Small Thermal Hysteresis Research Articles

Tuning the phase transitions of VO2 thin films on silicon substrates using ultrathin Al2O3 as buffer layers

High quality VO2 thin films have been fabricated on silicon substrates using magnetron sputtering by introducing Al2O3 thin films as a buffer. The ultrathin Al2O3 deposited by plasma-assisted atomic layer deposition leads to a greatly improved crystallinity and textures in VO2 films. Dramatic change in electrical resistivity (4 orders of magnitude) and a small thermal hysteresis loop (~4 K) are obtained across the metal–insulator phase transition (MIT). Remarkably, by applying perpendicular voltage to a VO2/Al2O3 based metal/VO2/semiconductor device, electrically driven MIT switching characteristics have been observed with a tiny tunneling leakage current of ~10 μA. These results show that an electric field alone is sufficient to trigger the MIT, and the realization of VO2 based ultrafast electrical switching devices on a silicon substrate is possible.

Grain size effect on the R-phase transformation of nanocrystalline NiTi shape memory alloys

Development of nanoscale actuators and sensors in recent years calls for functional materials with small dimensions and high strengths. High strength nanocrystalline NiTi alloys which experience the R-phase transformation with a small thermal hysteresis are ideal candidates for these applications. To facilitate the application of the R-phase transformation in nanocrystalline NiTi alloys, this study investigated the effect of grain size on the R-phase transformation of a nanocrystalline Ti-50.2at.%Ni alloy. The nanometric grain size was created by severe cold deformation and low temperature anneal. It was found that in the recrystallized state, achieving nanoscale grain sizes (<100 nm) was effective in suppressing the B2→B19’ martensitic transformation and revealing the B2↔R transformation. The B2↔R transformation temperature was found to increase with the decreasing grain size within the range of 22–155 nm. The suppression of the B19’ martensite in nanograins is attributed to the limited space within the grains to allow the formation of self-accommodation structures to contain the large lattice distortion of the martensite.

Spin Crossover Iron(II) Coordination Polymer Chains: Syntheses, Structures, and Magnetic Characterizations of [Fe(aqin)2(μ2-M(CN)4)] (M = Ni(II), Pt(II), aqin = Quinolin-8-amine)

New Fe(II) coordination polymeric neutral chains of formula [Fe(aqin)2(μ2-M(CN)4)] (M = Ni(II) (1) and Pt(II) (2)) (aqin = Quinolin-8-amine) have been synthesized and characterized by infrared spectroscopy, X-ray diffraction, and magnetic measurements. The crystal structure determinations of 1-2 reveal in both cases a one-dimensional structure in which the planar [M(CN)4](2-) (M = Ni(II) (1) and Pt(II) (2)) anion acts as a μ2-bridging ligand, and the two aqin molecules as chelating coligands. Examination of the intermolecular contacts in the two compounds reveals that the main contacts are ascribed to hydrogen bonding interactions involving the amine groups of the aqin chelating ligands and the nitrogen atoms of the two non bridging CN groups of the [M(CN)4](2-) (M = Ni(II) (1) and Pt(II) (2)) anion. The average values of the six Fe-N distances observed respectively at room temperature (293 K) and low temperature (120 K), that is, 2.142(3) and 2.035(2) Å for 1, and 2.178(3) and 1.990(2) Å for 2, and the thermal variation of the cell parameters (performed on 2) are indicative of the presence of an abrupt HS-LS spin crossover (SCO) transition in both compounds. The thermal dependence of the product of the molar magnetic susceptibility times the temperature (χmT), in cooling and warming modes, confirms the SCO behavior at about 145 and 133 K in 1 and 2, respectively, and reveals the presence of a small thermal hysteresis of about 2 K for each compound.

Recent Advances in Form-Stable Phase Change Materials of Polyethylene Glycol

The polyethylene glycol is a kind of phase change energy storage material with small thermal hysteresis effect, the phase change enthalpy and phase transition temperature varies with its different molecular weight. This article summarizes polyethylene glycol as research progress of phase change energy storage material, which mainly discusses the preparation methods and outlook about form-stable phase change materials.

