Structural Transformation Temperature Research Articles

Fabricating highly stable and reliable vanadium dioxide thin films: Insights from radio frequency magnetron sputtering

This study establishes an optimization protocol to fabricate a stoichiometric VO2 thin film onto the glass substrate using a single process step in RF magnetron sputtering. An interplay between the substrate temperature and the oxygen-to-argon ratio in process gas is shown to achieve a range of VOx composition (from oxygen-rich to oxygen-deficient phases). The re-sputtering phenomenon is found to be crucial at higher temperature (T ≥ 873 K), leading to a shift in stoichiometry from V2O5 to V2O3. Moreover, a process model for the fabrication of various VOx compounds in thin film form is also established in this study. The crystal structure transformation from M1 to R phase for these VO2 thin films deposited over a range of process conditions is found to be 339 ± 1 K, in close agreement with the reported value for the bulk VO2. In spite of the microstructural variations, these films are found to show consistent phase transition behavior, excellent stability, and reliability in repeated thermal cycles. These films show a semiconductor-to-metal transition, which is correlated to their structural phase transformation temperature. Moreover, this study also establishes a correlation between various external stimuli, widening the usage of this material in a range of applications.

Microstructure and texture control of Ni-Mn-Ga magnetic shape memory alloys manufactured by laser powder bed fusion

The Laser Powder Bed Fusion (LPBF) process was used to manufacture Ni-Mn-Ga polycrystalline samples. The initial powders with a fine particle size of about 20 µm were firstly prepared by a ball milling from melt-spun ribbons. The microstructure and texture evolution of Ni-Mn-Ga alloy manufactured by LPBF were investigated employing SEM, TEM, and synchrotron radiation diffraction. Using two different scanning strategies, optimization of laser parameters and post-processing heat treatment, a homogeneous microstructure with a strong crystallographic texture was obtained. The localized heating/cooling conditions allow obtaining a layered structure with preferred <100> fiber orientations along the growth direction. The crystal structure and crystallographic texture drastically change when the laser oscillation mode is chosen. The strong crystalline anisotropy and layered-type microstructure have a significant impact on twining flow behavior giving rise to an anisotropic mechanical response. The variant reorientation during mechanical training is realized by the so-called orthogonal shearing. It is also shown that the resulting crystal structure and characteristic transformation temperatures are strongly dependent on chemical composition related to Mn losses and internal stresses along with chemical order creating metastable phases due to the extremely high cooling rate upon the 3D printing. Therefore, all the above parameters are thoroughly monitored during the three-stage fabrication process.

Influence of heterogeneous inclusions on the process of formation, structural transformations, and growth of TiO2 nanocrystals

The processes of formation and growth of titanium dioxide nanocrystals with a certain structure, the phase transition of anatase to rutile in the presence of a heterogeneous inclusions during thermal and hydrothermal treatment are investigated in this work. Preformed TiO2 nanoparticles with anatase and rutile structure having different morphology were used as heterogeneous inclusions at the precipitation stage. It was shown that heterogeneous inclusions with this composition and structure could not be considered either as geometrical limitations preventing crystallization or as crystallization nuclei at the stage of titanium dioxide formation. A decrease in the temperature of the anatase-rutile structural transformation by 200 °C and a reduction in the average size of anatase crystallites in the presence of inclusions of the rutile modification in the initial material under hydrothermal conditions and during heat treatment were demonstrated. A conclusion was made about the effect of the size of anatase nanoparticles on their stability. The dependence of the morphological, dimensional, and surface characteristics of the resulting titanium dioxide nanoparticles on the parameters of hydrothermal synthesis and heat treatment, and the presence of inclusions in the reaction system is estimated.

