Low-temperature Phase Transition Research Articles

On the Origin of the Negative Thermal Expansion Behavior of YCu

Among the intermetallics and alloys, YCu is an unusual material because it displays negative thermal expansion without spin ordering. The mechanism behind this behavior that is caused by the structural phase transition of YCu has yet to be fully understood. To gain insight into this mechanism, we experimentally examined the crystal structure of the low-temperature phase of YCu and discuss the origin of the phase transition with the aid of thermodynamics calculations. The result shows that the high-temperature (cubic CsCl-type) to low-temperature (orthorhombic FeB-type) structural phase transition is driven by the rearrangement of three covalent bonds, namely, Y-Cu, Y-Y, and Cu-Cu, which compete for the bonding energy and phonon entropy. At low temperatures, the mixing of Y and Cu does not take place easily because of the weak attractive force between these atoms expected from the small negative mixing enthalpy. This causes all three interactions to take part in the bonding, and Y and Cu are segregated to form an FeB-type structure, which is stabilized by internal energy. At higher temperatures, Cu ions are bound loosely with Y ions due to the large Y-Cu distance (3.01 Å), which results in large vibration entropy and stabilizes a CsCl-type crystal structure. In addition, the CsCl-type structure is reinforced by the Y-Y interaction between next-nearest neighbors, resulting in a smaller unit cell volume. The crystal structure has the simple cubic framework of Y containing Cu ions bound loosely at the cavity sites. The calculated frequency of the Y-like phonon modes is much higher than that of the Cu-like modes, indicating the presence of Y-Y covalent interactions in the CsCl-type phase.

Experimental search for the origin of low-energy modes in topological materials

Point-contact spectroscopy of several non-superconducting topological materials reveals a low temperature phase transition that is characterized by a Bardeen-Cooper-Schrieffer-type of criticality. We find such a behavior of differential conductance for topological surfaces of non-magnetic and magnetic Pb$_{1-y-x}$Sn$_y$Mn$_x$Te. We examine a possible contribution from superconducting nanoparticles, and show to what extent our data are consistent with Brzezicki's et al. theory [arXiv:1812.02168], assigning the observations to a collective state adjacent to atomic steps at topological surfaces.

Controlled Thermoreversible Formation of Supramolecular Hydrogels Based on Poly(vinyl alcohol) and Natural Phenolic Compounds.

Supramolecular hydrogels have promising applications in a wide variety of fields including 3D bioprinting, sensors and actuators, biomedicine, and controlled drug delivery. This communication reports the facile reversible thermotriggered formation of novel pH-responsive supramolecular hydrogels based on poly(vinyl alcohol) (PVA) bonded via dynamic H-bridge with small phenolic biomolecules. PVA and phenolic compounds form a clear solution when they are physically mixed in water at high temperature, but a fast gelation is produced at room temperature through multiple strong H-bonding interactions. The structure and type of functional groups of different phenolic molecules allow preparing hydrogels with tailor-made viscoelastic properties, controlled low phase transition temperature, and pH-dependent swelling behavior. This combination makes these supramolecular networks very interesting candidates to be used in 3D bioprinting and topical drug delivery of thermolabile biomolecules.

High-Resolution Spectroscopy of Low-Temperature Phase Transitions in Copper Metaborate CuB2O4

The linear magnetic dichroism in the crystallographically isotropic ab plane of a tetragonal CuB2O4 single crystal has been investigated by high-resolution Fourier spectroscopy at temperatures of 1.4–22 K. Magnetic phase transitions are found at T < 2.0 K and their nature is discussed.

Вплив низькотемпературних фазових переходів на збереженість еритроцитів

Вплив низькотемпературних фазових переходів на збереженість еритроцитів

Biphenylyl-1,2,4-oxadiazole based liquid crystals – synthesis, mesomorphism, effect of lateral monofluorination

ABSTRACTAiming at developing a new type of materials for functional applications, three series of new compounds were prepared, 3-(biphenylyl)-1,2,4-oxadiazoles, Series I; laterally mono-fluorinated 3-(biphenylyl)-1,2,4-oxadiazoles, Series II; and 5-(biphenylyl)-1,2,4-oxadiazoles, Series III. Liquid crystalline properties of these compounds were studied using differential scanning calorimetry and polarizing optical microscopy. Ten of the thirteen compounds in Series I are mesogenic forming either N or SmA phases depending on the length-to-width molecular ratio. Mesogens of the positional isomers, Series III are both N and SmA. Monofluorination of the biphenylyl group at either the central or the terminal phenylene rings, Series III, resulted in the formation of nematic mesogens with low phase transition temperatures and broad nematic phase range.

