Low-temperature Form Research Articles

Liquid and crystallized phases stability in the sub-system H3PO4[sbnd]H4P2O7: Experimental determination and modeling

The study of the system formed by ortho- and pyrophosphoric acid was resumed in order to understand the crystallization conditions of these two compounds and to highlight the existence of their possible polymorphism. To this end, the solid-liquid equilibria (SLE) of pyrophosphoric acid was studied in depth. Contradictions in literature data were resolved through systematic experimentation: solubility measurements, differential scanning calorimetry (DSC), and X-ray diffraction (XRD). Calorimetric measurements confirmed the existence of two crystalline forms of pyrophosphoric acid, and their stability domains were determined. Furthermore, thermodynamic modeling of the SLE has led to a consistent and refined representation of the observed phenomena. In particular, the transition temperature from low-temperature (form I) to high-temperature form (form II) of pyrophosphoric acid was determined at 298.4 K and the coordinates of the eutectic point common between H3PO4 and H4P2O7 (I) were precisely determined. Modeling also confirms the non-negligible quantity of triphosphoric acid in the liquid state throughout virtually the entire compositional range. Finally, X-ray powder diffraction data were used to determine the cell parameters and space group of pyrophosphoric acid using EXPO 2014 software.

Revisiting the thermodynamic properties of the ZrCr2 Laves phases by combined approach using experimental and simulation methods

A comprehensive study of ZrCr2 Laves phases is important for both fundamental research and technological applications. The ZrCr2 compound is a typical precipitate observed in Zr-based alloys including the newly developed Cr-coated Zr cladding known as Accident Tolerant Fuel cladding. The brittle behavior of the Laves phases and the low melting point at the interface zone between the precipitates and the Zr matrix are considered as governing the thermomechanical behaviors of these newly developed nuclear fuel claddings for normal operating conditions but more dramatically in case of Loss Of Coolant Accident (LOCA) conditions. The aim of the present study is to investigate the various ZrCr2 Laves polymorphs and their thermodynamic features particularly where conflicts exist in the literature. Based on diffraction experiments of annealed samples, it has been established that the transformation from the C14 high-temperature form to the C15 low-temperature form is of displacive nature without the C36 polymorph forming as an intermediate phase. The stable and metastable domains of C14 and C15 and their thermodynamic properties have been determined. The specific heat of both C14 and C15 types was measured over a wide range of temperatures from 2 to 1063 K by coupling relaxation calorimetry and DSC. The experimental data were fitted using a modified Einstein model. The room temperature entropies of the C14 and C15 phases were evaluated. The enthalpy of formation was measured for the first time using drop solution calorimetry in liquid Al at 1173 K in a Tian–Calvet calorimeter. The experimental value is in good agreement with Density Functional Theory (DFT) calculations. Finally, all these new results are discussed regarding the abundant literature on the ZrCr2 Laves phases.

High-Resolution 17O Solid-State NMR as a Unique Probe for Investigating Oxalate Binding Modes in Materials: The Case Study of Calcium Oxalate Biominerals.

Oxalate ligands are found in many classes of materials, including energy storage materials and biominerals. Determining their local environments at the atomic scale is thus paramount to establishing the structure and properties of numerous phases. Here, we show that high-resolution 17O solid-state NMR is a valuable asset for investigating the structure of crystalline oxalate systems. First, an efficient 17O-enrichment procedure of oxalate ligands is demonstrated using mechanochemistry. Then, 17O-enriched oxalates were used for the synthesis of the biologically relevant calcium oxalate monohydrate (COM) phase, enabling the analysis of its structure and heat-induced phase transitions by high-resolution 17O NMR. Studies of the low-temperature COM form (LT-COM), using magnetic fields from 9.4 to 35.2 T, as well as 13C-17O MQ/D-RINEPT and 17O{1H} MQ/REDOR experiments, enabled the 8 inequivalent oxygen sites of the oxalates to be resolved, and tentatively assigned. The structural changes upon heat treatment of COM were also followed by high-resolution 17O NMR, providing new insight into the structures of the high-temperature form (HT-COM) and anhydrous calcium oxalate α-phase (α-COA), including the presence of structural disorder in the latter case. Overall, this work highlights the ease associated with 17O-enrichment of oxalate oxygens, and how it enables high-resolution solid-state NMR, for "NMR crystallography" investigations.

