Low-temperature Form Research Articles

High-Strength Titanium-Based Alloy for Low-Temperature Superplastic Forming

Reducing the deformation temperature is an important research task for superplastic forming of Ti-based alloys. This study demonstrates that the additions of Fe and B significantly improve microstructural homogeneity and superplastic performance, increase the post-forming mechanical strength, and reduce the superplastic deformation temperature of a Ti-Al-Mo-V alloy. The designed alloy exhibits an excellent superplastic deformation behavior with elongation of 590 to 1050 pct at 675 °C to 775 °C with a constant strain rate in a range of 5 × 10−4 to 5 × 10−3 s−1, and a high room temperature yield strength of 1020 MPa, a UTS of 1080 MPa, and elongation-to-failure of about 6 pct both after annealing and after superplastic deformation with a strain of 0.69 at 775 °C. The microstructure and the strain-induced changes in the size and shape of grains are discussed. The modification of the β-phase morphology leads to an increase in the curvature of interphase boundaries in the modified alloy. Advanced superplasticity and improved mechanical properties make the studied alloy a very attractive material for complex parts in numerous advanced applications.

Immobilizing Pertechnetate in Ettringite via Sulfate Substitution.

Technetium-99 immobilization in low-temperature nuclear waste forms often relies on additives that reduce environmentally mobile pertechnetate (TcO4-) to insoluble Tc(IV) species. However, this is a short-lived solution unless reducing conditions are maintained over the hazardous life cycle of radioactive wastes (some ∼10,000 years). Considering recent experimental observations, this work explores how rapid formation of ettringite [Ca6Al2(SO4)3(OH)12·26(H2O)], a common mineral formed in cementitious waste forms, may be used to directly immobilize TcO4-. Results from ab initio molecular dynamics (AIMD) simulations and solid-phase characterization techniques, including synchrotron X-ray absorption, fluorescence, and diffraction methods, support successful incorporation of TcO4- into the ettringite crystal structure via sulfate substitution when synthesized by aqueous precipitation methods. One sulfate and one water are replaced with one TcO4- and one OH- during substitution, where Ca2+-coordinated water near the substitution site is deprotonated to form OH- for charge compensation upon TcO4- substitution. Furthermore, AIMD calculations support favorable TcO4- substitution at the SO42- site in ettringite rather than gypsum (CaSO4·2H2O, formed as a secondary mineral phase) by at least 0.76 eV at 298 K. These results are the first of their kind to suggest that ettringite may contribute to TcO4- immobilization and the overall lifetime performance of cementitious waste forms.

Particle Brownian motion due to random classical radiation: superfluid-like behavior in the presence of classical zero-point radiation

Nonrelativistic classical mechanics allows no fundamental transition between low-velocity and high-velocity forms of behavior, nor between low-temperature and high-temperature forms. In contrast, classical electrodynamics, which is a relativistic theory, allows fundamental transitions in velocity. Furthermore, the inclusion within the classical theory of Lorentz-invariant classical zero-point radiation allows classical electrodynamics to distinguish high-temperature and low-temperature forms of behavior. Because electromagnetism is a relativistic theory, it may provide a thermal radiation bath which gives rise to phenomena in Brownian motion which are not included in a model with a thermal bath based upon nonrelativistic particles. Here we explore the Brownian motion of a classical electric-dipole particle in random classical radiation, making use of the calculations of Einstein and Hopf. The Brownian motion as a function of temperature is analyzed in terms of the mean-square velocity and the diffusion constant for four different classical radiation spectra: the Rayleigh–Jeans spectrum, the Planck spectrum without zero-point radiation, the zero-point radiation spectrum, and the Planck spectrum including zero-point radiation. We illustrate how the inclusion of classical electromagnetic zero-point radiation alters Brownian motion behavior between high-temperature and low-temperature forms. For the Planck spectrum with zero-point radiation, the high-temperature Brownian motion agrees with some aspects found from nonrelativistic mechanics, while the low-temperature behavior includes some aspects analogous to superfluid behavior. At sufficiently low temperatures, the Brownian particle has an increasing mean-square velocity and more rapid diffusion with decreasing temperature due to the increasing dominance of the classical electromagnetic zero-point radiation.