Effect of precipitation on the microstructure and the shape memory response of the Ni50.3Ti29.7Zr20 high temperature shape memory alloy

The effect of nano-precipitation on the shape memory characteristics and martensite microstructure of the Ni50.3Ti29.7Zr20 high temperature shape memory alloy was studied. With appropriate heat treatment, the alloy shows excellent dimensional stability during thermal cycling under tension. Fine precipitates are absorbed by martensite variants and do not significantly hinder martensite formation. These alloys demonstrated higher transformation strain, smaller thermal hysteresis and narrower transformation range than samples with larger precipitates, a few hundred nanometers, that interfere with martensite variant growth.

Syntheses, three types of hydrogen-bonded assembly structures, and magnetic properties of [FeIII(Him)2(hapen)]Y·solvent (Him = imidazole, hapen =N,N′-bis(2-hydroxyacetophenylidene)ethylenediamine, Y = BPh4−, CF3SO3−, PF6−, ClO4−, and BF4−)

Five iron(III) complexes of [FeIII(Him)2(hapen)]Y·solvent with five different counter anions were synthesized, where Him=imidazole, H2hapen=N,N′-bis(2-hydroxyacetophenylidene)ethylenediamine, Y=BPh4− (1), CF3SO3− (2), PF6− (3), ClO4− (4), and BF4− (5), and solvent=methanol or H2O. Each FeIII ion has octahedral coordination geometry with N2O2 donor atoms of the equatorial tetradentate ligand (hapen) and two nitrogen atoms of two imidazoles at the axial positions. The imidazole group is hydrogen-bonded to the phenoxo oxygen atom of hapen of the adjacent unit and/or to the counter anion or solvent molecule, to give three types of assembly structures involving linear dimer, cyclic dimer, and one-dimensional zigzag chain. Compound 1, with a cyclic dimer, and 2 and 3, with a one-dimensional chain, showed spin-equilibrium behavior between high-spin (S=5/2) and low-spin (S=1/2) states, whereas 4 and 5, with linear dimers, are high-spin complexes. Compound 3 shows abrupt spin transition with small thermal hysteresis.

Direct observation of negative thermal expansion in SrCu&lt;sub&gt;3&lt;/sub&gt;Fe&lt;sub&gt;4&lt;/sub&gt;O&lt;sub&gt;12&lt;/sub&gt;

We report a direct observation of the negative thermal expansion in an iron-based perovskite oxide SrCu3Fe4O12 by using a thermomechanical analyzer and strain gauge. Reversibility and small thermal hysteresis in the negative thermal expansion temperature range demonstrate that the negative thermal expansion of this compound is attributed to second-order phase transition.

Martensitic and magnetic transformation in Mn50Ni50−xSnx ferromagnetic shape memory alloys

A martensitic transformation (MT) from a body-centered-cubic austenitic phase to a tetragonal martensitic phase has been found in Mn50Ni50−xSnx (0 ≤ x ≤ 11) alloys. The martensitic transformation temperature can be decreased by about 71.6 K by increasing the Sn concentration by 1 at. %. For 9 ≤ x ≤ 11, Mn50Ni50−xSnx ferromagnetic shape memory alloys are obtained. Due to the large magnetization difference (ΔM = 60 emu/g) and small thermal hysteresis (ΔT = 6 K) in the Mn50Ni40Sn10 alloy, a two-way magnetic-field-induced martensitic transformation is observed with dT/dH = 2 K/T.

Enhanced magnetocaloric effects and tunable thermal hysteresis in transition metal pnictides

The effect of Co and Ni substitutions for Fe on the magnetocaloric properties of MnFe(P,Ge) compounds crystallizing in the hexagonal Fe2P type of structure was studied. It was found that, by changing composition, a small thermal hysteresis and a giant magnetocaloric effect can be obtained simultaneously in MnFe(P,Ge) compounds when the magnetic transition is controlled to be close to the border separating the first- and second-order transition regimes.