Optimizing Stainless Steel Bearings: Enhancement of Stainless Steel Bearing Fatigue Life by Low-Temperature Forming

A proposed low-temperature forging method is presented to enhance stainless steel bearings by creating a martensitic subsurface layer, significantly boosting bearing fatigue life due to increased surface hardness. This technique induces beneficial residual stresses, particularly in axial bearings, streamlining their construction and improving machine elements. Challenges persist, especially with radial bearings, but simplicity in axial bearing forging promotes compact, resource-efficient facility construction. Future research will focus on applying this technique to axial bearing washers, potentially replicating success in other bearing components. Despite the energy expenditure on cooling during forging, the substantial increase in bearing fatigue life offsets this, enhancing overall durability and reliability of critical machine components. Integration of this forging technique into bearing fabrication appears seamless, offering a promising trade-off between energy use and enhanced performance.

Cryo-compression and annealing hardening of 7075 aluminum alloy

7075 aluminum alloy, as a lightweight, high-strength, and corrosion-resistant structural material, is widely used in the aviation industry. Because 7075 aluminum alloy is commonly used in structural components, its plastic processing ability is highly required by the usage environment. Recent studies have shown that 7075 aluminum alloy exhibits better deformation ability under low temperature conditions. In this paper Cryo-compression experiments on 7075 aluminum alloy under different compression rates (0.001 s-1-0.5 s-1) in a low-temperature environment (soaked in liquid nitrogen for 30 minutes) has been discussed. Annealing process research was conducted on the low-temperature compressed samples, and the optimal annealing process parameters (400Co, insulation for 15 minutes) and optimal deformation amount (16%) were obtained through comparison of microstructure and properties. And it was found that 7075 aluminum alloy exhibited annealing hardening phenomenon when annealed at 250 Co, and the reason was the diffusion segregation of solid solution elements formed during low-temperature forming, which led to the precipitation of the second phase MgZn2 during low-temperature annealing.

Insights into promotional effects for ethylbenzene dehydrogenation to styrene with steam over Fe-K, Fe-K-Ce and Fe-K-Ce-Mo mixed oxide catalysts

In ethylbenzene dehydrogenation industrial applications, Fe-K based especially Fe-K-Ce and Fe-K-Ce-Mo based mixed oxides are the most widely used catalysts. But the corresponding reaction mechanism for promoters are still unclear. Herein, the model Fe-K, Fe-K-Ce, Fe-K-Ce-Mo catalysts were prepared by wet granulation method and the insights into promotional effects were clarified by using various characterizations and in situ DRIFTs studies. Our results suggest that Ce addition improved catalytic activity and Mo modification increased styrene selectivity. The Fe-K-Ce showed outstanding redox ability profited from abundant active oxygen species and reducible species. The Fe-K-Ce-Mo achieved optimal activity and selectivity because of the moderate redox behavior. The ethylbenzene adsorption at low temperature form coke deposits and enhanced oxidizing ability of modified catalysts facilitate CO2 producing thus improve catalyst coke resistance. Besides, the activation induction periods of modified catalysts were shortened based on accelerated formation of main active phase and redox equilibrium. Moreover, the steam significantly promoted styrene species generation and further shorten activation induction periods, which was involved in real reactions instead of just eliminating coke.

Fast heating inhibits endothermic solid-solid polymorphic transition giving a melting of low temperature polymorph with the next cold crystallization

Heating at the rate of several hundred Kelvin per second in fast scanning calorimetry (FSC) experiments was found to change the route of polymorphic transformation. Upon the increase of the heating rate first the endothermic solid-state polymorphic transition is hindered to give a consecutive melting of low-temperature form and crystallization of high temperature polymorph. At higher heating rates, only melting of low-temperature polymorph is observed much below an equilibrium melting point. This thermal behavior with depression of final melting point by 30 K was observed for tert‑butylthiacalix[4]arene tetrasubstituted with (ethoxycarbonyl)methoxy groups (1), which does not sublimate at heating. A similar shift of the endothermic solid-state polymorphic transition by temperature at fast heating was found for caffeine (2), however the melting of low-temperature polymorph is not observed due to sublimation of the compound at elevated temperatures. The heating rates, at which a significant depression of the melting points of 1 and 2 may be expected in absence of sublimation, were calculated using equations of thermal kinetics derived from the data of conventional differential scanning calorimetry. The results give a practical guide for the preparation of glassy forms of the substances capable of endothermic solid-solid transition below melting point of high-temperature form.

Entropic Control of Bonding, Guided by Chemical Pressure: Phase Transitions and 18-n+m Isomerism of IrIn3.