Structural, Magnetocaloric, and Magnetic Properties in Heusler Ni50Mn35In10X5 (X = Ga, Fe and Al) Alloys

The structural, magnetocaloric, and magnetic characteristics in Heusler Ni50Mn35In10X5 (X = Ga, Fe, and Al) alloys were examined using X-ray diffraction and field-dependent magnetization measurements. All samples exhibited a mixture structure of cubic L21 and tetragonal L10 and underwent second-order magnetic transitions at TC(Al5) = 220 K, TC(Ga5) = 252 K, and TC(Fe5) = 298 K. The Ga5 alloy exhibited structural change as indicated by a thermal hysteresis that may be seen in the saturation magnetic field in the M(T) dependences. The transition at the TC point from a ferromagnetic to a paramagnetic state caused a drop in magnetization, supported by thermal hysteresis, at a low magnetic field (0.01 T). On the other hand, the Fe5 alloy presented a gradual decrease in magnetization with similar hysteresis behavior, also at a low magnetic field (0.01 T), whereas at 0.1 T of field, no features characteristic of this transition were detected. This could be due to a large difference in the metallic radius of Fe compared to that of In. Otherwise, magnetic investigations demonstrated that the replacement of In with Al may cause the structural transformation temperatures and TC to be shifted to low temperatures. The present results imply that the structural transformation temperatures and the transition itself are highly dependent on chemical composition. Furthermore, under a magnetic field change of 5 T, the maximum magnetic entropy changes of 0.6 J/kg K, 1.4 J/kg K, and 2.71 J/kg K for the Ga5, Fe5, and Al5 alloys, respectively, were determined by their TC. Refrigeration capacity values were found to be 25 J/kg, 74 J/kg, and 98 J/kg at µ0∆H = 5 T. These ribbons are viable candidates for multifunctional applications due to their cheaper cost and their physical characteristics disclosed during the magnetostructural transition, which takes place close to the room temperature.

Influence of γ phase on the magnetostructural transition and magnetocaloric properties of Ni38Co12Mn41Sn9 melt-spun ribbons

In the present work, Ni38Co12Mn41Sn9 melt-spun ribbons were annealed under different conditions to make samples free of secondary γ phase precipitates or get them with various average grain sizes. A comparative study was carried out to demonstrate how γ phase formation affects the martensitic transformation (MT) and the magnetocaloric properties linked to the first-order phase transition. The γ phase significantly reduces the structural transformation temperatures, while slightly increases the thermal hysteresis. The γ phase also decreases the maximum entropy change (|ΔS|max) and the effect is more significant for smaller γ phase precipitates. As compared with those with coarse γ phase grains, the ribbons containing smaller ones possess a lower |ΔS|max but a smaller maximum magnetic hysteresis loss.

Comparative analysis of intercellular lipid organization and composition between psoriatic and healthy stratum corneum

The lipid composition and organization of the stratum corneum (SC) in patients with psoriasis and healthy subjects were compared using X-ray diffraction, Fourier-transform infrared spectroscopy (FT-IR), and ultraperformance liquid chromatography, combined with time-of-flight mass spectrometry (UPLC-TOFMS). In healthy SC (HSC), SC lipids formed two lamellar phases (long and short periodicity phases). Hexagonal and orthorhombic hydrocarbon-chain packing were observed in the lateral lipid organization at 30 °C via X-ray diffraction. In HSC, the lamellar phases and the hydrocarbon-chain packing organizations changed with elevated temperatures and finally disappeared. In these behaviors, the high-temperature hexagonal hydrocarbon-chain packing organization, which appeared above the orthorhombic hydrocarbon-chain packing organization, transformed to the liquid phase at about 90 °C in HSC. In psoriatic SC (PSC), hexagonal hydrocarbon-chain packing organization disappeared at about 65 °C with elevated temperatures. No high-temperature hexagonal hydrocarbon-chain packing organization were observed in PSC during heating process. Disorder of the hydrocarbon-chain packing of SC lipids was observed in PSC via FT-IR. In UPLC-TOFMS, free fatty acid (FFA) and ceramide (CER) compositions differed between patients with PSC and HSC. Specifically, the levels of ultra-long chain fatty acids containing CER and phytosphingosine-containing CER were decreased, while those of sphingosine and dihydrosphingosine-containing CER and unsaturated FFA were increased in PSC. Furthermore, FFA and CER carbon chain lengths decreased in patients with PSC. These results suggest that the alteration of SC lipid composition and the reduction of carbon chain lengths in PSC lowered the structural transformation temperature, thereby reducing barrier function.