A novel Na0.5K0.5NbO3–La(Zn0.5Ti0.5)O3 ceramics with excellent dielectric properties at wide temperature range

Abstract Ceramic-based dielectrics are considered as the best candidates for high temperature capacitors because of their outstanding mechanical and electrical properties. Nevertheless, conventional barium titanate-based capacitors show narrow operating temperature ranges owing to the low tetragonal-cubic phase transition temperature. In order to increase the working temperature and relative permittivity, a novel (1-x)Na0.5K0.5NbO3- xLa(Zn0.5Ti0.5)O3 (NKN-xLZT) ceramics were chosen to meet the targets in this work. The NKN-xLZT ceramics with sub-micrometer grains (0.2–0.4 μm) were synthesized via a conventional solid-state sintering route. A relative permittivity (e’ = 1560 ± 15%) with low loss tangent over wide temperature range from 96 °C to 350 °C was obtained in the x = 0.02 ceramics. Additionally, the crystal structure distortion and conduction behaviors of the NKN-xLZT ceramics were systematically studied. The decrease of oxygen octahedron distortion induced a weak polarization, and the high resistance (9 × 106 Ωcm at 400 °C) greatly suppressed the long-term migration of defective ions in the ceramics. Therefore, the low loss tangent and high permittivity were still stabilized at the high temperature. It believes that the NKN-xLZT ceramic system in this work will become one of the most promising candidates for high-temperature capacitor devices.

Effects of phase transition temperature and preheating on residual stress in multi-pass & multi-layer laser metal deposition

To investigate the influences of phase transition temperature and preheating on the residual stress of multi-layer and multi-pass laser metal deposition (LMD), the multi-layer and multi-pass LMD, with and without preheating, were performed using five kinds of alloy with different phase transition features, and their residual stresses were measured using the hole drilling method. A finite-element (FE) model incorporating the phase transition was developed based on experimentally obtained physical property data. The results demonstrated that the low-temperature solid phase transition has a tensile stress relaxation effect, which leads to the formation of a compressive stress area. This relaxation effect was observed to decrease with the increase of the phase transition temperature. The high-temperature solid phase transition has no significant tensile stress relaxation effect during the multi-layer and multi-pass LMD process, which is different from the single track LMD. when the solid phase transition temperature is low, the preheating can improve the uniformity of the stress field only to a certain extent. However, when the preheating increases the lowest temperature of the thermal cycle and makes it higher than the starting point temperature of the solid phase transition, the tensile stress relaxation effect of the solid phase transition can be brought into full play.

Preparation of smart glass with superhydrophobic and thermochromic properties

Abstract The practical application of VO2 smart glass is usually restricted by poor thermochromic property and contaminant. In this study, the VOx/SiO2 composite films were fabricated using magnetron sputtering and sol-gel preparations. Compared with VOx-based films, the VOx/SiO2 composite films showed the superhydrophobic property, and its water contact angle was up to 165°. At the same time, the VOx/SiO2 composite films retained the original excellent thermochromic property with solar regulation of 13.95%, luminous transmittance of 37.34% and the low phase transition temperature around 38℃. This study provided guidance for the combined application of VO2 smart glass and superhydrophobic coating.

Investigation of the changes in hydrogen bonding accompanying the structural reorganization at 103 K in ammonium iodate.

Neutron powder diffraction has been used to observe the changes in hydrogen bonding that occur as a function of temperature in ND4IO3 and, thus, determine the structural features that occur during the low-temperature (103 K) phase transition. It is shown that in the deuterated material the change is not a phase change per se but rather a structural reorganization in which the hydrogen bonding becomes firmly locked in at the phase transition temperature, and stays in this configuration upon further cooling to 4.2 K. In addition, both the differences and changes in the axial thermal expansion coefficients in the region 100-290 K can be explained by the changes involving both the hydrogen bonding and the secondary I...O halogen bonds.