As with other electron counting rules, the 18-n rule of transition metal-main group (T-E) intermetallics offers a variety of potential interatomic connectivity patterns for any given electron count. What leads a compound to prefer one structure over others that satisfy this rule? Herein, we investigate this question as it relates to the two polymorphs of IrIn3: the high-temperature CoGa3-type and the low-temperature IrIn3-type forms. DFT-reversed approximation Molecular Orbital analysis reveals that both structures can be interpreted in terms of the 18-n rule but with different electron configurations. In the IrIn3 type, the Ir atoms obtain largely independent 18-electron configurations, while in the CoGa3 type, Ir-Ir isolobal bonds form as 1 electron/Ir atom is transferred to In-In interactions. The presence of a deep pseudogap for the CoGa3 type, but not for the IrIn3 type, suggests that it is electronically preferred. DFT-Chemical Pressure (CP) analysis shows that atomic packing provides another distinction between the structures. While both involve tensions between positive Ir-In CPs and negative In-In CPs, which call for the expansion and contraction of the structures, respectively, their distinct spatial arrangements create very different situations. In the CoGa3 type, the positive CPs create a framework that holds open large void spaces for In-based electrons (a scenario suitable for relatively small T atoms), while in the IrIn3 type the pressures are more homogenously distributed (a better solution for relatively large T atoms). The open spaces in the CoGa3 type result in quadrupolar CP features, a hallmark of low-frequency phonon modes and suggestive of higher vibrational entropy. Indeed, phonon band structure calculations for the two IrIn3 polymorphs indicate that the phase transition between them can largely be attributed to the entropic stabilization of the CoGa3-type phase due to soft motions associated with its CP quadrupoles. These CP-driven effects illustrate how the competition between global and local packing can shape how a structure realizes the 18-n rule and how the temperature can influence this balance.

Anapole, chiral, and orbital states in Mn3Si2Te6

The ferrimagnet Mn3Si2Te6 attracts attention because of a recently discovered colossal magnetoresistance (CMR) with unique magnetic field properties. An improved magnetic structure for the material has emerged from a neutron diffraction study linked to understanding the CMR. A deeper theoretical investigation of the magnetic structure has now revealed anapole, chiral and orbital states of manganese ions not previously mentioned. Moreover, it is shown that existence of these states in the low temperature form of Mn3Si2Te6, with a magnetic field applied, can be tested by neutron and resonant x-ray diffraction.

Ag-Ion Dynamics in the Low-Temperature Form of Ag2S as Studied by Impedance Spectroscopy

In this study, Ag2S samples are characterized using X-ray diffraction, X-ray photoelectron spectroscopy, differential scanning calorimetry, and impedance spectroscopy. At 447 K, an AC conductivity jump of about two orders of magnitude is observed, and an endothermic peak is measured by differential scanning calorimetry, apparently resulting from the β-to-α phase transition, i.e., the order–disorder transition. A silver vacancy proportion of 2.2% is obtained by refinement of the X-ray diffraction profile. The dielectric loss peaks and analysis of the observed complex impedance peaks indicate four different relaxation processes, namely, P1, P2, P3, and P4, from low to high temperature. P1 and P2 are obviously the migration of silver ions from two types of lattice sites to their nonequivalent nearest-neighbor vacancies. P3 involves the migration of interstitial silver ions to adjacent interstitial sites. An Arrhenius-to-VFTH crossover of the P4 relaxation time is inferred to the hopping of the individual silver ion and then the cooperative jumps of several ions in the disordered region of the silver sublattice upon heating.

Sintering and Crystallization Intensifiers for Production of Ceramic Paving Blocks by Vibropressing Technology

The article presents the results of research on application of sintering and crystallization initiators based on a composition of blast-furnace granulated slag and glass wastes in the ceramic masses for production of ceramic paving blocks by vibro-pressing. The leading role of sintering and crystallization initiators in assuring the strong and dense structure of ceramic pieces was updated. The main laws of changes in physical and mechanical properties of ceramic paving stone samples depending on the amount of sintering and crystallization initiators in the burning temperature range 950–1000 °C have been established. It was determined that the availability of finely dispersed glass powder (fraction less than 0.1 mm) as a component of crystallization sintering initiators contributes to early emergence of liquid phase in the ceramic mass, as softening temperature of glass powder begins already at 720–750 °C. According to the results of X-ray phase and electron microscopic analysis it was determined that crystallization of low-temperature form of ß-wollastonite (CaSiO3) is observed in the samples burnt at the temperature range of 950–1000 °C. It was proved that the availability of ß-wollastonite in the ceramic mass serves as a reinforcing component. It has been established that high strength values are achieved in those compositions where ß-wollastonite crystallization in the burning products is the highest. As a result of scientific and experimental work the feasibility of producing the ceramic paving stones by vibropressing containing sintering and crystallization initiators that meet the requirements of quality, aesthetics, environmental friendliness, resource- and energy-saving was proved.

