Low Bulk Modulus Research Articles

Toward Superior Stiffness in Carbon: The Interplay between Bond Strength and Anisotropic Response.

Carbon's ability to form diverse structures with varying hybridizations motivates the search for novel materials surpassing diamond's exceptional mechanical rigidity. We analyze previously predicted superhard phases and find that some exhibit average bond stiffness and density higher than those of diamond, hinting at potentially superior stiffness. However, these structures show a lower bulk modulus. We delve into this contradiction and demonstrate that it stems from the structures' anisotropic response to isotropic deformation. The concept of strain anisotropy is introduced to quantify this phenomenon. Our findings reveal a key connection between bond stiffness and bulk modulus in carbon materials, highlighting the importance of considering anisotropic behavior for a comprehensive understanding of the mechanical properties.

An experimental study on the influence of pressure on permeability and seismic velocity in the Montserrat geothermal field (West Indies)

Volcanic regions are prime areas for harnessing geothermal energy. However, the geological complexity of these regimes can hinder the efficient exploitation of this renewable resource. As useful as geophysical surveys are in understanding the subsurface, models can be unconstrained due to the lack of realisitic data. Here, we present the results of a new laboratory study measuring permeability and P-wave velocity on altered lavas, volcaniclastic sediments and limestones samples obtained from three cores retrieved from the MON-3 well, drilled in the Montserrat geothermal field. By measuring these parameters at increasing effective pressure varying from 10 to 70 MPa, we found permeability stress senstivity factors varying from 0.006 to 0.122 MPa−1 and increases in P- wave velocity between 2 and 20 %. The highest permeability stress senstivity factor and largest increase in P-wave velocity found in altered lavas were interpreted to be as a result of the easy closure and deformation of slit shaped microfractures and smectite clays characteristic of low bulk modulus. Conversely, lowest permeability stress senstivity factors and smallest increases in P-wave velocity found in volcaniclastic sediments, were coincident with intergranular pore geometries and higher bulk modulus minerals such as quartz and illite-smectite that may enhance the rigidity of the rock. Additionally, by measuring permeability and P-wave velocity on samples cored perpendicular (vertical) and parallel (horizontal) to the axis of the core, we found higher permeability senstivity factors (0.001–0.05 MPa−1 higher) and P-wave velocity increases (1–4 % higher) on horizontal samples, which is consistent with the preferential closure of horizontally oriented pore spaces. From this experimental study, we provide implications for enhancing geothermal energy recovery in Montserrat. Overall, our findings can be utilized to help improve on geophysical and numerical models of volcanic geothermal regimes.

Thermodynamic and elastic properties of laves phase AB2-based alloys and their hydrides: A density functional theory (DFT) study

A first-principles method based on the density functional theory (DFT) was employed in conjunction with quasi-harmonic Debye model to investigate the structural, elastic, and thermodynamic properties of a series of AB2-based Laves phases alloys and their hydrides. Simulation results revealed that the studied materials possess the Laves phase structure with lattice parameters comparable to experimental findings. For the first time, to the best of our knowledge, mixing entropy determined using Debye model was used to classify alloys into (Low- Medium-High Entropy Alloys) LEA, MEA and HEA categories rather than the commonly used ideal solution model, which is often inaccurate and ignores the impact of temperature. Lattice analyses of the alloy materials indicated that cell volume increases with the addition of elements, while the enthalpy of hydride formation indicates that hydrogen absorption in these alloys is exothermic and that the 3.00 H/F.U configuration is energetically stable. The alloys and their hydrides are metallic with no band gap at the Fermi level. The thermodynamic properties were studied using the quasi-harmonic Debye model and it was found that Bulk modulus decreases with increasing volume, and the hydrides possess lower bulk modulus compared to their metallic counterparts. Moreover, Debye temperature decreases with the gradual addition of elements, indicating weaker chemical bonds in ternary and other alloys. All hydrides have lower Debye temperature than their parent alloy materials. Finally, The alloys are classified into low-, medium-, and high-entropy alloys based on mixing entropy calculated using the Debye model. TiMn2 is classified as a low-entropy alloy, ZrMn2, Ti0.5Zr0.5Mn2, and Ti0.5Zr0.5MnFe as medium-entropy alloys, and Ti0.5Zr0.5(MnCr)2, Ti0.5Zr0.5Mn2/3Fe2/3Cr2/3, and Ti0.5Zr0.5Mn0.5Fe0.5Cr0.5Ni0.5 as high-entropy alloys.