Magnetocaloric effects in Mn1.35Fe0.65P1−xSix compounds

The structural and magnetocaloric properties of Mn1.35Fe0.65P1−xSix compounds are investigated. The Si-substituted compounds, Mn1.35Fe0.65P1−xSix with x = 0.52, 0.54, 0.55, 0.56, and 0.57, are prepared by high-energy ball milling and the solid-state reaction. The X-ray diffraction shows that the compounds crystallize into the Fe2P-type hexagonal structure with space group P6̄2m. The magnetic measurements show that the Curie temperature of the compound increases from 253 K for x = 0.52 to 296 K for x = 0.56. The isothermal magnetic-entropy change of the Mn1.35Fe0.65P1−xSix compound decreases with the Si content increasing. The maximal value of the magnetic-entropy change is about 7.0 J/kg·K in the Mn1.35Fe0.65P0.48Si0.52 compound with a field change of 1.5 T. The compound quenched in water possesses a larger magnetic entropy change and a smaller thermal hysteresis than the non-quenched samples. The thermal hysteresis of the compound is less than 3.5 K. The maximum adiabatic temperature change is about 1.4 K in the Mn1.35Fe0.65P0.45Si0.55 compound with a field change of 1.48 T.

Stable magnetostructural coupling with tunable magnetoresponsive effects in hexagonal ferromagnets

The magnetostructural coupling between the structural and the magnetic transition has a crucial role in magnetoresponsive effects in a martensitic-transition system. A combination of various magnetoresponsive effects based on this coupling may facilitate the multifunctional applications of a host material. Here we demonstrate the feasibility of obtaining a stable magnetostructural coupling over a broad temperature window from 350 to 70 K, in combination with tunable magnetoresponsive effects, in MnNiGe:Fe alloys. The alloy exhibits a magnetic-field-induced martensitic transition from paramagnetic austenite to ferromagnetic martensite. The results indicate that stable magnetostructural coupling is accessible in hexagonal phase-transition systems to attain the magnetoresponsive effects with broad tunability.

Ultrasonic Relaxation in Phase Transition Region in Ferroelectric Semiconductors of Sn&lt;sub&gt;2&lt;/sub&gt;P&lt;sub&gt;2&lt;/sub&gt;S&lt;sub&gt;6 &lt;/sub&gt;Family

The paper presents recent results of ultrasonic investigation of Sn2P2S6 family ferroelectric crystals and their solid solutions in the temperature range 100-300 K. It was shown that in Sn2P2(S,Se)6 crystals the critical ultrasonic velocity slowing down for longitudinal waves propagating along main crystallographic directions is quite sharp and large. The relative change of longitudinal ultrasonic velocity along z-axis at the phase transition gradually increased from 10 % in pure Sn2P2S6 till 25 % for sample with 0.4 content of Se. Such large velocity change causes the large ultrasonic attenuation anomaly. The increase of relaxation time: τ=τ0/(TC-T) leads to the increase of attenuation. Prefactor τ0 was shown to be very small and the critical attenuation anomaly arises in the narrow temperature range close to phase transition. In the 0.4 Se sample the phase transition is of the first order because small thermal hysteresis exists. The ultrasonic velocity behaviour in the ferroelectric phase was described using Landau theory and free energy expansion including sixth order terms. For (Sn,Pb)2P2S6 system the critical ultrasonic anomalies were smaller and the phase transition temperature substantially decreased (for 0.45 Pb sample the phase transition point was at Tc =140 K). The ultrasonic anomalies at phase transition in (PbxSn1-x)2P2S6 have large hysteresis showing that transition is of the first order, far from the critical point.

From first-order magneto-elastic to magneto-structural transition in (Mn,Fe)1.95P0.50Si0.50 compounds

We report on structural, magnetic, and magnetocaloric properties of MnxFe1.95−xP0.50Si0.50 (x ≥ 1.10) compounds. With increasing the Mn:Fe ratio, a first-order magneto-elastic transition gradually changes into a first-order magneto-structural transition via a second-order magnetic transition. The study also shows that thermal hysteresis can be tuned by varying the Mn:Fe ratio. Small thermal hysteresis (less than 1 K) can be obtained while maintaining a giant magnetocaloric effect. This achievement paves the way for real refrigeration applications using magnetic refrigerants.