Observation of multigap and coherence peak in the noncentrosymmetric superconductor CaPtAs: As75 nuclear quadrupole resonance measurements

We present synthesis and $^{75}$As-nuclear quadrupole resonance (NQR) measurements for the noncentrosymmetric superconductor CaPtAs with a superconducting transition temperature $T_c$ of $\sim 1.5$ K. We discovered two different forms of CaPtAs during synthesis; one is a high-temperature tetragonal form that was previously reported, and the other is a low-temperature form consistent with the orthorhombic structure of CaPtP. According to the $^{75}$As-NQR measurement for superconducting tetragonal CaPtAs, the nuclear spin-lattice relaxation rate $1/T_1$ has an obvious coherence peak below $T_c$ and does not follow a simple exponential variation at low temperatures. These findings indicate that CaPtAs is a multigap superconductor and a large $s$-wave component.

Noncovalent bonding assessment by pair distribution function.

Noncovalent interactions are essential in the formation and properties of a diverse range of materials. However, reliably identifying noncovalent interactions remains challenging using conventional methods such as X-ray diffraction, especially in nanocrystalline, poorly crystalline or amorphous materials which lack long-range lattice periodicity. Here, we demonstrate the accurate determination of deviations in the local structure and tilting of aromatic rings during the temperature-induced first order structural transition in the 1 : 1 adduct of 4,4'-bipyridinium squarate (BIPY:SQA) from the low temperature form HAZFAP01 to high temperature HAZFAP07 by X-ray pair distribution function. This work demonstrates how pair distribution function analyses can improve our understanding of local structural deviations resulting from noncovalent bonds and guide the development of novel functional materials.

Di-Iron(II) [2+2] Helicates of Bis-(Dipyrazolylpyridine) Ligands: The Influence of the Ligand Linker Group on Spin State Properties.

Four bis[2-{pyrazol-1-yl}-6-{pyrazol-3-yl}pyridine] ligands have been synthesized, with butane-1,4-diyl (L1 ), pyrid-2,6-diyl (L2 ), benzene-1,2-dimethylenyl (L3 ) and propane-1,3-diyl (L4 ) linkers between the tridentate metal-binding domains. L1 and L2 form [Fe2 (μ-L)2 ]X4 (X- =BF4 - or ClO4 - ) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe2 (μ-L1 )2 ][BF4 ]4 exhibit three different helicate conformations, which differ in the torsions of their butanediyl linker groups. The solvates exhibit gradual thermal spin-crossover, with examples of stepwise switching and partial spin-crossover to a low-temperature mixed-spin form. Salts of [Fe2 (μ-L2 )2 ]4+ are high-spin, which reflects their highly twisted iron coordination geometry. The composition and dynamics of assembly structures formed by iron(II) with L1 -L3 vary with the ligand linker group, by mass spectrometry and 1 H NMR spectroscopy. Gas-phase DFT calculations imply the butanediyl linker conformation in [Fe2 (μ-L1 )2 ]4+ influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms.

Superplastic forming of titanium alloys at 700°C

The formability of the novel Ti-1.5Al-1.5V-2.75Fe-3.0Mo-0.25Ni-0.1B alloy and the commercial Ti-6Al-4V alloy at 700°С was evaluated in the present study. Experiments have demonstrated the possibility of superplastic forming of both alloys with a fine-grained microstructure at 700°C and a constant argon gas pressure p = 2 MPa. However, the forming time for the novel alloy was only a few minutes (τ = 235 s), while for the Ti-6Al-4V it amounted almost 2.5 hours (τ = 8700 s). The better formability of the novel alloy at 700°С compared to the commercial one is in good agreement with the results of computer simulations performed earlier and is mainly due to an increased content of the β phase. Microstructural analysis showed that low-temperature superplastic forming was accompanied by the development of spheroidization and recrystallization processes. It is important that recrystallization did not lead to a significant grain growth and provided an increase in the structure homogeneity and randomization of the rolling texture. The most significant improvement of microstructure homogeneity after forming at 700°C was observed in the Ti-6Al-4V alloy with the initial partially recrystallized structure.