Critical Role of Framework Flexibility and Disorder in Driving High Ionic Conductivity in LiNbOCl4.

Understanding Li-ion transport is key for the rational design of superionic solid electrolytes with exceptional ionic conductivities. LiNbOCl4 is reported to be one of the most highly conducting materials in the recently realized new class of soft oxyhalide solid electrolytes, exhibiting an ionic conductivity of ∼11 mS·cm-1. Here, we apply X-ray/neutron diffraction and pair distribution function analysis─coupled with density functional theory/ab initio molecular dynamics (AIMD)─to determine a structural model that provides a rationale for the high conductivity that we observe experimentally in this nanocrystalline solid. We show that it arises from unusually high framework flexibility at room temperature. This is due to isolated 1-D [NbOCl4]- anionic chains that exhibit energetically favorable orientational disorder that is─in turn─correlated to multiple, disordered, and equi-energetic Li+ sites in the lattice. As the Li ions sample the 3-D energy landscape with a fast predicted diffusion coefficient of 5.1 × 10-7 cm2/s at room temperature (σicalc = 17.4 mS·cm-1), the inorganic polymer chains can reorient or vice versa. The activation energy barrier for Li migration through the frustrated energy landscape is especially reduced by the elastic nature of the NbO2Cl4 octahedra evident from very widely dispersed Cl-Nb-Cl bond angles in AIMD simulations at 300 K. The phonon spectra are predominantly influenced by Cl vibrations in the low energy range, and there is a strong overlap between the framework (Cl, Nb) and Li partial density of states in the region between 1.2 and 4.0 THz. The framework flexibility is also reflected in a relatively low bulk modulus of 22.7 GPa. Our findings pave the way for the investigation of future "flex-ion" inorganic solids and open up a new direction for the design of high-conductivity, soft solid electrolytes for all-solid-state batteries.

Electronic, elastic, and thermodynamic properties of complex hydrides XAlSiH (X = Sr, Ca, and Ba) intended for hydrogen storage: an ab-initio study

The mechanical and thermodynamic properties of polyanionic hydrides XAlSiH (X = Sr, Ca, and Ba) were evaluated using density functional theory (DFT). The thermal parameters of XAlSiH hydrides, such as the Grüneisen parameter γ, heat capacity, and thermal expansion coefficient, were computed for the first time. The quasi-harmonic Debye model was used to determine these parameters over a range of pressures (0–40 GPa) and temperatures (0–1000 K). The gravimetric hydrogen storage capacities for BaAlSiH, SrAlSiH, and CaAlSiH were found to be 0.52%, 0.71%, and 1.05%, respectively. The hydrogen desorption temperatures for these compounds were also simulated at 748.90 K, 311.57 K, and 685.40 K. Furthermore, semiconducting behavior with an indirect bandgap value between 0.2 and 0.7 eV was exhibited by these compounds using the GGA and LDA approximation, and between 0.7 and 1.2 eV using the mBJ-GGA and mBJ-LDA approximation. Accurate elastic constants for single crystals were obtained from the calculated stress–strain relationships. The elastic constants for the XAlSiH compounds were significantly higher than those for other hydrides. The [001] direction was more compressible than the [100] direction in the hexagonal structure of XAlSiH. A lower bulk modulus than metallic hydrides was exhibited by these materials, indicating that XAlSiH compounds (X = Sr, Ca, and Ba) were highly compressible. The melting temperature for CaAlSiH was higher than that for SrAlSiH and BaAlSiH. Consequently, the decomposition temperature for XAlSiH (X = Sr and Ba) at which hydrogen was released from a fuel cell was lower than that for CaAlSiH. The bonding behavior of CaAlSiH was more directional than that of SrAlSiH and BaAlSiH. Brittle materials were XAlSiH (X = Sr, Ca, and Ba). Our PBE calculations yield linear compressibility and orientation-dependent Young’s modulus. Materials composed of hexagonal XAlSiH (where X represents Sr, Ca, or Ba elements) exhibit anisotropy in Young’s modulus but isotropy in bulk modulus.

