β-fractions with golden ratio base β. We show that if the continued β-fraction expansion of a non-negative real number is eventually periodic, then it is the root of a quadratic irreducible polynomial with the coefficients in Z[β] and we conjecture the converse is false, which is different from Lagrange’s theorem for the regular continued fractions. We prove that the set of Lévy constants of the points with eventually periodic continued β-fraction expansion is dense in [c, +∞), where c=12logβ+2−5β+12

Octahedral tilting dominated phase transition in compressed double perovskite Ba2_2SmBiO6_6

The comprehension of structural behaviors in double perovskites is crucial for their functional optimization, especially when applying external regulations. Here, to inquire about potential structures with better magnetic performance, high-pressure phase transformation in double perovskite Ba$_2$SmBiO$_6$ was first investigated up to 50 GPa via in situ high-pressure x-ray diffraction and Raman spectroscopy. A pressure-induced phase transition from cubic Fm-3m to orthorhombic Pnma is discovered at 4.8 GPa, accompanied by the splitting of the diffraction peaks. Above 19.8 GPa, the new phase becomes distorted as shown by the peak recombination and broadening. The variation of Raman spectra also confirms the formation and distortion of the high-pressure phase during compression, through the evolution of Bi–O stretching, Bi–O bending, octahedral rotation, and Ba-sites translation modes. The analysis of tilt angles and distortion factor evinced that the octahedral BiO$_6$ tilting is the key factor for the phase transition occurrence. Based on the Mulliken populations analyses, the Bi–O bonds undergo a covalent-ionic-antibonding transition across the phase transition under compression. Our exploration of the phase transition mechanism guides the modulation of the magnetic and electronic properties under extreme conditions

Strategies for hardening purity metallic materials by high pressure and high temperature quenching method

Purity metallic materials are increasingly demanded in modern manufacturing industries, but their applications are limited owing to their poor wear resistance and mechanical properties. Therefore, exploring an efficient hardening method to significantly enhance the hardness of pure metals is emergent in materials science. In this work, a series of high pressure and high temperature (HPHT) quenching experiments were carried out on several pure metals, with a maximum hardening factor exceeding 10. The results indicated that pressure has an unusual effect on refining grains and increasing the Hall–Petch coefficient ky. The ky value of pure Fe is 49.5 GPa*μm1/2 with a quenching pressure of 5 GPa, which is two orders higher than that of the untreated polycrystalline sample (0.2 GPa*μm1/2). In addition, we report an extreme hardness of 8.34 GPa in pure Ti induced by HPHT quenching, and the unprecedented hardening comes from the formation of the twin and lath martensitic substructures. The hardening mechanism of the HPHT quenching method is a combination of Hall–Petch hardening and work-hardening. Our results provide a practical route to achieve attractive mechanical properties in pure metals and shine a light on the hardening mechanism of metallic materials

An electrically conductive and ferromagnetic nano-structure manganese mono-boride with high Vickers hardness

The combination of various desired physical properties greatly extends the applicability of materials. Magnetic materials are generally mechanically soft, yet the combination of high mechanical hardness and ferromagnetic properties is highly sought after. Here, we report the synthesis and characterization of nanocrystalline manganese boride, CrB-type MnB, using the high-pressure and high-temperature method in a large volume press. CrB-type MnB shares the specificity of large numbers of unpaired electrons of manganese ions and strong covalent boron zigzag chains. Thus, manganese mono-boride exhibits “strong” ferromagnetic, magnetocaloric behavior, and possesses high Vickers hardness. We demonstrate that zigzag boron chains in this structure not only play a pivotal role in strengthening mechanical properties but also tuning the exchange correlations between manganese atoms. Nontoxic and Earth-abundant CrB-type MnB is much more incompressible and tougher than traditional ferromagnetic materials. The unique combination of high mechanical hardness, magnetism, and electrical conductivity properties makes it a particularly promising candidate for a wide range of applications

Hydrogen Evolution Reaction of γ-Mo0.5W0.5 C Achieved by High Pressure High Temperature Synthesis

For the first time, the hydrogen evolution reaction (HER) electrocatalytic performances of incompressible γ-Mo0.5W0.5C, prepared by high-pressure, high-temperature (HPHT) synthesis, were investigated in the electrolyte. The polarization curve of the γ-Mo0.5W0.5C cathode exhibits the current density of 50 mA∙cm−2 at an overpotential value of 320 mV. The corresponding Tafel slope of the incompressible γ-Mo0.5W0.5C is 74 mV∙dec−1. After a 1000-cycle test, and then exposure to the air for six months, the γ-Mo0.5W0.5C electrode performed a current density of 50 mA∙cm−2 at an overpotential of 354 mV, which was close to the initial one

Pressure Dependence of Structural Behavior and Electronic Properties in Double Perovskite Ba2_2SmSbO6_6

