High-density Phase Research Articles

Investigating densification processes of amorphous silica phases through activation energy distribution

We investigated thermal stability of silica glass that underwent high-temperature (HT), high-pressure (HP) densification followed with electron irradiations. We performed isothermal annealing and in situ Raman spectroscopy to observe changes in the silica structure. By determining the activation energy distributions at the root of the densification process, we aimed to gain insights into the relaxation pathway of densified silica and its relationship with the structural properties. Of particular interest we observed a non monotonous behavior of D2 band in Raman spectra in HP-HT samples exposed at low dose ≤107 Gy, which we attribute to presence of a specific phase of silica glass known as the high-density amorphous phase. Our study highlights the impact of irradiation on the ratio between high-density amorphous and low-density amorphous phases, and on relative activation energy distributions. These findings have important implications for development of high-performance optical devices based on silica densification for a wide range of applications.

Programmed Pore-Shape Fixing in a Soft Porous Molecular Crystal-Navigating the Phase Interconversion Landscape.

Pore-shape fixing effects (PSFEs) in soft porous crystals are a relatively unexplored area of materials chemistry. We report the PSFE in the prototypical dynamic van der Waals solid p-tert-butylcalix[4]arene (TBC4). Starting with the high-density guest-free phase, two porous shape-fixed phases were programmed using the stimuli of CO2 pressure and temperature. A suite of complementary in situ techniques, including variable-pressure (VP) single-crystal X-ray diffraction, VP powder X-ray diffraction, VP differential scanning calorimetry, volumetric sorption analysis, and attenuated total reflectance Fourier-transform infrared spectroscopy was used to track dynamic guest-induced transformations, providing molecular-level insight into the PSFE. The interconversion between the two metastable phases is particle size dependent, making this the second example of the PSFE by crystal downsizing, and the first example involving a porous molecular crystal: larger particles undergo reversible transitions while smaller particles remain fixed in the metastable phase. A complete phase interconversion scheme was constructed for the material, thus allowing navigation of the phase interconversion landscape of TBC4 using the easily applied stimuli of CO2 pressure and thermal treatment.

Microstructure and Properties of Non-Equiatomic Ni10Cr6WFe9TiAlx High-Entropy Alloys Combined with High Strength and Toughness

High-entropy alloys (HEAs) with excellent mechanical properties have broad application scope and application prospects. However, it is difficult to obtain the optimized element composition, based on the traditional equiatomic or near-equiatomic statistical analysis of the phase selection rules. The non-equiatomic HEAs have abundant constituents combination by optimizing the type and content of elements. In this study, Ni10Cr6WFe9TiAlx (x = 0, 1.0 and 1.5, at.%) HEAs were prepared by vacuum arc melting. The effect of Al content x on microstructure and mechanical properties of HEAs was systematically studied. The results show that the HEAs are composed mainly of face-centered cubic (FCC) with hexagonal Al2W phase. The increase of Al content promotes the formation of the hexagonal Al2W phase. When the Al mole content is 1.0, the Ni10Cr6WFe9TiAl HEA material has achieved superior mechanical properties. The alloy exhibited a high ultimate tensile strength of 741 MPa and a large total elongation of 46%. The improvement in the mechanical properties of the Ni10Cr6WFe9TiAl HEA is mainly attributed to the precipitation strengthening of the high-density Al2W phase. This work provides a reference for the future design of Al2W precipitation-strengthened non-equiatomic HEAs with ideal properties.

A novel Al-Mg-Zn(-Cu) crossover alloy with ultra-high strength

Improving the strength continues to be the key to the development of high-performance Al-Mg-Zn(-Cu) crossover alloys. Here, a novel ultra-high strength Al-5.64 Mg-5.43Zn-0.51Cu (wt.%) crossover alloy was designed by utilizing CALPHAD (Calculation of Phase Diagram) assisted design. The yield strength, tensile strength and elongation are 570.7 ± 2.7 MPa, 628.0 ± 9.6 MPa and 9.5 ± 0.2 %, respectively. Compared with reported Al-Mg-Zn based crossover alloys and typical ultra-high strength 7075-T6 alloy, the developed alloy demonstrates the highest yield strength (70.7 MPa higher than the second) with an acceptable elongation. This is because, the high-solute supersaturation of Mg, Zn and Cu elements promotes the precipitation of the high-density T-type phases (Mg32(Al,Zn,Cu)49), thus hindering dislocation movement and increasing the strength of the alloy. The developed ultra-high strength novel Al-Mg-Zn(-Cu) crossover alloy is expected to provide a desirable material option for the lightweight design of transportation industries.