Tailoring the Mechanical Properties of X–C–Si (X = Al, Mg) Alloys for Aerospace and Automotive Industries: An ab Initio Study

Silicon carbide (SiC) is a strong and adaptable material that has a good number of uses, including in the automotive industry for power electronics and electric vehicle components; abrasive material used in grinding and cutting; and in the production of refractory materials. These applications are due to the superior properties of SiC, which include high hardness, chemical inertness, and mechanical strength. However, its brittleness, low fracture toughness, as well as relatively high density hinder it from being applied in other areas such as in making bodies of airplanes as well as automobiles. Through careful alloying with lighter materials, this shortcoming can be addressed. Using an ab Initio approach, this study examined the mechanics of magnesium (Mg) and aluminum (Al) alloys with SiC as a potential candidate for the automotive and aerospace industries. Al or Mg atoms were substituted for some of the carbon atoms in the SiC to complete the alloying process. The results showed that the alloys had lower bulk moduli, shear moduli, Youngs moduli, and density compared to those of SiC. However, their ductility and fracture toughness increased. Although the mechanical properties reduced, they were found to be still much better than those of the common alloys for the aerospace. The alloys were thus found to be suitable for the construction of the shell of the airplanes, owing to their superior ductility and fracture toughness.

Stress redistribution in a multilayer chamber for compressed air energy storage in abandoned coalmine: Elastic analytical insights and material choice

AbstractCompressed air energy storage (CAES) is attracting attention as one of large‐scale renewable energy storage systems. Its gas storage chamber is one of key components for its success. A successful utilization of an abandoned coalmine roadway depends on the stability of the gas storage chamber. The chamber is a multilayer structure and the redistribution of the stress and displacement in each layer is critical to the chamber stability. So far, this redistribution mechanism under any lateral pressure coefficient and internal air pressure has been unclear and thus a quick and easy evaluation on the CAES chamber stability becomes difficult. In this study, the redistributions of stress and displacement in each layer are analytically solved based on complex variable function theory. First, the stress and displacement in three CAES multilayer structures are obtained under any lateral pressure coefficient and internal air pressure. Then, a comprehensive parameter b1 is obtained to describe the redistribution of stress and displacement in rock mass, lining layer, and grout layer. Its linkage with lateral pressure coefficient, internal air pressure, and material properties is analytically expressed. Thirdly, an analytical expression is obtained for the influence range of internal air pressure on chamber stress. Finally, the role and selection of lining and grouting layers are explored at different lateral pressure coefficient and internal air pressure. It is found that the CAES chamber stability can be effectively and quickly evaluated by these analytical solutions and comprehensive parameters and enhanced by a material with low bulk modulus but high shear modulus.

Simulation of ammonia injection system for a compression ignition engine

Ammonia in anhydrous and aqueous forms, are attractive carbonless fuels for future marine diesel engines. The aim of this study was to determine if these fuels could be injected into a two-stroke marine diesel engine using a unit injector system. In this study, existing in service injector hardware was used to develop a dynamic numerical injector model. The results of the simulation were then validated against injector performance literature obtained for an engine, using conventional diesel fuel oil. Using the developed numerical model, the injection performance of anhydrous and aqueous ammonia solutions could then be investigated. The study found good correspondence between the model and the pressure trace found in literature. In terms of fuel injection performances, it was found that both fuels were injectable with a unit injector, however the correct volume and mass required were not achieved. Furthermore, injection timing and duration were affected marginally. For aqueous ammonia, with the higher bulk modulus, injection was advanced by 0.21 CA degrees and for anhydrous ammonia with a lower bulk modulus; it was delayed by 0.21 CA degrees. Overall proper operation of the injection system was observed. This outcome is promising, especially in shifting the maritime industry towards zero carbon ammonia engines.