Effect of Ta Content on Microstructure and Phase Transformation Temperatures of Ti75.5-Nb25.5 (%at.) Alloy

β-titanium shape memory alloys are used as shape memory-based biomedical applications and superelastic materials because they are made up of non-toxic components. Also, their behaviors are more semilar with that of human bone and dental biological tissues, such as biocompatibility, superior corrosion resistance, low density, and low modulus of elasticity. In the current study, the effect of Ta additive on the physical properties of Ti-Nb alloy has been investigated. Ti-Nb-Ta alloy samples were produced such that the Tantalum (Ta) element was substituted instead of Niobium (Nb), and the induction arc melting was used to melting the mixed powders. To investigate the crystal structure, microstructure, and phase transformation temperatures of all samples, the X-Ray Diffraction (XRD), Scanning Electron Microscopic (SEM), and Diffraction Scanning Calorimetry (DSC) measurements have been used. Both β-rich and α peaks were observed in the XRD and SEM measurements. Also, the DSC results revealed that the Ti-Nb-Ta alloys have only austenite transformation temperatures and their phase austenite phase transformation were decreased by the effect of Ta additive, additionally, the enthalpy change in〖 E〗_3 sample ( Ti_75.5 Nb_22.5 Ta_2 (%at)) has the highest value, while E_4 sample ( Ti_75.5 Nb_21.5 Ta_3 (%at)) has the lowest respective value.

Influence of doping additive on thermophysical and rheological properties of halogen-free polymer composition for cable insulation and sheaths

Introduction. The demand for halogen-free fire-resistant compositions for the manufacture of fire-retardant wires and cables is constantly growing. Problem. Therefore, the creation and further processing of these materials is an urgent problem. Goal. The aim of the article is to study the effect of the doping additive on the thermophysical and rheological properties of halogen-free compositions for power cables with voltage 1 kV with the determination of both the temperatures of phase and structural transformations of polymer compositions. Methodology. Experiments investigating the phase transformations were carried out with the help device of thermogravimetric analysis and differential scanning calorimetry TGA/DSC 1/1100 SF of METTLER TOLEDO company. Rheological studies of polymeric materials were conducted by using the method of capillary viscosimetry in the device IIRT–AM. Results. The influence of the doping additive on the formation of the supramolecular structure of the filled polymer compositions for cable products was determined, that resulted in the temperature increase of the decomposition beginning by 11 °С and the end of decomposition by 7 °С. Originality. The effect of a doping additive on reducing the effective melt viscosity of a polymer composition from 6·104 to 1·104 Pa·s with increasing shear rate has been shown for the first time. The shear rate of the polymer composition containing the doping additive increases from 0.5 to 20 s–1 with increasing shear stress. Practical value. The research results provide an opportunity to reasonably approach the development of effective technological processes for the manufacture of the insulation and sheaths of power cables from halogen-free polymer compositions.

Heusler‐Based Cylindrical Micro‐ and Nanowires

Wire‐shaped materials are ideal form for rapid, cheap, and scalable production and application. Herein, the most recent results on Heusler‐based cylindrical micro‐ and nanowires are reviewed, emphasizing spintronic, magnetocaloric, and shape memory applications. Herein, how the Heusler alloy's composition can be tuned to achieve a material with repeatable magnetic and mechanical properties is showed. A wide range of possible Heusler compositions allows us to tune the structural transformation temperatures by alloying the original composition with a few percent of metalloid. Wire shape brings additional effect—well‐defined easy axis change due to a structural transformation that enhances the magnetocaloric effect at low‐magnetic fields. The unique wire's shape properties can be downscaled to nanoscale range by electrodeposition. It is showed that such a method allows the production of a highly ordered array of nanowires. As a result, the magnetocaloric effect of an array of nanowires can be one order higher than that obtained by other production methods, where the nanowires are oriented randomly. Finally, some possible applications of shape memory or magnetocaloric actuators and the possibility of implementing high spin polarization for the construction of spintronic devices are showed.

Study of martensitic transition temperature on Ni54Fe19Ga23X4 Heusler glass-coated microwires doped by X = B, Al, Ga, in

The influence of alloying in Ni54Fe19Ga23X4 Heusler glass-coated microwires doped by X = B, Al, Ga, In is presented in this work. Even though the alloys exhibit different chemical compositions, the valence electron concentration (e/a) was constant. Furthermore, basic magnetic and resistivity measurements have been performed to determine the chemical doping effect on the magnetic properties and the associated structural transformation. Results point to the fact that the fine-tuning of the chemical composition may be a suitable tool for adjusting the Curie temperature, transformation temperature, and magnetocaloric properties of the glass-coated microwires. In the case of Ga and the Al-doped alloy, a thermal hysteresis can be distinguished in the M(T) dependences at saturation magnetic field, pointing to the presence of the structural transformation. Moreover, the 4% Al doping shifts the structural transformation temperature and the Curie temperature almost to the same position. On the other hand, B and In doping may lead to the disappearance or shift of the structural transformation out of the measurement range (100–400 K). Resistivity measurements proved these results.