Skyrmion crystal phases in Kondo lattice model on triangular lattices

Very often the magnetic skyrmions (topologically protected spin textures) form a triangular crystal in chiral magnets. Here we study the effect of itinerant electrons on the structure of skyrmion crystal (SkX) on triangular lattice using Kondo lattice model in the large coupling limit and treating the localized spins as classical vectors. To simulate the system, we employ hybrid Markov Chain Monte Carlo method (hMCMC) which includes electron diagonalization in each MCMC update for classical spins. We present the low-temperature results for system at electron density which show a sudden jump in skyrmion number reducing the size of the skyrmions when we increase the hopping strength of the itinerant electrons. We find that this high skyrmion number SkX phase is stabilized by a combined effect: lowering of density of states at electron filling and also pushing the bottom energy states further down. We show that these results hold for larger system using traveling cluster variation of hMCMC. We expect that itinerant triangular magnets might exhibit the possible transition between low-density to high-density SkX phases by applying external pressure.

Advanced Phase Change Materials from Natural Perspectives: Structural Design and Functional Applications.

Phase change materials have garnered extensive interest in heat harvesting and utilization owing to their high energy storage density and isothermal phase transition. Nevertheless, inherent leakage problems and low heat storage efficiencies hinder their widespread utilization. Nature has served as a great source of inspiration for addressing these challenges. Natural strategies are proposed to achieve advanced thermal energy management systems, and breakthroughs are made in recent years. This review focuses on recent advances in the structural design and functions of phase change materials from a natural perspective. By highlighting the structure-function relationship, advanced applications including human motion, medicine, and intelligent thermal management devices are discussed in detail. Finally, the views on the remaining challenges and future prospects are also provided, that is, phase change materials are advancing around the biomimicry design spiral.

Boosting the power factor and thermoelectric performance in eco-friendly Cu3SbS4 by twin boundary and grain boundary phase

Te/Se-free Cu3SbS4 is one of the most promising thermoelectric materials because of its earth-abundant, low-cost and environment-friendly characteristics. However, its thermoelectric performance is relatively low due to the high electrical resistivity. In this work, dense twin boundaries and grain boundary phases were constructed in the Ge-doped Cu3SbS4 materials by the melting reaction, sulfuration process, and plasma activated sintering. Density Functional Theory (DFT) calculations demonstrate that the Ge doping makes the Fermi level (EF) move from the forbidden-band into the valence band, thus significantly increasing carrier concentration. These twin boundaries don’t degrade the carrier mobility, leading to the enhanced electrical conductivity, thus obtaining a higher power factor of ∼ 14.4 μW cm−1 K−2 at 623 K. As a result, a peak ZT value of ∼ 0.78 at 623 K is achieved in the Cu3Sb0.975Ge0.025S4 sample, which is much higher than those of most metal sulfides. Moreover, high-density twin boundaries and grain boundary phases can inhibit the propagation of fatigue crack and enhance the fracture toughness. This modified approach can efficiently construct dense nanotwin boundaries in metal sulfides for high-performance thermoelectrics.

Experimental results of embedded phase change material capsules for increasing the performance of a wrapped heat pump water heater

To enhance the capacity of heat pump water heaters (HPWHs), high energy density phase change material (PCM) was used in two experimentally tested HPWH prototypes. Preliminary findings showed that in the PCM, stored thermal potential energy could be effectively converted into sensible heat in the water over a slight temperature difference, with the goal of increasing the capacity of hot water above a desired cut-off temperature. This exchange of energy results in an increased amount of hot water necessary for mixing at the location utilized by the consumer. It was also found that for a wrapped HPWH, there was an average increase of 27.7% in the capacity when 7.6 L of novel PCM filled spherical capsules were added to a 22.7 L tank downstream of the HPWH. In the second design (Beta), PCM capsules were immersed into the tank with a cylindrical geometry, and the UEF was increased by 5.5% over the baseline HPWH test result. As for the Beta design, heat loss was reduced significantly compared to a typical HPWH and the Alpha design. Food-grade PCMs were selected as appropriate. The novel encapsulation design increased the effective conductivity inside the capsule by adding metal fins.