Structural, electronic, and elastic properties of RbI using the FP-LAPW method

The structural, mechanical, and electronic properties of rubidium iodide (RbI) have been extensively investigated utilizing the generalized gradient approximation (GGA) and the full-potential linearized augmented plane wave (FP-LAPW) approach. The potential was roughly calculated using a modified Becke–Johnson (mBJ) approximation, which increased the precision of the electronic properties. In this study, RbI is analyzed in a wide range of crystal structures, including topologies like rock salt (RS), CsCl, zinc blende (ZB), NiAs, and wurtzite (WZ), among others. Our research shows a strong relationship between the material’s physical properties and the conclusions drawn from both theoretical and experimental studies. Significantly, our results show that the RS form corresponds to RbI’s ground state. All the aforementioned topologies display wide-bandgap semiconductor capabilities, according to further examination of their electronic band structures. Notwithstanding these findings, it was discovered that RbI has a poor fracture resistance due to its low bulk modulus. Born’s stability analysis has shown that RbI is stable in the RS, CsCl, ZB, NiAs, and WZ structures. All RbI structures were discovered to have ionic bonding and to be ductile, and every stabilized system displayed anisotropic stability. Using the Cauchy pressure and Poisson’s ratio, the stiffness of the systems was evaluated, with the RS structure proving to be the stiffest. Overall, the findings illuminate the physical properties of RbI, providing valuable insights that could facilitate the creation and refinement of novel materials possessing desirable characteristics.

Mysteries of Water and Other Anomalous Liquids: “Slow” Sound and Relaxing Compressibility and Heat Capacity (Brief Review)

Reasons for the existence of “fast” sound at terahertz frequencies in various liquids have been analyzed. It has been shown that the fast sound speed is described well by the conventional formula from the theory of elasticity, where ρ is the density of a liquid andandare the bulk and shear moduli at the frequency ω, respectively. The excess of the speed of fast sound over the speed of normal sound in “normal” liquids is 10–20% and is almost completely determined by the contribution of the shear modulusat high frequencies, and vanishes on the Frenkel line. At the same time, the huge excess (50–120%) of the fast speed of sound over the speed of normal sound in some liquids (called “anomalous”), such as water and tellurium melt, is due mainly to the strong frequency dependence of the bulk modulus. Anomalously low relaxing bulk moduli were studied in our previous works for many oxide and chalcogenide glasses near smeared pressure-induced phase transitions. In anomalous liquids, smeared phase transitions also occur in a wide temperature and pressure region, which sharply reduces the bulk moduli and speeds of sound. Thus, the record large difference between speeds of fast and normal sound in anomalous liquids is due not to anomalously fast sound but to the fact that normal sound in such liquids is anomalously “slow” and bulk moduli are anomalously low. Ultrasonic studies of low- and high-density amorphous water ices show that their bulk moduli are indeed a factor of 4–5 higher than the bulk modulus of water. In addition, because of smeared phase transitions, the heat capacities of water and tellurium melt are a factor of 1.5–2 higher than those for normal liquids; i.e., anomalous liquids are characterized not only by an anomalous (nonmonotonic) behavior but also by anomalous magnitudes of physical quantities for most of the available measurement methods. A similar anomalous increase in the compressibility and heat capacity is observed for all fluids in the close vicinity of the liquid–gas critical point. In this case, anomalously fast sound is observed at terahertz frequencies, which is also due to a sharp increase in the bulk modulusat high frequencies. At the same time, high compressibility and heat capacity, as well as a large excess of the speed of fast sound over the speed of normal sound, for anomalous liquids and glasses near smeared phase transitions are not necessarily due to the proximity of critical points and occur in any scenario of the smeared phase transition.