Ordering mechanism and kinetics in Ni2Mn1−xCuxGa ferromagnetic shape memory alloys

The effect of atomic order on the martensitic and magnetic transformations undergone by Ni2Mn1−xCuxGa ferromagnetic shape memory alloys has been investigated. Different degrees of order have been induced by performing selected thermal treatments, and it has been found that, although both the structural (martensitic) and magnetic transformation temperatures increase with the improvement of the atomic order, each of these temperatures evolve with different kinetics. Reinforcing this result, during post–quench heating of Ni2Mn1−xCuxGa alloys two consecutive DSC exothermic peaks are observed, while the ternary Ni–Mn–Ga, Co–doped Ni–Mn–Ga and even Cu–doped Ni–Mn–Ga alloy when Cu replaces Ni show a single peak. The kinetics of the post–quench ordering processes have been studied using the Kissinger’s method, and two different activation energies have been obtained: 1.16 eV for the process that is common to all alloys and 1.35 eV for the one that is only observed in Ni2Mn1−xCuxGa alloys. The former, responsible for the change in Curie temperature, is attributed to the improvement of L21 order mainly due to Mn diffusion; for the second, which underlies the rise of martensitic transformation temperatures, diffusion of Cu atoms, misplaced in the Ni sublattice after quench, towards their most favourable sites in the Mn sublattice is proposed as the responsible mechanism.

Martensitic transformation and magnetocaloric effect of Mn&amp;lt;sub&amp;gt;1&amp;minus;&amp;lt;/sub&amp;gt;&amp;lt;sub&amp;gt;&amp;lt;italic&amp;gt;x&amp;lt;/italic&amp;gt;&amp;lt;/sub&amp;gt;Bi&amp;lt;sub&amp;gt;&amp;lt;italic&amp;gt;x&amp;lt;/italic&amp;gt;&amp;lt;/sub&amp;gt;CoGe alloys

<p indent="0mm">Quaternary alloys of MnCoGe with the partial addition of other elements can realize coupling between structural and magnetic transformations, and it can present apparent magnetocaloric effects during martensitic transformation. In this paper, we have prepared Mn₁₋_{<italic>x</italic>}Bi_{<italic>x</italic>}CoGe (<italic>x</italic>=0, 0.03, 0.04, 0.05, 0.055, 0.06) series alloys using arc melting in a protective argon atmosphere. Experimental results indicate that the structural transformation temperature (<italic>T</italic>_M) of the samples shifts to low temperatures with increasing elemental Bi doping. The Mn_0.945Bi_0.055CoGe alloy (valence electron concentration<italic>e</italic>/<italic>a</italic>=6.63) undergoes a first-order magnetostructural transition (FOMST) near room temperature, and a large magnetic entropy change of <sc>36.8 J kg⁻¹ K⁻¹</sc> is obtained under an external magnetic field of <sc>0–5 T,</sc> which proves that elemental Bi doping in MnCoGe alloys can effectively tune the martensitic transformation temperature and further improve the magnetic entropy change.

Synthesis of highly dense spark plasma sintered magnetocaloric Ni-Mn-Sn alloys from melt-spun ribbons

In this work, polycrystalline melt-spun ribbons of the magnetocaloric Ni50Mn37Sn13 Heusler alloy were used as precursor material in order to fabricate a nearly 98% dense piece by means of spark plasma sintering. SEM and XRD data show that consolidation was achieved through ribbons surfaces preserving the structural and microstructural properties. Structural transformation temperatures and magnetic properties for the as-sintered SPS sample remained similar to the precursor. A moderate shifting in the martensitic transformation to higher temperatures (8 K) for the sintered piece was observed. Furthermore, higher values of maximum entropy change were calculated for the sintered sample (10.1 Jkg−1K−1 at 5 T), which basically doubles that of the precursor ribbons.