Specific heat studies on pristine and oxygenated iron tellurides

Specific heat studies carried out on Fe 1.1Te and oxygenated Fe 1.1Te and FeTe 2 in the range 77–300 K exhibit interesting behavior. The specific heat of the pristine sample reveals a well known structural transition associated with antiferromagnetic ordering near 67 K with a small thermal hysteresis of ∼1 K. Contrastingly, the oxygenated samples exhibit a phase transition with a very large thermal hysteresis of ∼100 K. The specific heat transition observed at the 150 and 260 K regions in the oxygenated Fe 1.1Te sample could not be captured by the magnetization measurements indicating that specific heat transitions seen in oxygenated samples may not be of magnetic origin.

Spin crossover in di-, tri- and tetranuclear, mixed-ligand tris(pyrazolyl)methane iron(ii) complexes

A series of polynuclear mixed-ligand tris(pyrazolyl)methane iron(II) complexes displaying high temperature spin crossover behaviour has been synthesised. These complexes are of the type [(Fe((3,5-Me(2)pz)(3)CH))(n)(μ-L)](BF(4))(2n), where μ-L is one of five bridging ligands X(CH(2)OCH(2)C(pz)(3))(n), (X = the central linking moiety, pz = pyrazolyl ring and n = 2 (ditopic), 3 (tritopic) or 4 (tetratopic)). Throughout the series the terminal tris(3,5-dimethylpyrazolyl)methane co-ligand (3,5-Me(2)pz)(3)CH and the BF(4)(-) counter anion were kept constant while variations in the central linking moiety have produced three dinuclear complexes and a trinuclear and tetranuclear complex, all isolated as solvates. The three dinuclear complexes are a 1,4-xylene-bridged complex 1·2DME, a 2,6-naphthalene-bridged complex 2·2.5MeCN.2DME and a 1,4-butene-bridged complex 3·2DME. The trinuclear complex 4·solvent (solvent undefined) has a 1,3,5-mesitylene core and the tetranuclear complex, 5·8MeCN.2(t)BuOMe, has a 1,2,4,5-tetramethylbenzene core (DME = dimethoxyethane, (t)BuOMe = tertiarybutyl-methylether). The trinuclear cluster has a "3-up" cup shape with the cups arranging themselves in pairs to form capsules that contain anion guests. All the solvated compounds have been structurally characterised and both the solvated and desolvated versions have had their magnetic and thermal properties thoroughly investigated by variable temperature magnetic susceptibility, differential scanning calorimetric and Mössbauer spectral methods. They all display typical low spin iron(II) magnetic behaviour at room temperature and all undergo a spin state transition to high spin iron(II) above room temperature. In particular, complex 1·2DME shows an abrupt spin transition which shifts, upon desolvation, to a lower value of T(1/2) and in addition displays a small thermal hysteresis.

Co doping enhanced giant magnetocaloric effect in Mn1−xCoxAs films epitaxied on GaAs (001)

A giant magnetocaloric effect was found in series of Mn1−xCoxAs films epitaxied on GaAs (001). The maximum magnetic entropy change caused by a magnetic field of 4 T is as large as 25 J/kg K around room temperature, which is about twice the value of pure MnAs film. The observed small thermal hysteresis is more suitable for practical application. Growing of layered Mn1−xCoxAs films with Co concentration changing gradually may draw layered active magnetic regenerator refrigerators closer to practical application. Our experimental result may provide the possibility for the combination of magnetocaloric effect and microelectronic circuitry.

Giant magnetic entropy change in colossal magnetoresistance in La0.7Ca0.3MnO3 material in low field

The structural, magnetic, and magnetocaloric properties of the manganite La0.7Ca0.3MnO3 have been studied. Change in the giant magnetic entropy was observed without any noticeable magnetic hysteresis but with small thermal hysteresis losses. We observed a first order magnetic phase transition around 251 K. The magnetic entropy change observed in this work is estimated to be 5.27 J/kg K for field changes from 0 to 1.5 T based on magnetization measurements. This value is about twice as large as those for other perovskite manganites and is even larger than for Gd-based magnetic materials at low fields. In addition, the entropy change was estimated by using the heat capacity method, which can be well explained by the Maxwell relation.