Difference in magnetic and ferroelectric properties between rhombohedral and hexagonal polytypes of AgFeO2 : A single-crystal study

We have investigated magnetic and dielectric properties of rhombohedral $3R\ensuremath{-}{\mathrm{AgFeO}}_{2}$ and hexagonal $2\mathrm{H}\ensuremath{-}{\mathrm{AgFeO}}_{2}$ by using magnetic and dielectric bulk measurements and a neutron diffraction experiment with single crystals grown by hydrothermal synthesis. Magnetic phase transitions occur at $T=14.0$ K and $T=6.0$ K in $3R\ensuremath{-}{\mathrm{AgFeO}}_{2}$ and $T=17.0$ K and $T=9.5$ K in $2H\ensuremath{-}{\mathrm{AgFeO}}_{2}$ under zero magnetic field. Multistep metamagnetic phase transitions were observed in $3R\ensuremath{-}{\mathrm{AgFeO}}_{2}$ in magnetization measurements up to 60 T, while a single phase transition occurs in $2H\ensuremath{-}{\mathrm{AgFeO}}_{2}$. The ferroelectric polarization parallel and perpendicular to the triangular lattice plane appears below $T=6.0\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in $3R\ensuremath{-}{\mathrm{AgFeO}}_{2}$, which is concomitant with the onset of the cycloid magnetic ordering with the propagation vector $\mathbit{k}=(\ensuremath{-}\frac{1}{2},q,\frac{1}{2};q\ensuremath{\simeq}0.2)$ and the magnetic point group polar $m{1}^{\ensuremath{'}}$. On the other hand, the ferroelectric polarization is absent even below the lower phase transition temperature in $2H\ensuremath{-}{\mathrm{AgFeO}}_{2}$, which can be explained by the proper screw magnetic structure with $\mathbit{k}=(0,q,0;q\ensuremath{\simeq}0.4)$ and the nonpolar ${2221}^{\ensuremath{'}}$ point group. Although the two-dimensional triangular lattice layers of ${\mathrm{Fe}}^{3+}$ are common in the two polytypes, the magnetic and ferroelectric properties are significantly different. The emergence of ferroelectric polarization which is not confined to be within the plane of the cycloid for $3R\ensuremath{-}{\mathrm{AgFeO}}_{2}$ can be explained by the extended inverse Dzyaloshinskii-Moriya effect with two orthogonal components, ${\mathbit{p}}_{1}\ensuremath{\propto}{\mathbit{r}}_{ij}\ifmmode\times\else\texttimes\fi{}[{\mathbit{S}}_{i}\ifmmode\times\else\texttimes\fi{}{\mathbit{S}}_{j}]$ and ${\mathbit{p}}_{2}\ensuremath{\propto}{\mathbit{S}}_{i}\ifmmode\times\else\texttimes\fi{}{\mathbit{S}}_{j}$. Unlike other delafossite compounds, the ${\mathbit{p}}_{2}$ component is not allowed in the proper screw phase of $2H\ensuremath{-}{\mathrm{AgFeO}}_{2}$ due to the symmetry restriction of the parent space group.

Searching for Order Parameter of Low-temperature Phase Transitions in Divalent Nitrates

Searching for Order Parameter of Low-temperature Phase Transitions in Divalent Nitrates

Low-temperature phase transitions of sodium aluminate solutions

A series of low-temperature phase transitions of sodium aluminate solutions were studied by differential scanning calorimetry (DSC) and Raman spectroscopy. The results indicate that NaOH concentration is a primary impact factor for the binary eutectic point and ice melting temperature of sodium aluminate solutions with low NaOH concentration. In addition, the phase transition process of sodium aluminate solutions with low NaOH concentration from 123.15 to 283.15 K is divided into four steps: non-crystal to crystal, ternary eutectic reaction, binary eutectic reaction and ice melt. The projection phase diagram of NaOH-Al(OH)3-H2O system at low temperature was plotted, in which the ternary eutectic temperature for sodium aluminate solutions is 183.15 K.

Cool-SPS stabilization and sintering of thermally fragile, potentially magnetoelectric, NH4FeP2O7

Cool-SPS conditions (low temperature/high pressure) can be used to densify materials with limited thermodynamical stability, due to low temperature phase transition, melting, decomposition… We recently demonstrated the potential of SPS for the stabilization and densification of diversified materials at very low temperatures. Here we report the sintering of a potentially magnetoelectric pyrophosphate with low temperature decomposition. Characterization of its magnetic properties are presented, and dielectric/magnetoelectric characterization could also be performed thanks to the high densification obtained at temperatures as low as 200 °C.