A dislocation density-based model considering dynamic strain aging for annealed and water-quenched aluminum alloys under low-temperature conditions

Low-temperature forming is a novel manufacturing method for aluminum alloy components. To better understand the flow characteristics and deformation behavior of aluminum alloys at low temperatures, uniaxial tensile tests were conducted with an initial strain rate range of 2.5 × 10−4–1 × 10−2 s−1 and a temperature range of 77–298 K for annealed (O) and water-quenched (WQ) 2060 Al–Cu–Li alloys. The results showed that the two tempered alloys exhibited excellent work hardening at lower temperatures, resulting in improved ductility, especially in the WQ-tempered alloy. Dynamic strain aging (DSA) caused by interactions between solute atoms and moving dislocations occurred at higher temperatures and lower strain rates, which resulted in serrated flow. Because moving dislocations were additionally pinned by solute clusters, the flow stress increased. The stress increase and negative strain rate caused by DSA could not be captured using the typical dislocation density-based K-M model. Thus, a modified K-M model that considered DSA was proposed to describe the tensile behavior of the studied alloys, and the stress-strain values predicted by the proposed model agreed well with the experimental ones.

Structural phase transition, optical and electrical properties of the hybrid material [(C2H5)4N]2ZnI4

The bis (tetra-ethylammonium) tetraiodo-zincate (II) was prepared by slow evaporation an aqueous stoichiometric mixture of tetra-ethylammonium iodide and zinc iodide at room temperature. This compound has been characterized by X-ray diffraction (XRD), thermal analysis, UV–vis spectroscopy and complex impedance spectroscopy. Single crystal X-ray diffraction analysis shows that the title compound crystallizes in the tetragonal system with space group P4¯21m.The differential thermal analysis (DTA) and temperature-controlled X-ray diffraction study reveal a reversible first order phase transition at about 444 ​K between low temperature-tetragonal and high temperature-orthorhombic forms of [(C2H5)4N]2ZnI4. Below 444 ​K, the low temperature form exhibits two linear regimes of thermal expansion with a change in the slope around 373 ​K which is ascribed to a second-order phase transition. The diffuse reflectance indicates the existence of an optical direct transition with the band gap energy is equal to 3.9 ​eV. The spectra of complex impedances obtained in the temperature range from 403 to 468 ​K were modelled by a combination of R//CPE elements. . A change of conduction regime around 444 ​K is noted in the variation of the Ln (σDC.T) versus reciprocal temperature. In both regimes, the conductivity obeys the Arrhenius law with activation energies of 1.37 ​eV in regime I and 2.37 ​eV in regime II. Frequency dependence of the AC conductivity followed the universal Jonscher's power law.

A Vanadium Dioxide-PMMA Composite For Microwave Radiation Switching.

Reconfigurable radio-frequency components are in high demand for modern communication systems as they can be involved in multiband and multistandard electronic devices. The key part of such components is an active switching element. This work offers a way to obtain an efficient microwave switch using vanadium dioxide-poly (methyl methacrylate) composite. Differential scanning calorimetry, SQUID magnetometery, and impedance spectroscopy measurements were used to characterize the phase transition in the proposed composite. Temperature induced metal-insulator transition occurs at technologically attractive 341 K. The transition leads to a change of microwave transmission trough VO2 -PMMA composite from -4.9 dB for low-temperature monoclinic form to -5.8 dB for high-temperature rutile form. This provides an ability to tune the material's transparency in the microwave range, while the shaping polymer matrix provides the proper mechanical processability of the switching element.

Single-crystal-to-single-crystal phase transition of 18β-glycyrrhetinic acid isopropyl ester.

Due to the destruction of the integrity of the parent crystal, single-crystal-to-single-crystal phase transition in organic compounds is still a relatively rare phenomenon. The phase transition in glycyrrhetinic acid isopropyl ester is triggered by temperature change. The increasing volume of the isopropyl substituent as a result of increasing temperature forces a remodelling of the structural motifs. These changes cause a single-crystal-to-single-crystal phase transition. The low-temperature form is isostructural with glycyrrhetinic acid methanol solvate, while the high-temperature phase is isostructural with the ethyl ester of this acid.

High-Pressure Structural Behavior of para-Xylene

A high-pressure neutron powder diffraction study was conducted on perdeuterated para-xylene (C8D10). para-Xylene crystallizes in the monoclinic crystal system (space group P21/n) at ambient temperature and ca. 0.1 GPa. The structure is consistent with the known low-temperature form. No further phase transitions were observed in the pressure range 0.11(1)–4.72(2) GPa. A complementary high-pressure single-crystal diffraction experiment was performed on hydrogenous para-xylene confirming the assigned space group P21/n. An isothermal equation of state was obtained [bulk modulus, B0 = 3.5(4) GPa] and structural changes of the material have been investigated as a function of pressure. This experimental study is supported by dispersion-corrected density functional theory calculations.