Understanding the structural behavior of double perovskites plays a pivotal role in optimizing their optical, electrical, and magnetic properties, especially when the effects of external parameters are considered. In this work, we report the high-pressure phase transition, the light absorption, and the bandgap of double perovskite Ba2SmSbO6 investigated by using in situ high-pressure synchrotron X-ray diffraction and Raman and ultraviolet–visible (UV–vis) absorption spectroscopy measurements up to 40 GPa. We found that pressure induces the phase transition from a cubic Fm-3m to a tetragonal I4/m at 8.6–12.8 GPa, as accompanied by the splitting and broadening of the diffraction peaks. The evolution of various modes in the Raman spectra and the enthalpy calculations support the phase transition of BaBa$_2$SmSbO$_6$ under compression. The analysis of UV–vis absorption spectroscopy reveals that the bandgap as a pressure of function is closely related to the phase transition. Calculation results demonstrate that the pressure-induced variation of the electronic structure mainly stems from the contribution of conduction states in BaBa$_2$SmSbO$_6$. Our investigations provide a fundamental understanding of the structure–property modulation in Ba2SmSbO6 under high pressure and will functionalize a new application─pressure sensor

Transparent ββ-SiN4_4 and γγ-Si3_3N4_4 compacts synthesized with mixed-size precursor under high pressure and high temperature

Transparent polycrystalline ceramics exhibit improved mechanical and optical properties. However, synthesizing transparent ceramics without additives is nontrivial. Herein, we report the synthesis of two transparent ceramics ($β$-SiN$_4$ and $γ$-Si$_3$N$_4$) under high pressure and high temperature from a pure Si$_3$N$_4$ precursor with nano-/micro-dual grain sizes. Synthesized $β$-SiN$_4$ exhibited a significantly enhanced Vickers hardness reaching 24.2 GPa (at 10 N load) when transparency was achieved. Transparent nano-grained $γ$-Si$_3$N$_4$ exhibited a Vickers hardness of 37.3 GPa. These are the highest hardness values reported for these two phases at a 10 N load. Density and microstructure measurements suggest that the hardness and transparency of the specimens correlate with both the grain size and porosity/density. The negligible amount of pores accounts for the superior optical transparency and high hardness of two Si$_3$N$_4$ allotropes. As higher pressures can effectively suppress grain growth and minimize pores between grains, high-pressure sintering is demonstrated as an effective way to realize highly dense transparent ceramics

Diffusion-induced stress optimization by boosted surface Li-concentration for single-crystal Ni-rich layered cathodes

Nickel-rich layered LiNi$_x$Co$_y$Mn$_{1-x-y}$O$_2$ (NCM, x ≥ 0.83) is considered as a promising cathode material for lithium-ion batteries (LIBs), owing to its satisfying specific energy. However, the undesired phase transformation from layered into rock-salt at the NCM surface will easily induce Li concentration gradient during cycling, which hinders the Li-ions diffusion and leads to the gradual accumulation of inner stress with the appearance of microcracks. Herein, we propose a surficial engineering strategy to promote the Li-ions transmission for single-crystalline LiNi$_{0.83}$Co$_{0.11}$Mn$_{0.06}$O$_2$ (SCNCM) via Li$_{1.3}$In$_{0.3}$Ti$_{1.7}$(PO$_4$)$_3$ (LITP) modification. It is noted that, as the fast Li-ion conductor, LITP can accelerate the Li-ions diffusion and alleviate the electrode–electrolyte side reaction simultaneously. More importantly, LITP can serve as the Li-ions regulator, ensuring the homogeneous distribution of Li-ions and minimizing the concentration difference at SCNCM surface. It can relieve the stress induced by the inconsistent Li-ions dispersion, which effectively decreases the degree of structural disordering and lattice mismatch at surface, eventually maintaining the high structure integrity during long-term cycling. As anticipated, even under the harsh testing conditions, the LITP modified SCNCM still can achieve a satisfied reversible capacity of 196.4 mAh g$^{−1}$ under potential range of 2.75–4.6 V after 200 cycles at 25 °C in coin-type half-cells. Furthermore, it provides an extraordinary capacity retention of 88% in 2.75–4.3 V after 400 cycles at high temperature of 45 °C in pouch-type full-battery

Pressure-induced bandgap engineering and photoresponse enhancement of wurtzite CuInS2CuInS_2 nanocrystals

Wurtzite CuInS2 exhibits great potential for optoelectronic applications because of its excellent optical properties and good stability. However, exploring effective strategies to simultaneously optimize its optical and photoelectrical properties remains a challenge. In this study, the bandgap of wurtzite CuInS2 nanocrystals is successfully extended and the photocurrent is enhanced synchronously using external pressure. The bandgap of wurtzite CuInS$_2$ increases with pressure and reaches an optimal value (1.5 eV) for photovoltaic solar energy conversion at about 5.9 GPa. Surprisingly, the photocurrent simultaneously increases nearly 3-fold and reaches the maximum value at this critical pressure. Theoretical calculation indicates that the pressure-induced bandgap extention in wurtzite CuInS$_2$ may be attributed to an increased charge density and ionic polarization between the In–S atoms. The photocurrent preserves a relatively high photoresponse even at 8.8 GPa, but almost disappears above 10.3 GPa. The structural evolution demonstrates that CuInS$_2$ undergoes a phase transformation from the wurtzite phase (P6$_3$mc) to the rock salt phase (Fm$\bar{3}$m) at about 10.3 GPa, which resulted in a direct to indirect bandgap transition and fianlly caused a dramatic reduction in photocurrent. These results not only map a new route toward further increase in the photoelectrical performance of wurtzite CuInS$_2$, but also advance the current research of A$^{I}$–B$^{III}$–C$^{VI}$$_2$ materials