High-throughput thermodynamic computation and experimental study of solid-state phase transitions in organic multicomponent orientationally disordered phase change materials for thermal energy storage

Understanding the behavior of solid-solid phase transformation in phase change materials is crucial to design advanced thermal energy storage materials. It is however challenging to study the complex solid-solid phase transition and predict the optimal composition with reliable performances (such as high energy storage at invariant phase transition temperature) in multi-component systems based on traditional empirical rules. Therefore, the high-throughput computational framework by coupling thermodynamic calculation via CALPHAD (CALculation of PHAse Diagrams) methodology with key experimental validation is firstly proposed for ternary Pentaglycerine-Tris-(hydroxymethyl)-aminomethane-2-amino-2-methyl-1,3-propanediol (PG-TRIS-AMPL) system. A self-consistent thermodynamic database of PG-TRIS-AMPL ternary system has been firstly assessed and validated by experimental measurements from in-situ X-Ray-Diffraction (XRD) and Differential Scanning Calorimetry (DSC). Using this thermodynamic database via CALPHAD method, the high-throughput calculation has been performed to seek the optimal composition in PG-TRIS-AMPL ternary system. It is found that two optimal ternary compositions in PG-TRIS-AMPL ternary system are designed as PG0.33TRIS0.07AMPL0.60 (ΔHmax = 137.5 kJ/kg) during the 1st invariant reaction αAMPL-rich+βPG-rich→δTRIS-rich+γAMPL-rich at 326.5 K (53.35 °C) and PG0.58TRIS0.069AMPL0.351 (ΔHmax = 52.44 kJ/kg) during the 2nd invariant reaction γAMPL-rich+βPG-rich→δTRIS-rich+γ’PG-rich at 338.4 K (65.25 °C), respectively. This finding shows the good balance between high latent heat storage and low invariant phase transition temperature at mid-temperature (20–100 °C) application. Also, the present high-throughput computation approach can be extended into other multicomponent systems for various temperature range.

Tungsten doped M-phase VO2 mesoporous nanocrystals with enhanced comprehensive thermochromic properties for smart windows

The phase transition temperature (~68 °C) of M-VO2 film can be lowered significantly by tungsten (W) doping into the crystal lattice of VO2 due to the reduction of the strength of V-V pair interaction. However, W doping was always coupled with a serious weakening of luminous transmittance and solar modulation efficiency because W dopants can increase the electron concentration of VO2 film. Herein, the simultaneous introduction of W dopants and mesopores into M-VO2 nanocrystals was employed to prepare VO2 film. Interestingly, the obtained 0.4 at% W-doped mesoporous VO2 nanocrystals based composite films exhibited enhanced comprehensive thermochromic performance with excellent solar modulation efficiency (ΔTsol = 11.4%), suitable luminous transmittance (Tlum = 61.6%) and low phase transition temperature around 43 °C, much lower than 65.3 °C of undoped VO2. It was demonstrated that the lower phase transition temperature of VO2 can be primarily attributed to abundant lattice distortion after W doping, whereas the mesoporous structure can facilitate the uniform distribution of W dopants in VO2 nanocrystals, enhance the luminous transmittance and guarantee enough VO2 nanocrystals in the composite film to keep relatively high solar modulation efficiency. Therefore, this work can provide a new way to balance the three important parameters for the thermochromic performance of VO2 film (ΔTsol, Tlum and Tc) and probably promote the application of VO2 nanocrystals in the energy efficient windows.

New insights into the thermal behavior and decomposition of sodium propionate

Na-propionate (NaC2H5CO2) was synthesized from Na2CO3 dissolved in propionic acid. Its thermal behavior was studied in argon and in air. In both atmospheres, two solid-solid phase transitions occur below 250 °C and are followed by melting near 290 °C. Thermal decomposition begins with release of CO2 attributed to the decomposition of a limited amount of propionyl radicals. Elemental carbon results from this process. This initial reaction is followed by a faster conversion reaction into Na2CO3 with evolution of 3-pentanone, CO2, CO, CH4 and C2H4 as found during pyrolysis in argon. In air, oxidative decomposition takes place with CO2 and H2O as main released gases. The stability of Na-propionate is reduced in air compared to argon atmosphere. The reaction paths are discussed and compared with previously published results on Na-propionate and other propionate metalorganic compounds. The potential of using the low temperature phase transitions of Na-propionate for thermal energy storage (ΔH ≈ 245 J/g) was evaluated by submitting samples to 25 heating/cooling cycles.