Two polymorphs of a new intermetallic Ce2Rh2Ga – crystal structure and physical properties

We report on the synthesis and physical properties of the new ternary intermetallic Ce2Rh2Ga. Its formation and dimorphism have been investigated by powder and single crystal X-ray diffractometry, as well as by differential thermal analyses. LT- and HT-Ce2Rh2Ga can be obtained by the chosen respective thermal treatment. The high-temperature form (HT-Ce2Rh2Ga), stable above 864(5)°C, is orthorhombic La2Ni3-type [Cmce, oS20, a = 5.851(2) Å, b = 9.6184(19) Å, c = 7.4871(17) Å, V = 421.3(2) Å3]; the low-temperature form (LT-Ce2Rh2Ga), stable below this temperature, is monoclinic Pr2Co2Al- (Ca2Ir2Si-) type [C2/c, mS20, a = 10.0903(6) Å, b = 5.6041(3) Å, c = 7.8153(4) Å, β = 104.995(3)°, V = 426.88(4) Å3]. The magnetic measurements were conducted on two different samples, namely on the LT-Ce2Rh2Ga and the other on the HT-Ce2Rh2Ga sample. The HT-Ce2Rh2Ga compound is found to exhibit a phase transition at 128.5 K. By virtue of the sharp peak anomaly in the temperature dependence of the magnetic susceptibility this phase transition is plausibly of antiferromagnetic origin, but occurs at a remarkably high temperature compared to what is typical for Ce binary and ternary compounds. LT-Ce2Rh2Ga has a ground state of ferromagnetic nature which sets in at a paramagnetic-to-ferromagnetic phase transition at 5.2 K.

75LiF + 25SmF3 Eutectic Composite and Ionic Conductivity of SmF3 near the Polymorphic α–β Transition

75 mol % LiF + 25 mol % SmF3 eutectic composite is obtained by fusing components in a CF4 atmosphere. The composite is multiphase and consists of β-SmF3 (sp. gr. Pnma, a = 6.6703(2) A, b = 7.0557(2) A, c = 4.4005(2) A), LiF (sp. gr. $$Fm\bar {3}m$$, a = 4.0234(3) A), and a small amount (up to 4.5% of the total trifluoride content) of α-SmF3 (sp. gr. $$P\bar {3}c1$$, a = 6.946(1) A, c = 7.136(1) A). The conductometric measurements are carried out in a heating–cooling cycle in the range of 463–827 K. A first-order phase transition is detected upon heating at Ttr = 763 ± 15 K, which increases the conductivity σdc of the composite by a factor of ~10. After the experiment, the α-SmF3 phase content in the composite increases by a factor of almost 3. According to the differential scanning calorimetry data, the polymorphic transition in pure SmF3 component occurs at Tβ−α = 764.6 ± 0.2 K. The first-order transition in the composite is assigned to the polymorphic β-SmF3 → α-SmF3 transition. Upon cooling, a reverse transition from the high-temperature α-SmF3 (LaF3 type) to the low-temperature form β-SmF3 (β-YF3 type) is kinetically retarded. The enthalpy of σdc activation for α-SmF3 is ΔHσ = 0.37 ± 0.05 eV.