Modulating Liquid-Liquid Transitions and Glass Formation in Zeolitic Imidazolate Frameworks by Decoration with Electron-Withdrawing Cyano Groups.

The liquid phase of metal-organic frameworks (MOFs) is key for the preparation of melt-quenched bulk glasses as well as the shaping of these materials for various applications; however, only very few MOFs can be melted and transformed into stable glasses. Here, the solvothermal and mechanochemical preparation of a new series of functionalized derivatives of ZIF-4 (Zn(im)2, where im- = imidazolate and ZIF = zeolitic imidazolate framework) containing the cyano-functionalized imidazolate linkers CNim- (4-cynanoimidazolate) and dCNim- (4,5-dicyanoimidazolate) is reported. The strongly electron-withdrawing nature of the CN groups facilitates low-temperature melting of the materials (below 310 °C for some derivatives) and the formation of microporous ZIF glasses with remarkably low glass-transition temperatures (down to only about 250 °C) and strong resistance against recrystallization. Besides conventional ZIF-4, the CN-functionalized ZIFs are so far the only MOFs to show an exothermic framework collapse to a low-density liquid phase and a subsequent transition to a high-density liquid phase. By systematic adjustment of the fraction of cyano-functionalized linkers in the ZIFs, we derive fundamental insights into the thermodynamics of the unique polyamorphic nature of these glass formers as well as further design rules for the porosity of the ZIF glasses and the viscosity of their corresponding liquids. The results provide new insights into the unusual phenomenon of liquid-liquid transitions as well as a guide for the chemical diversification of meltable MOFs, likely with implications beyond the archetypal ZIF glass formers.

Multi-principal element alloys with High-density nanotwinned 9R phase

Nanostructure design is an effective strategy for strength enhancement for metals and alloys. Nevertheless, stacking faults and nanotwins are not thermodynamically favoured. Here, it is demonstrated that high-density 9R phase with a columnar nanotwinned structure can be obtained through rational compositional design in a novel NiMnAlFeCr multi-principal element alloy. A combined density functional theory (DFT) study and molecular dynamics simulation (MD) was also performed to study the impact of alloy composition on the stability of 9R phase and its consequences on strength. The results showed that the energy barrier of formation of stacking fault can be obviously reduced by introduction of Fe or Cr in NiMn solid solution and its stability can be enhanced by simultaneously addition a certain range of Fe and Cr, which in favoring formation of high-density 9R phase.The experimental and simulation results demonstrate the substantial impact of 9R phase on the mechanical properties of the (Ni50Mn30Al20)100-x(FeCr)x alloys. This study provides a novel insight into nanotwinned structure as well as a promising strategy to design materials with high-strength.

Hydrogen Production via Cerium-Promoted Dehydrogenation of Formic Acid Catalyzed by Carbon-Supported Palladium Nanoparticles

Formic acid is a promising hydrogen carrier owing to its high hydrogen density and liquid phase. Therefore, the production of hydrogen from formic acid via catalytic dehydrogenation reaction has been extensively studied. Various methods such as alloying of the active Pd metal with other metals and adding dopants (e.g., the amine-functional group, nitrogen, or metal oxide) have been suggested to enhance the catalytic activities for formic acid dehydrogenation. In this study, we synthesized carbon-supported palladium nanoparticles coexisting with ceria particles and investigated the role of cerium in this reaction over Pd/C catalysts. We made an effort not to change the particle size and the electronic state of Pd significantly while varying Ceria/Pd molar ratios in the catalysts, ruling out their effects on the reaction. Ceria was impregnated on carbon by incipient wetness impregnation; subsequently, Pd was impregnated on Ceria/C by deposition–precipitation. The catalytic tests demonstrated that ceria improved the reaction rate by producing formate anions, the main reactant of this reaction, as it was dissolved by formic acid during the reaction. More importantly, the dissolved cerium ions accelerated the reaction by interacting with the formate anion to change its electronic charge, thereby reducing the activation barrier for C–H bond cleavage. The carbon-supported palladium nanoparticles promoted by cerium could be used to develop a highly efficient hydrogen extraction system from formic acid.