Post-annealing effects on structural and magnetic properties of pulsed laser deposition grown Co–Ni–Al ferromagnetic shape memory alloys thin films

Co–Ni–Al is considered as a suitable ferromagnetic shape-memory alloy and has significant control over structural transformation temperature, magnetic parameters, and ductility. In this paper, the fabrication of Co–Ni–Al alloy thin films on Si (1 0 0) substrate was achieved via pulsed laser deposition (PLD) technique. The developed films annealing was done at 573, 673, and 773 K temperatures and inveterate their crystalline structure, phase analysis, and magnetic nature of the annealed Co38Ni35Al27 films. X-ray diffraction analysis at 773 K annealing temperature reveals that, the Co38Ni35Al27 film shows high crystallinity, and all annealed films have a body-centered cubic phase structure. The crystallite size estimated from Scherrer rule is noticed to increase from 10 to 22 nm when the annealing temperature raised to 773 K. From the SEM images, we infer that there is an increase in grain size when the annealing temperature increases and the grains shape looks spherical. The magnetic study of the samples shows that the film possesses highest magnetic saturation and lowest coercivity at 773 K annealing temperature. Also, the film annealed at 773 K exhibits a wide range of transformation temperatures and has a curie temperature of 180 K.

Significant enhancement of magnetocaloric effect in a NiMnCuGa Heusler alloy through textural modification

Magnetocaloric materials are of increasing interest to bring magnetic refrigeration to everyday households and drastically impact the energy demands for temperature control devices. In this work, a polycrystalline Heusler alloy of composition Ni2Mn0.76Cu0.24Ga with coinciding structural and magnetic transformation temperatures was subjected to compressive stress assisted thermal cycling (SATC) to enhance the magnetic properties by inducing a preferred orientation in the martensite. Isofield magnetization measurements showed a sharpening of the transformation between ferromagnetic martensite and paramagnetic austenite due to SATC. In isothermal magnetization measurements, SATC was seen to increase the magnetostructural coupling. With a 2 T applied magnetic field, the magnetocaloric effect (MCE) increased from ∼10 to ∼25 J/kg K and the refrigeration capacity (RC) almost doubled due to SATC. Heat capacity measurements were largely unaffected by SATC. The change in adiabatic temperature was estimated by using Cp and change in magnetic entropy (ΔSM) calculations. SATC was seen to increase ΔTad from ∼1.2 K to 2 K for an applied magnetic field of 2 T. Neutron diffraction measurements revealed highly textured martensite in the as received state that rotated to a more ideal preferred orientation after SATC that enhanced the magnetostructural transformation; and thus, improving the MCE and ΔTad.

Transition temperature tuning of Ni2FeGa based Heusler alloys in form of glass-coated microwires

We report on the possibility of tuning the martensitic transition temperature in Ni2FeGa Heusler alloys prepared in the form of glass coated microwires. Glass-coated microwires with different valence electron concentration (e/a) and possible structural transformation in the temperature range from 100 K to 400 K have been prepared (Ni51+xFe22-xGa27; x = 0, 2, 4). Magnetic and electrical resistance measurements demonstrated a linear dependence of structural transformation temperature on valence electron concentration TT(e/a). In order to prove the correctness of the TT(e/a) dependence, a glass-coated microwire with the transformation temperature 310 K (human body temperature) was selected, prepared and experimentally investigated. The effect on magnetocaloric response is also shown. Our theoretical prediction together with the experimental results point to the fact that the e/a parameter is the key parameter for transition temperature tuning in Ni2FeGa Heusler-based glass-coated microwires.

Anomalous effect of Si additions upon the paramagnetic-to-antiferromagnetic transition entropy in fcc high-Mn steels

Fe–Mn based alloys with more 12% at. of Mn are usually called High-Mn steels. Several technological properties of High-Mn steels are explained by the presence of the fcc-hcp martensitic transformation. This transition is strongly affected by the para-to-antiferromagnetic ordering transition present in the fcc structure. The magnetic ordering temperature depends on the alloy chemical composition, and strongly stabilizes the fcc austenitic phase, leading to a decrease of the structural transformation temperatures. In several cases, this effect is strong enough to inhibit the martensitic transformation. In the present study, the effect on the magnetic transition of adding Si to high-Mn Fe–Mn alloys is analyzed. Calorimetric measurements are performed in order to obtain the entropy change associated to the magnetic ordering transition. The results are discussed in the light of the available information in the literature. Striking effects are reported, which indicate that Si addition strongly affects the driving force of the martensitic transition.