Crystal-to-crystal transitions in nylon M5T

Crystal-to-crystal transitions of a high-temperature nylon, poly(2-methtyl pentamethylene terephthalamide, nylon M5T), were studied by conventional and modulated temperature DSC (DSC and MT-DSC, respectively), as well as modulated thermomechanical analysis (MT-TMA), wide-angle X-ray scattering (WAXS), dynamic mechanical analysis (DMA) and hot stage microscopy. This polymer can easily be quenched to the completely amorphous phase at a cooling rate as low as 80 °C min−1. This polymer has three different crystal forms plus a smectic phase, and these crystal forms change into each other during a continuous heating of the amorphous polymer as evidenced by MT-DSC, MT-TMA, DMA and WAXS. The crystal-to-crystal transitions of two vastly different molecular masses were investigated: One was prepared by polycondensation followed by chain extension (see Menczel et al. J Thermal Anal 46:753, 1996), and the other sample was made by simple polycondensation. Tg of the amorphous polymer (including the as-spun fiber) is 143 °C, which is about 25 °C higher for the drawn fiber. The crystal-to-crystal transitions take place between 170 and 290 °C. MT-DSC and DMA measurements (the reversing heat flow signal and the tensile storage modulus of the as-spun fiber) proved that these crystal-to-crystal transitions take place through melting. Efficient nucleating agents (carbon nanofibers) do affect the temperature of these transitions as well as the melt-to-crystal transition during cooling of the melt and also during heating of the amorphous polymer. Polarization optical microscopy observations suggest that the highest temperature crystal form obtainable in high-temperature melt isothermal crystallization is probably a smectic, and this phase changes to some spherulitic structure during further cooling. Non-isothermal and isothermal crystallization measurements let us conclude that the two high-temperature crystal forms (smectic and crystal form C) are thermodynamically stable (enantiotropic), while the two low-temperature crystal forms are metastable (monotropic). The Hoffman–Weeks method gave 339 °C for the equilibrium melting point of this polymer. DMA experiments gave three secondary transitions. A transition at ~ − 120 °C was assigned to a local mode relaxation of the methylene groups. The transition at − 65 °C corresponds to the onset of rotation of the amide groups, while another secondary transition at 63 °C is the onset of rotation of the phenylene groups.

An Overview of the Thermomechanical Processing of α/β Titanium Alloys: Current Status and Future Research Opportunities

Current understanding of the principles underlying the thermomechanical processing (TMP) of α/β titanium alloys is reviewed. Attention is focused on the formulation of constitutive descriptions for plastic flow under hot-working conditions, the evolution of microstructure, the occurrence of defects, and novel/emerging TMP techniques. With regard to constitutive behavior, descriptions of the plastic flow of the individual phases and two-phase alloys per se are summarized. The important influence of phase morphology, size, and volume fraction on plastic flow is emphasized. Mechanisms which underlie microstructure evolution include beta recrystallization (in the high-temperature β field), the development of dislocation substructure and its effect on dynamic and static spheroidization of colony microstructures (in the two-phase field), static and dynamic coarsening of primary α, and the development of deformation and transformation textures. In the area of defects, the effect of TMP variables and starting microstructure on the formation of cavities, the persistence of microtexture, and the development of undesirably-coarse β grain structures are described. The current status of relatively new processing techniques for α/β titanium alloys such as low-temperature superplastic forming and solid-state joining (via linear friction or friction-stir methods) are also briefly reviewed. Last, R&D which could help to resolve deficiencies in the current knowledge base for TMP of α/β titanium alloys are summarized for each of the areas.

Experimental consequences of Bogoliubov Fermi surfaces

Superconductors involving electrons with internal degrees of freedom beyond spin can have internally anisotropic pairing states that are impossible in single-band superconductors. As a case in point, in even-parity multiband superconductors that break time-reversal symmetry, nodes of the superconducting gap are generically inflated into two-dimensional Bogoliubov Fermi surfaces. The detection and characterization of these quasiparticle Fermi surfaces requires the understanding of their experimental consequences. In this paper, we derive the low-energy density of states for a broad range of possible nodal structures. Based on this, we calculate the low-temperature form of observables that are commonly employed for the characterization of nodal superconductors, i.e., the single-particle tunneling rate, the electronic specific heat and Sommerfeld coefficient, the thermal conductivity, the magnetic penetration depth, and the NMR spin-lattice relaxation rate, in the clean limit. We also address the question whether the topological invariant of the Bogoliubov Fermi surfaces is associated with topologically protected surface states, with negative results. This work is meant to serve as a guide for experimental searches for Bogoliubov Fermi surfaces in time-reversal-symmetry-breaking superconductors.