The effect of annealing on mechanical properties of selective laser melted CoCrNi medium-entropy alloy

The mechanical properties were evaluated in the selective laser melted CoCrNi medium-entropy alloy after the annealing at 800 °C. The as-selective laser melted sample delivered the yield strength of 820 MPa, the ultimate tensile strength of 1090 MPa and the fracture strain of 34%, While corresponding value for annealed sample at 800 °C for 4 h was 630 MPa, 870 MPa and 42%, respectively. High density dislocation induced high strength of as-selective laser melted sample, and higher ratio of the low-angle grain boundaries in sample annealed at 800 °C for 1 h impeded the dislocation movement and improved the strength. Moreover, high density dislocation and phase transformation from a face-centered cubic phase to a hexagonal close-packed phase in better recrystallization sample annealed at 800 °C for 4 h synergistically induced strength-ductility trade-off.

Nanoscale Phase Change Material Array by Sub-Resolution Assist Feature for Storage Class Memory Application

High density phase change memory array requires both minimized critical dimension (CD) and maximized process window for the phase change material layer. High in-wafer uniformity of the nanoscale patterning of chalcogenides material is challenging given the optical proximity effect (OPE) in the lithography process and the micro-loading effect in the etching process. In this study, we demonstrate an approach to fabricate high density phase change material arrays with half-pitch down to around 70 nm by the co-optimization of lithography and plasma etching process. The focused-energy matrix was performed to improve the pattern process window of phase change material on a 12-inch wafer. A variety of patternings from an isolated line to a dense pitch line were investigated using immersion lithography system. The collapse of the edge line is observed due to the OPE induced shrinkage in linewidth, which is deteriorative as the patterning density increases. The sub-resolution assist feature (SRAF) was placed to increase the width of the lines at both edges of each patterning by taking advantage of the optical interference between the main features and the assistant features. The survival of the line at the edges is confirmed with around a 70 nm half-pitch feature in various arrays. A uniform etching profile across the pitch line pattern of phase change material was demonstrated in which the micro-loading effect and the plasma etching damage were significantly suppressed by co-optimizing the etching parameters. The results pave the way to achieve high density device arrays with improved uniformity and reliability for mass storage applications.

Improved strength, creep resistance and recyclability of polyisoprene vitrimers by bottom-up construction of inhomogeneous network

Vitrimer elastomers bring huge chance to solve the recycling problem of conventional rubbers. However, their application is extremely limited by their lack of creep resistance. Although the crosslinking density can significantly affect the relaxation behavior of dynamic covalent networks, but limited by the tradeoff between mechanical strength and creep resistant properties. Here, we introduce inhomogeneous dynamic covalent network with regions of different crosslinking densities, in which the low crosslinking density mainly contribute to the creep resistance and the high crosslinking density contribute to the mechanical strength. In detail, block copolymers with polymyrcene and polyisoprene blocks and its control sample polyisoprene were prepared and epoxidized. The polymyrcene block is a double bond-rich block compared to polyisoprene block, which will become an epoxy group-rich block after epoxidation and a high cross-link density phase region after crosslinking with a dicarboxylic acid agent. Therefore, the cross-linked block copolymer will have an inhomogeneous dynamic covalent network, which were confirmed by the Multi-quantum (MQ) NMR tests, AFM images and tanδ measurement. And the heterogeneous structure becomes more obvious with the increased length of polymyrcene block. WAXD results show that cross-linked block copolymers have lower strain-induced crystallization onset strain and higher crystallinity, making them far stronger and tougher. In addition, block copolymer samples demonstrate lower creep and lower permanent set due to slow relaxation of the low crosslinking density regions. The recycling efficiencies of mechanical strength of block copolymer samples were higher than that of homopolymer. This work provides a useful approach for improving the mechanical properties and dimensional stability of vitrimer elastomers simultaneously.