Investigation of stability, electronic, optical and mechanical properties of honeycomb BeN2 monolayer: A DFT study

In this work, from the DFT calculations, we investigate the physical properties of the 2D BeN2 monolayer with a honeycomb lattice. The dynamic stability of the BeN2 monolayer is demonstrated by the presence of real modes in its phonon spectrum. The theoretical electronic band structure and density of states reveal the semiconductor nature of the BeN2 sheet. The semi-local PBE results show a bandgap of 1.32 eV, while HSE hybrid functional computation yields a higher bandgap of 3.16 eV. Our results suggest that HSE approximation employing ultrasoft pseudopotentials can accurately predict the most important electronic property of BeN2 structure that is not only in excellent agreement with the most accurate GW predictions but also better than those reported before with the same functional. The wide-bandgap semiconductor properties make BeN2 nanostructure a highly promising 2D material for innovative applications in nanoelectronic devices. The optical analysis obtained from HSE calculations reveals that the BeN2 nanostructure has optical absorption in the violet range of visible light with a low static dielectric constant of 2.55. Further, our results indicate that this material presents low bulk and shear moduli of 61.3 and 12.0 N/m, respectively.

Investigation of Structural, Mechanical, Optoelectronic, and Thermoelectric Properties of BaXF3 (X = Co, Ir) Fluoro-Perovskites: Promising Materials for Optoelectronic and Thermoelectric Applications.

Coded within Wien2K, we carry out DFT-based calculations for investigations of the structural, elastic, optoelectronic, and thermoelectric properties of BaXF3 (X = Co, Ir) fluoro-perovskites. The Birch-Murnaghan fit to the energy-vs-volume data and formation energy shows that these fluoro-perovskites are structurally stable. The phonon calculation confirms the thermodynamic stability, while the relation between elastic constants such as C 11 - C 12 > 0, C 11 > 0, C 11 + 2C 12 > 0, and B > 0 validates the mechanical stability of the compounds. BaIrF3 exhibits a strong ability to endure compressive and shear stresses. BaCoF3 shows a weaker capacity of withstanding changes in volume, attributed to a lower bulk modulus. Demonstrating a higher G-modulus of rigidity than the BaIrF3, BaCoF3 demonstrates stronger resistance to change the shape and both compounds are found to be anisotropic and brittle. The determined band structure profiles reveal that both BaCoF3 and BaIrF3 demonstrate a metallic nature. In addition, the metallic nature of BaCoF3 and BaIrF3 is reinforced by the density-of-states (DOS) study, where Co and F atoms contribute significantly to the total DOS in the valence band in the case of BaCoF3, while that of BaIrF3 is predominated by the Ba and F atoms. The computed values of ε1(0) for BaCoF3 and BaIrF3 are approximately 30 and 19, respectively, which are in line with Penn's model. The researched materials are confirmed to be strong contenders for optoelectronics by the lack of absorption in the visible range. For their potential use in thermoelectric device applications, thermoelectric parameters such as temperature-dependent Seebeck coefficient, specific heat capacity, thermal conductivity, power factor, and figure of merit are also investigated, which show that these materials are thermally stable and promising for applications in thermoelectric devices.

Pressure-Induced Monoclinic to Tetragonal Phase Transition in RTaO4 (R = Nd, Sm): DFT-Based First Principles Studies

In this manuscript, we report the density functional theory-based first principles study of the structural and vibrational properties of technologically relevant M′ fergusonite (P2/c)-structured NdTaO4 and SmTaO4 under compression. For NdTaO4 and SmTaO4, ambient unit cell parameters, along with constituent polyhedral volume and bond lengths, have been compared with earlier reported parameters for EuTaO4 and GdTaO4 for a better understanding of the role of lanthanide radii on the primitive unit cell. For both the compounds, our calculations show the presence of first-order monoclinic to tetragonal phase transition accompanied by nearly a 1.3% volume collapse and an increase in oxygen coordination around the tantalum (Ta) cation from ambient six to eight at phase transition. A lower bulk modulus obtained in the high-pressure tetragonal phase when compared to the ambient monoclinic phase is indicative of the more compressible unit cell under pressure. Phonon modes are calculated for the ambient and high-pressure phases with compression for both the compounds along with their pressure coefficients. One particular IR mode has been observed to show red shift in the ambient monoclinic phase, possibly leading to the instability in the compounds under compression.