Xonoltlite aggregate – part II. Properties of synthesis products

The aim of the work was to obtain crystalline xonotlite from cheap and commonly available raw materials, such as hydrated burnt lime and quartz flour, and properties of synthesis products. Crystalline xonotlite, without any unwanted carbonate phases, was only obtained after applying a protective atmosphere during on-going synthesis for 10 and 24 hours, at a pressure of 3 MPa. The product of autoclaving displays thermal stability up to 700°C and after exceeding this temperature, it is gradually dehydroxylated, and next, transformed into the low-temperature polymorphous form of wollastonite. The obtained xonotlite is characterized by slight thermal expansion, indicating its good dimensional stability up to a temperature of crystallization of wollastonite, reaching about 800°C, which makes it a good half-product for the production of light insulating materials.

News about thallium arsenates(V)

Systematic phase formation studies in the system Tl/As/O/(H) led to crystal growth of some previously reported and also to numerous novel hydrous and anhydrous phases. Their crystal structures were determined by single crystal X-ray diffraction. This includes the low-temperature forms of TlIH2AsO4 (isotypic with the corresponding phosphate homologues), the hydrogen arsenate TlI2HAsO4 (unique structure), the acidic phases TlIH2AsO4·H3AsO4 and TlI2(H6As4O14)·2H3AsO4 (both unique), the cyclo-tetrametaarsenate TlI4As4O12 (isotypic with its phosphate homologue), TlI8As8O24 (unique with an arsenate anion of novel constitution), the mixed-valent metaarsenate TlITlIIIAs4O12 (unique) and variscite-type TlIIIAsO4·2H2O. TlIH2AsO4, TlI2HAsO4, TlIH2AsO4·H3AsO4, TlI4As4O12 and TlIIIAsO4·2H2O comprise solely of tetrahedral AsO4 units, TlI2(H6As4O14)·2H3AsO4 and TlI8As8O24 comprise of tetrahedral AsO4 and octahedral AsO6 units, and TlITlIIIAs4O12 contains only AsO6 units. The TlI–O bond lengths span a wide range from ∼2.5–3.5 Å associated with coordination numbers between 4 and 12, whereas the TlIII–O bond length are characterized by a narrow range of 2.18–2.32 Å within octahedral coordination spheres.

Lattice thermal conductivity of quartz at high pressure and temperature from the Boltzmann transport equation

The thermal conductivities along the basal and hexagonal directions of α-quartz silica, the low-temperature form of crystalline SiO2, are predicted from the solution of the Boltzmann transport equation combined with the van Beest, Kramer, and van Santen potential for the temperature up to 900 K and the pressure as high as 4 GPa. The thermal conductivities at atmospheric pressure, which show a negative and nonlinear dependence on temperature, are in reasonable agreement with the experimental data. The influence of pressure on thermal conductivity is positive and linear. The pressure (P) and temperature (T) dependences of the thermal conductivity (λ) in basal and hexagonal directions are fitted to a function of the form λ=(b+cP)Ta. The thermal conductivity, influenced by temperature and pressure, is analyzed based on phonon properties, including spectral thermal conductivity, dispersion relation, phonon density of states, phonon lifetime, and phonon probability density distribution function.

Flow Stress and Hot Deformation Activation Energy of 6082 Aluminium Alloy Influenced by Initial Structural State

Stress-strain curves of the EN AW 6082 aluminium alloy with 1.2 Si-0.51 Mg-0.75 Mn (wt.%) were determined by the uniaxial compression tests at temperatures of 450–550 °C with a strain rate of 0.5–10 s−1. The initial structure state corresponded to three processing types: as-cast structure non-homogenized or homogenized at 500 °C, and the structure after homogenization and hot extrusion. Significantly higher flow stress appeared as a result of low temperature forming of the non-homogenized material. Hot deformation activation energy Q-values varied between 99 and 122 kJ·mol−1 for both homogenized materials and from 200 to 216 kJ·mol−1 for the as-cast state, while the Q-values calculated from the measured steady-state stress were always higher than those calculated from the peak stress values. For the extruded state of the 6082 alloy, the physically-based model was developed to reliably predict the flow stress influenced by dynamic softening, temperature, strain rate, and true strain up to 0.6.