The Influence of Vacancy Concentration of Low-Stability Pre-Transitional Structural-Phase States and Energy Characteristics of NiAl Intermetallide

Using the Monte Carlo method, the influence of vacancy concentration on the structural-phase states and energy characteristics is investigated by the example of an intermetallic compound NiAl in the course of its heating and cooling. According to the analysis, the availability and concentration of vacancies are important factors in the pre-transitional low-stability structural-phase states prior to transformation. On the one hand, neither the vacancies nor their concentration affect the temperature ranges of structural-phase transformations, on the other hand, they essentially influence both the pre-transitional low-stability structural-phase states and the rate of diffusion processes. The temperature behavior of the short-range order parameter suggests that the higher the vacancy concentration (i.e., system’s defectiveness), the higher the temperatures at which the tendencies for increasing atomic ordering would be manifested due to intensified diffusion. This, in turn, evidences of a higher starting structural transformation temperature with an increase in the number of defects in the alloy during cooling. An analysis of the temperature curves of the long-range order parameter of the NiAl intermetallide allows making a conclusion that an increased vacancy concentration (i.e., the alloy’s defectiveness) gives rise to a logical result – decreased long-range ordering in the system in the region of low-stability pre-transitional states and increased starting transformation temperature.

Microstructure, magnetic and magnetocaloric properties in magnetic-field-induced martensitic transformation Mn1−xCo1+xGe alloy ribbons

The sharp field-induced metamagnetic martensitic transformation plays a pivotal role in magnetoresponsive effects for ferromagnetic shape memory alloys (FSMAs). Homogeneous polycrystalline Mn1−xCo1+xGe alloy ribbons are prepared by using the arc-melting and melt-spinning technique in this work. The microstructure, crystalline structure, magnetic and magnetocaloric properties are studied in these ribbons. The substitution of Co for Mn reduces the structural transformation temperature, which leads to the realization of coincident magnetostructural transformation. Strikingly, a magnetic-field-induced martensitic transformation is observed in these ribbons. Therefore, large magnetocaloric effect, including maximum magnetic entropy change (ΔSM,max) value of ∼28 and 25 Jkg−1K−1 and effective refrigeration capacity (RCeff) of ∼168 and 181 Jkg−1 are obtained for x=0.035 and 0.09 samples, respectively, under the field change of μ0ΔH=0–7T. Furthermore, the average hysteresis loss of these ribbons reduces largely in contrast to their bulk counterparts. These results would provide the new experimental data for magnetic refrigerant materials and pave the way for the further investigation of other FSMAs.

Effect of cobalt doping on martensitic transformations and the magnetic properties of Ni50−xCoxMn37Sn13 (x = 1, 2, 3) Heusler ribbons

The effect of cobalt doping on martensitic transformations and the magnetic properties of Ni50−xCox Mn37Sn13 (x = 1, 2, 3) magnetic shape memory alloys obtained by melt spinning in the form of ribbons is studied. The crystallographic structure of all of the ribbons at room temperature is austenite cubic L21. SEM micrographs indicate the formation of textured ribbons with columnar grains growing up perpendicular to the ribbon plane. For all of the ribbons, DSC cyclic scans reveal the martensitic transformation on cooling and the reverse austenitic transformation upon heating below room temperature. In addition, they indicate that the martensitic transformation temperatures decrease with an increasing cobalt content.Thermomagnetic curves reveal the coexistence of AFM and FM exchange interactions at low temperatures. Likewise, the magnetization change (ΔM) between the martensite and austenite phase, as well as the austenite Curie temperature (TCA), increase with increasing cobalt content in the ribbons. Structural transformations are also sensitive to the external applied magnetic field. This fact suggests that the structural transformation temperatures of the ribbons could be tuned to the desired functional temperature by controlling the cobalt amount through the replacement of Ni as well as by changing the applied magnetic field, which could lead to an enhancement of the magnetic properties.