One-Dimensional Spin-Crossover Iron(II) Complexes Bridged by Intermolecular Imidazole−Pyridine NH···N Hydrogen Bonds, [Fe(HLMe)3]X2 (HLMe = (2-Methylimidazol-4-yl-methylideneamino-2-ethylpyridine; X = PF6, ClO4, BF4)

2-Methylimidazol-4-yl-methylideneamino-2-ethylpyridine (abbreviated as HL(Me)) is the 1:1 condensation product of 2-methyl-4-formylimidazole and 2-aminoethylpyridine and functions as a bidentate ligand to the iron(II) ion to produce the 3:1 complexes together with anions, [Fe(HL(Me))(3)]X(2) (X = PF(6) (1), ClO(4) (2), BF(4) (3)). The magnetic susceptibilities, differential scanning calorimetric measurements, and Mossbauer spectral measurements demonstrated that complexes 1, 2, and 3 showed a steep one-step spin crossover (SCO) between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states with small thermal hysteresis. Three complexes have an isomorphous structure and are crystallized in the same monoclinic space group, C2/c, both in the HS and LS states. The iron(II) ion has the octahedral coordination geometry of a facial isomer with N(6) donor atoms of three bidentate ligands, in which an imidazole and an imine nitrogen atom per ligand participate in the formation of the coordination bond, but the pyridine nitrogen is free from coordination. The complex cation fac-[Fe(HL(Me))(3)](2+) is a chiral species with a Delta or Lambda isomer, and the adjacent Delta and Lambda isomers are linked alternately by an intermolecular imidazole-pyridine NH...N hydrogen bond to produce an achiral 1D chain. The two remaining imidazole moieties per complex are hydrogen-bonded to the anions that occupy the space among the chains. The SCO profile becomes steeper with the decrease of the anion size (73.0 A(3) for PF(6)(-), 54.4 A(3) for ClO(4)(-), and 53.4 A(3) for BF(4)(-)). The SCO transition temperature T(1/2) of the PF(6) (1), ClO(4) (2), and BF(4) (3) salts estimated from the magnetic susceptibility measurements are (T( downward arrow) = 151.8 K, T( upward arrow) = 155.3 K), (T( downward arrow) = 184.5 K, T( upward arrow) = 186.0 K), and (T( downward arrow) = 146.4 K, T( upward arrow) = 148.2 K), respectively, indicating that the T(1/2) value is not in accord with the anion size.

Dissolution, regeneration and ion-gel formation of agarose in room-temperature ionic liquids

The suitability of several ionic liquids, containing imidazolium or pyridinium cations with different alkyl chains and anions ranging from small hydrogen-bond acceptors to those of a large and non-coordinating nature, has been tested for solubilization of a widely used biopolymer, agarose. The solubility of agarose was found to depend on both the nature of anion and amphiphilicity of the cation. Dissolved agarose was regenerated using methanol, and ionic liquids were recovered and recycled for different experiments. Regenerated agarose largely maintained the features of native agarose in terms of molecular weight, polydispersity, thermal stability and crystallinity but varied slightly in conformation preferences. Subsequently, agarose-based highly conducting soft ion-gels having small thermal hysteresis were prepared and characterized. Such ion-gels have possible applications as electrochemical devices.

First order ferromagnetic transition in binaryCeIn2

Measurements of the magnetic, thermal, and transport properties of the ${\text{CeIn}}_{2}$ binary alloy are consistent with a paramagnetic-ferromagnetic transition at ${T}_{C}=22\text{ }\text{K}$. A discontinuity in the magnetic entropy, electrical resistivity and thermal expansion, and a huge anomaly in the specific heat of 113 J/mol K $(\ensuremath{\Delta}{c}_{mag}=103\text{ }\text{J}/\text{mol}\text{ }\text{K})$, at the magnetic transition, are observed. In addition, the Arrott plots show negative slope at low fields, the field-cooling and field-warming magnetization present irreversibility, and both the susceptibility and the resistivity evidence a small thermal hysteresis of 0.05 K. Moreover, the values of the entropy change calculated from the magnetization data using the Clausius-Clapeyron equation are in good agreement with those directly obtained from the specific-heat data. The joint analysis of all these results provides evidence for the first order character of this magnetic transition in ${\text{CeIn}}_{2}$.