Preparation, identification, and low-temperature infrared spectra of two elusive crystalline nitrile ices

Infrared (IR) spectra of the alkyl nitrile ices CH3CN and CH3CH2CN are relevant to the study of the low-temperature chemistry of objects within and beyond the Solar System, but the thermodynamically favored low-temperature crystalline phases of these compounds have not been presented and identified in the planetary-sciences literature. Moreover, there is a large variation in the published IR spectra of these two icy solids, including spectra that are used for the analyses of spacecraft data from the Voyager and Cassini missions. Here the IR spectra of the low-temperature crystalline forms of CH3CN and CH3CH2CN, which are the thermodynamically stable phases at Titan temperatures, are presented for the first time with all samples being made by vapor-phase deposition. Conditions are described for producing these ice phases for both compounds, and new measurements are reported of ice density and refractive index, quantities needed to compute IR absorption coefficients, band strengths, optical constants, and, ultimately, nitrile abundances in extraterrestrial environments. Comparisons are drawn between CH3CN and CH3CH2CN, an earlier prediction on their similar crystallization behaviors is verified, and previous work is examined in light of these new results, including a counter-intuitive observation in which the low-temperature phase of an ice is made by heating a high-temperature phase. Applications and extensions are described.

Analysis of Dihydrogen Bonding in Ammonium Borohydride

The structural and vibrational properties of ammonium borohydride, NH4BH4, have been examined by first-principles density functional theory (DFT) calculations and inelastic neutron scattering (INS). The H disordered crystal structure of NH4BH4 is composed of the tetrahedral complex ions NH4+ and BH4-, which are arranged as in the fcc NaCl structure and linked by intermolecular dihydrogen bonding. Upon cooling, the INS spectra revealed a structural transition between 45 and 40 K. The reversible transition occurs upon heating between 46 and 49 K. In the low-temperature form reorientational dynamics are frozen. The libration modes for BH4- and NH4+ are near 300 and 200 cm(-1), respectively. Upon entering the fcc high-temperature form, NH4+ ions attain fast reorientational dynamics, as indicated in the disappearance of the NH4+ libration band, whereas BH4- ions become significantly mobile only at temperatures above 100 K. The vibrational behavior of BH4- ions in NH4BH4 compares well to the heavier alkali metal borohydrides, NaBH4-CsBH4. DFT calculations revealed a nondirectional nature of the dihydrogen bonding in NH4BH4 with only weak tendency for long-range order. Different rotational configurations of complex ions appear quasi-degenerate, which is reminiscent of glasses.

The Yttrium, Lanthanum, and Lanthanide Trifluorides: Internal Periodicity of Phase Transitions

Internal periodicity of phase transitions (fusion and polymorphism) has been revealed in trifluorides of Y, La, and 14 lanthanides. This periodicity is determined by the internal periodicity of filling of the 4f-electron subshell of rare-earth elements (REEs, R), in which cerium 58Се–64Gd and terbium 65Тb–71Lu subsets can be selected. The lanthanide compression of ionic radii R3+ (15% towards the smaller one) induces the formation of three structure types: LaF3, β-YF3, and α-YF3 (α-UO3). The formula volumes Vform of REE trifluorides of the LaF3 and β-YF3 structure types only partially obey the internal periodicity of lanthanides with an increase in Z. The internal periodicity of the fusion and polymorphism of REE trifluorides (without ScF3) divides RF3 into four structural subgroups: A (R = La–Nd), B (R = Pm–Gd), C (R = Tb–Ho), and D (R = Er–Lu, Y), strictly specifying the percentage and elemental composition of each subgroup. The periodicity of the RF3 polymorphism in the products manifests itself in the crystallization of the melts prone to supercooling in the form of a periodic change of single crystals with coarse- or fine-grained blocks of the low-temperature forms.