The novel rufigallol-based liquid crystals with cholesterol units: synthesis, mesomorphic and photophysical properties

ABSTRACTThree novel rufigallol derivatives with four alkyl chains and two cholesteric units 6a, 6b and 6c were prepared in yields of 83–88%. The mesomorphic properties were investigated by DSC, POM and XRD analysis, suggesting that compounds 6a, 6b and 6c possess the ordered hexagonal columnar mesophase. The long alkyl chains contribute to the low phase transition temperature. Sample 6c with four dodecyl chains and two cholesterol units is a room temperature liquid crystal between 31°C and 87°C. Compounds 6a, 6b and 6c also have good fluorescence emission between 400 nm and 500 nm. The studies on CD spectra implied that the chiralities of cholesterol moieties are successfully transferred to the liquid crystalline core. Samples 6a, 6b and 6c are the first fluorescence liquid crystal based on rufigallol core with chiral cholesterol units.

Insight into understanding structural relaxation dynamics of [NH2NH3][Mn(HCOO)3] metal-organic formate

We report the synthesis, thermal and dielectric measurements of [NH2NH3][Mn(HCOO)3] (HyMn) compound. Above room temperature this polymeric material undergoes two phase transitions at ∼360 K and ∼298 K, as observed via DSC and BDS spectra. The first high temperature phase transition is associated with the paraelectric to ferroelectric transition of perovskite HyMn. The low temperature phase transition corresponds to the paraelectric to antiferroelectric transition of chiral HyMn. However, mechanisms hidden behind the observed two polymorphic phases are not completely clear yet. Dielectric spectroscopy measurements have revealed formation of clusters and superclusters as well as the relaxor-like behavior of this compound in wide temperature range resulting from both chiral and perovskite phases. Analysis of dielectric permittivity spectra obtained for the investigated material showed the generalized Mittag-Leffler two-power-law relaxation pattern that was interpreted by means the stochastic scenario of correlated-clusters. The proposed approach brought into light the presence of polar nanoregions in the material suggesting the relaxor nature of the existing ferroelectricity.

Medium-Chain Polyprenols Influence Chloroplast Membrane Dynamics in Solanum lycopersicum.

The widespread occurrence of polyprenols throughout the plant kingdom is well documented, yet their functional role is poorly understood. These lipophilic compounds are known to be assembled from isoprenoid precursors by a class of enzymes designated as cis-prenyltransferases (CPTs), which are encoded by small CPT gene families in plants. In this study, we report that RNA interference (RNAi)-mediated knockdown of one member of the tomato CPT family (SlCPT5) reduced polyprenols in leaves by about 70%. Assays with recombinant SlCPT5 produced in Escherichia coli determined that the enzyme synthesizes polyprenols of approximately 50-55 carbons (Pren-10, Pren-11) in length and accommodates a variety of trans-prenyldiphosphate precursors as substrates. Introduction of SlCPT5 into the polyprenol-deficient yeast Δrer2 mutant resulted in the accumulation of Pren-11 in yeast cells, restored proper protein N-glycosylation and rescued the temperature-sensitive growth phenotype that is associated with its polyprenol deficiency. Subcellular fractionation studies together with in vivo localization of SlCPT5 fluorescent protein fusions demonstrated that SlCPT5 resides in the chloroplast stroma and that its enzymatic products accumulate in both thylakoid and envelope membranes. Transmission electron microscopy images of polyprenol-deficient leaves revealed alterations in chloroplast ultrastructure, and anisotropy measurements revealed a more disordered state of their envelope membranes. In polyprenol-deficient leaves, CO2 assimilation was hindered and their thylakoid membranes exhibited lower phase transition temperatures and calorimetric enthalpies, which coincided with a decreased photosynthetic electron transport rate. Taken together, these results uncover a role for polyprenols in governing chloroplast membrane dynamics.