Microstructure and phase evolution of micronized ceramic colorants from a pilot plant for inks production

The advent of inkjet printing as digital decoration of ceramic materials has irreversibly modified the industrial decoration technology, imposing companies to change the colorant production process. The inkjet application requires micronized particles in the ultrafine particle size range (smaller than 1 μm). Particles size reduction of ceramic colorants is performed by a high-energy comminution process in wet-operated bead mills, affecting colorants properties. Since a deep knowledge of milling-induced microstructural changes is still lacking, the micronization effects on a set of five industrial ceramic colorants are thoughtfully investigated in this work by simulating the industrial process at a pilot plant. Particle size distribution and energy consumption are monitored during the comminution process. The compositional (including crystallite size and microstrain analysis of the main phases) and morphological variation of four ceramic pigments (yellow zircon, brown spinel, pink malayaite, and green eskolaite) and one dye (blue olivine) is investigated by XRPD (Rietveld method) and SEM analyses. The analytical approach combined with a physical/semiempirical modelling of the colorants elastic features versus the energy demand for particle reduction has yielded details on the nature of the micronization-induced microstructural changes in ceramic colorants. Specifically, the comminution efficiency as well as the crystalline phase stability are related to the intrinsic properties of each colorant. Brittle breakage rather than plastic deformation on comminution are also system dependent. When an euhedral to subhedral crystal habit is maintained a brittle fracture is preserved throughout the comminution progress, while the formation of flake-like particles and particle agglomeration are strong evidences of plastic deformation. The last evidence deals with the material elastic features. Materials with high bulk modulus convert the grinding energy to lattice defects that lead to particle breakage by brittle fractures, while materials with lower bulk modulus convert/dissipate part of the supplied energy in plastic deformations, drastically decreasing the comminution process efficiency.

Physical Mechanism and Chemical Trends in the Thermal Expansion of Inorganic Halide Perovskites.

The considerable thermal expansion of halide perovskites is one of the challenges to device stability, yet the physical origin and modulation strategy remain unclear. Herein, we report first-principles calculations of the thermal properties of halide perovskites at 300 K using oxides as a reference. We found that the large thermal expansion of halide perovskites can mainly be attributed to their low bulk modulus and volumetric heat capacity because of the soft crystal lattice, whereas composition-dependent anharmonicity emerges as the most important factor in determining thermal expansion with the same structure. We discovered that thermal expansion of halide perovskites can be decreased by weakening the B-X bond to promote the octahedral anharmonicity. We further proposed an effective thermal expansion coefficient descriptor of halide perovskites with a Pearson correlation coefficient of nearly -80%. Our findings provide insights into the underlying mechanisms and chemical trends in the thermal expansion behavior of halide perovskites.

Measurement of Grain Bulk Modulus on Sandstone Samples From the Norwegian Continental Shelf

AbstractThe grain bulk modulus is a crucial property of a rock frame, determining poroelastic responses such as seismic velocity. Conventionally, the grain bulk modulus is estimated with bulk moduli of compositional minerals. A systematic measurement is still scarce in answering what factors control the grain bulk modulus. We test 38 sandstone samples with porosity ranging from 4% to 35% from three fields on the Norwegian continental shelf through unjacketed tests. The measured grain bulk moduli range from 18 to 61 GPa. The deposition and composition from thin sections and X‐ray diffraction data can interpret abnormally low or high grain bulk moduli of samples compared to quartz. Carbonate cement is a factor correlating the low porosity with the high grain bulk modulus in a range of 46–61 GPa for tight sandstones. Illite/chlorite coating on the quartz grains can enhance the result to 48 GPa; nevertheless, bearing clay at the grain‐to‐grain contact correlates with the low grain bulk modulus. The measured grain bulk modulus overall increases with decreasing porosity. Specifically, cementation and composition, such as calcite and clays, account for the correlation. Therefore, when people model the elastic properties of sandstones, direct usage of the quartz bulk modulus may cause significant errors. We suggest the unjacketed test as the most reliable way to determine grain bulk moduli of sandstones with different geological features.