Simulation of pressure tank leakage based on PHAST

Chemical plants usually store hazardous chemicals in high-pressure or low-pressure storage tanks in the form of pressurization, liquefaction, and low temperature. The poisonous gas in the tank is easily diffused. If a leakage accident occurs, it will cause personal injury and environmental pollution. In this paper, based on the propane acrylonitrile project in an industrial park, Major hazard identifications were identified for the tank area (risk level is Level 1), then PHAST software was used to simulate the leakage and diffusion of acrylonitrile storage tanks. It was concluded that the greater the wind speed, the leakage pore and the more stable atmosphere, the greater the leakage and diffusion of hazardous areas. And timely and intuitive prediction of the diffusion process and scope of toxic gases provide a scientific reference for taking corresponding countermeasures and on-site control after the accident.

Development of Metal Composite Powder Non-corrosive Flux for Low Temperature Forming of the Aluminum Brazing Filler Material

Development of Metal Composite Powder Non-corrosive Flux for Low Temperature Forming of the Aluminum Brazing Filler Material

Thermal analysis and crystal growth of doped Nb2O5

The systems Nb2O5–Ta2O5 and Nb2O5–V2O5 were investigated using thermal analysis, X-ray powder diffraction and thermodynamic simulations. Solid solution formation is possible for both systems; furthermore, both contain one intermediate compound, VNb9O25 or Ta2Nb4O15, respectively. Phase relationships for pure niobium(V)-oxide and tantalum(V)-oxide were studied under ambient pressure. It was found that both compounds can occur in two stable solid modifications. For niobium(V)-oxide this are the monoclinic high-temperature modification (H-Nb2O5) and an orthorhombic low-temperature modification (T-Nb2O5) and for tantalum(V)-oxide a tetragonal high-temperature form (α-Ta2O5) and an orthorhombic low-temperature form (β-Ta2O5). Based on these results, crystal growth experiments with various compositions from both systems were carried out using the optical floating zone (OFZ) technique.

A structural exploration of anisole accessed through extreme crystallisation conditions

A novel high pressure polymorph of anisole has been discovered and investigated with respect to the known low temperature form.

Photorefractive effect of low-temperature-grown aluminum gallium arsenide

The ternary compound aluminum gallium arsenide is an important material that can be used in all-optical solid-state ultrafast diagnostic technology. The low-temperature-epitaxially-grown AlGaAs (LT-AlGaAs) not only has the characteristics of ultra-short carrier lifetime of low-temperature-grown gallium arsenide (LT-GaAs), but also possesses the advantage of adjustability of band gap, which will provide great flexibility for the design of ultra-fast diagnostic systems. We use low-temperature epitaxial growth technology to grow AlGaAs on a GaAs substrate. The low-temperature-grown AlGaAs can effectively absorb 400 nm pump light to generate excess carrier. Therefore, we use a femtosecond laser with a wavelength of 800 nm and a pulse width of 200 fs as a light source to generate 400-nm pump light after passing through the BBO crystal, and 800 nm light without frequency doubling as the probe light. Using such a light source, we build a pump probe experimental platform to test the LT-AlGaAs. We normalize the experimental results and deconvolute it with the normalized laser pulses to obtain the response function of the semiconductor to the pump light. Therefore, we know that the nonequilibrium carrier relaxation time is less than 300 fs, and the nonequilibrium carrier recombination time is 2.08 ps. Due to the special passivation process, the effect of surface recombination on the carrier decay process is greatly reduced. The As clusters introduced by low-temperature epitaxial growth form deep level defects are the main factor for accelerating carrier recombination. In order to understand the complex process of photogenerated nonequilibrium carriers in depth, we use the indirect recombination theory of single recombination center to calculate the carrier recombination process, and establish an LT-AlGaAs carrier evolution model. Thus we obtain the key physical parameter related to the recombination rate, which is the carrier trapping area. We also use a theoretical model of carrier-regulated refractive index to calculate the effect of carrier concentration on the amount of change in refractive index. Combining our AlGaAs carrier evolution model, we simulate the refractive index change process of LT-AlGaAs after being illuminated by pump light. The simulation results are in good agreement with the experimental results. The method can be used for the quantitative analysis of carrier evolution characteristics of semiconductor materials, and it can conduce to the optimization and improvement of ultra-fast response semiconductor materials.