Structural transformation and anomalous diffusion in network forming liquid GeO2

In this study, we simulated the structural transformation and self-diffusion mechanism in liquid GeO2 oxide system. Under compression, structure of liquid GeO2 model tends to transform gradually from low-density phase to high-density phase. Concentration of basic structural units can be determined via density of material model. The average Ge–O bond distance in GeO4 tetrahedra is smaller than the ones in GeO5 pentahedra and GeO6 octahedra. Each GeOx polyhedron always exists a Ge–O bond with the longest length (the weakest bond). The diffusion mechanism in liquid GeO2 oxide system is via breaking the weakest bonds that accompany the change local coordination environment of Ge cations. The longest Ge–O bond in a GeO5 pentahedron is very weak in comparison to the one in other polyhedra. The diffusivity is significantly dependent on the number of GeO5 pentahedra. The increase of GeO5 under compression is the origin of anomalous diffusion in liquid GeO2 oxide. The increase of average Ge–O bond distance under compression is also clarified in this work.

Microstructure evolution and deformation mechanism of coherent L12-strengthened high-entropy alloy during sliding wear

Face-centered-cubic high-entropy alloys (HEAs) strengthened with coherent ordered nanoprecipitates have demonstrated excellent strength-ductility synergy, even at elevated temperature. However, there still lacks fundamental understanding on their microstructure evolution and deformation mechanisms during dry sliding wear. Herein, we systematically investigated the friction and wear behaviors of CoCrNi2(Al0.2Nb0.2) alloy with high-density coherent L12 nanoprecipitates during sliding at room and elevated temperatures, with particular focus on wear-induced microstructure evolution. The alloy shows a low wear rate of 1.80 × 10−5 mm3/(N⋅m) at room temperature (RT) and even an ultralow wear rate of the order of 10−6 mm3/(N⋅m) at 600 °C. Detailed TEM analyses reveal that sliding-induced stacking faults (SFs) and dislocation cells play important roles in the formation of the gradient microstructure at RT and 600 °C, respectively. The superior wear resistance at RT is mainly attributed to the precipitation strengthening of high-density coherent L12 phase and the dynamic work-hardening of SF networks near the sliding surface. However, at 600 °C, the reduced wear rates and coefficients of friction are associated with the formation of glaze layer and the high resistance to thermal softening. This work provides significant insight into the sliding-induced microstructure evolution and deformation mechanism of L12-strengthened high-entropy alloys during sliding wear.

Testing the phase transition parameters inside neutron stars with the production of protons and lambdas in relativistic heavy-ion collisions

We demonstrate the consistency of the quark deconfinement phase transition parameters in the beta-stable neutron star matter and in the nearly symmetric nuclear matter formed in heavy-ion collisions (HICs). We investigate the proton and $\mathrm{\ensuremath{\Lambda}}$ flow in $\mathrm{Au}+\mathrm{Au}$ collisions at 3 and $4.5\text{ }\text{ }\mathrm{GeV}/\mathrm{nucleon}$ incident beam energies with the pure hadron cascade version of a multiphase transport model. The phase transition in HICs and neutron stars is described based on a class of hybrid equations of state from the quark mean-field model for the hadronic phase and a constant-speed-of-sound parametrization for the high-density quark phase. The measurements of the anisotropic proton flow at $3\text{ }\text{ }\mathrm{GeV}/\mathrm{nucleon}$ by the STAR Collaboration favor a relatively low phase transition density lower than $\ensuremath{\sim}2.5$ times saturation density indicated by the gravitational wave and electromagnetic observations of neutron stars. And the proton flow data at the higher energy of $4.5\text{ }\text{ }\mathrm{GeV}/\mathrm{nucleon}$ can be used to effectively constrain the softness of high-density quark matter equations of state. Finally, compared to the proton flow, the $\mathrm{\ensuremath{\Lambda}}$ flow is found to be less sensitive and not constraining to the equations of state.