An investigation on the dynamic characteristics of a two-dimensional piston flowmeter

The immediate measurement of dynamic flow rate is of importance in the modern hydraulic system. Following a brief review and analysis on measuring methods of dynamic flow rate, authors introduced the structure and working principle of a two-dimensional piston dynamic flowmeter. Consequently, a mathematical model was carefully established based on its unique mechanical properties due to the two degrees of working freedom. Meanwhile, the transform function is obtained to describe the dynamic characteristics and used to discuss influences from various design parameters. Finally, a 3D printed prototype was designed and fabricated to measure the sinusoidal flow rate with the frequency range of 1–7 Hz on a calibration test rig. The discrepancy between the experimental and the simulated date is quite foreseeable considering the major influence of low oil bulk modulus due to air bubbles. Although the magnitude reaches −3 dB at 4 Hz, the dynamic characteristics are acceptable and can verify the feasibility of applying the two-dimensional piston flowmeter on the dynamic measurement field. The shortcomings of this prototype are well discussed in the conclusion part to provide guidance to enhance the design ability of next-generation prototypes.

Discontinuous yielding transition of amorphous materials with low bulk modulus

The yielding transition of amorphous materials is studied with a two-dimensional Hamiltonian model that allows both shear and volume deformations. The model is investigated as a function of the relative value of the bulk modulus B with respect to the shear modulus μ. When the ratio B/μ is small enough, the yielding transition becomes discontinuous, yet reversible. If the system is driven at constant strain rate in the coexistence region, a spatially localized shear band is observed while the rest of the system remains blocked. The crucial role of volume fluctuations in the origin of this behavior is clarified in a mean field version of the model.

Highly compressible ceramic/polymer aerogel-based piezoelectric nanogenerators with enhanced mechanical energy harvesting property

Ceramic piezoelectric materials have orders of magnitude higher piezoelectric coefficients compared to polymers. However, their brittleness precludes imposition of large strains in mechanical energy harvesting applications. We report here that ice templating affords low bulk modulus lead-free aerogel piezoelectric nanogenerators (PENG) with unprecedented combination of flexibility and high piezoelectric response (voltage and power density). A modified ice templating protocol was used to fabricate piezoelectric nanocomposites of surface modified BaTiO3 (BTO) nanoparticles in crosslinked polyethylene imine. This protocol allowed incorporating a significantly high fraction of BTO particles (up to 83 wt %) in the aerogel, while retaining remarkably high compressibility and elastic recovery up to 80% strain. The output voltage, at an applied compressive force of 20 N (100 kPa), increased with BTO loading and a maximum output voltage of 11.6 V and power density of 7.22 μW/cm2 (49.79 μW/cm3) was obtained for PENG aerogels containing 83 wt% BTO, which is orders of magnitude higher than previously reported values for foam-based piezoelectric energy harvesters. The BTO/PEI PENGs also showed cyclic stability over 900 cycles of deformation. PENGs with higher porosity showed better elastic recovery and piezoelectric properties than lower porosity and higher BTO content aerogels. To the best of our knowledge, this is the first report to demonstrate the piezoelectric properties of high ceramic content aerogels having very high compressibility and elastic recovery.

A pressure induced reversal to the 9R perovskite in Ba3MoNbO8.5

The pressure response of Ba3MoNbO8.5 reveals a structural transformation, which acts to increase the energy barriers to migration along all available transport pathways, and an exceptionally low bulk modulus.