Relativistic Hybrid Stars with Sequential First-order Phase Transitions in Light of Multimessenger Constraints

In this work, we consider the properties of compact stars in which quark matter has low- and high-density phases that are separated by a first-order phase transition. Thus, unlike the commonly considered case of a single phase transition from hadronic to quark matter, our models of hybrid stars contain sequential phase transitions from hadronic matter to low- and then to high-density quark matter phases. We extend our previous study of the parameter space of hybrid stars with a single phase transition to those with sequential phase transitions, taking into account the constraints on the mass and radius of neutron stars from the NICER experiment, the experimental inferences of the neutron skin thickness of the lead nucleus by the PREX-II experiment, and constraints on the tidal deformability from the gravitational-wave event GW170817. We determine the range of the masses for which both twin and triplet configurations, i.e., identical-mass stars with two and three different values of radii, arise.

Liquid-liquid phase separation in supercooled water from ultrafast heating of low-density amorphous ice

Recent experiments continue to find evidence for a liquid-liquid phase transition (LLPT) in supercooled water, which would unify our understanding of the anomalous properties of liquid water and amorphous ice. These experiments are challenging because the proposed LLPT occurs under extreme metastable conditions where the liquid freezes to a crystal on a very short time scale. Here, we analyze models for the LLPT to show that coexistence of distinct high-density and low-density liquid phases may be observed by subjecting low-density amorphous (LDA) ice to ultrafast heating. We then describe experiments in which we heat LDA ice to near the predicted critical point of the LLPT by an ultrafast infrared laser pulse, following which we measure the structure factor using femtosecond x-ray laser pulses. Consistent with our predictions, we observe a LLPT occurring on a time scale < 100 ns and widely separated from ice formation, which begins at times >1 μs.

Porous Single Crystals at the Macroscale: From Growth to Application.

ConspectusPorous materials have wide applications in the fields of catalysis, separation, and energy conversion and storage. Porous materials contain pores that are specifically designed to achieve expectant performance. The solid phases in porous materials are normally completely continuous to form the basic porous frame while the pores are fluid phase within the solid phase. Single crystals are macroscopic materials in three spatial dimensions with the constituent atoms, ions, molecules, or molecular assemblies arranged in an orderly repeating pattern with the ordered structures. The growth of single crystals is indeed a process to arrange these constituents in three dimensions into a repeating pattern within the materials. Today the applications of single crystals are exponentially growing in wide fields, and single crystals are therefore unacknowledged as the pillars of our modern technology. Introducing porosity into single crystals would be expected to create a new kind of porous material in which the basic porous frames are single-crystalline and free of grain boundaries. The structural symmetry is completely maintained within the basic porous frames which are a continuous solid phase, but it is completely lost inside the pores. The porous architecture is free of grain boundaries, and the fully interconnected skeletons are in single-crystalline states within the basic porous frames. Single crystals with porosities can therefore be considered to be a new kind of porous material, but they are single-crystal-like because the structural symmetry is maintained only in the skeletons and completely lost within the pores. We therefore call them porous single crystals or consider them in porous single-crystalline states to stand out with their structural features. Porous single crystals at the macroscale combine the advantages of porous materials and single crystals to incorporate both porosity and structural coherence in a porous architecture, leading to invaluable opportunities to alter the material's properties by controlling the unique structural features to enhance its performance. However, the growth of single crystals in three dimensions reduces the formation of porosities, leading to a fundamental challenge for introducing porosity into single crystals in a traditional process of crystal growth. In this Account, we report the rational design, growth methodology, and microstructural engineering of porous single crystals in a solid-solid transformation. We rationally design a high-density mother phase in a single-crystalline state and transform it into a low-density new phase in a single-crystalline state to introduce porosities into single crystals even incorporating the removal of specific compositions from the mother phase during the growth of porous single crystals. The porosity can be tailored by controlling the change in relative densities from the mother phase to the porous single crystals while the pore size can be engineered by controlling the fabrication conditions. Considering the unique structural features, we explore their functionalities and applications in photoelectrochemical energy conversion, electrochemical alkane conversion, and electrochemical energy storage. We believe that the materials, if tailored into porous single-crystalline states, would not only find a broad range of applications in other fields but also enable a new path for material innovations.