Synthesis,characterization, and thermodynamic parameters of vanadium dioxide

A novel process was developed for synthesizing pure thermochromic vanadium dioxide (VO₂) by thermal reduction of vanadium pentoxide (V₂O₅) in ammonia gas. The process of thermal reduction of V₂O₅ was optimized by both experiments and modeling of thermodynamic parameters. The product VO₂ was characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TG), and differential scanning calorimetry (DSC). The experimental results indicated that pure thermochromic VO₂ crystal particles were successfully synthesized. The phase transition temperature of the VO₂ is approximately 342.6 K and the enthalpy of phase transition is 44.90 J/g.     

Thermochromic VO2 nanorods and other vanadium oxides nanostructures

Thermochromic VO₂ nanorods were prepared via thermal conversion of the metastable VO₂–B phase synthesized by hydrothermal methods. We observe an increased thermochromic transition temperature to ∼75–80 °C by variable-temperature infrared spectroscopy. Nano- and sub-micron structures of other vanadium oxides (V₃O₇, (NH₄)_0.5V₂O₅, and V₂O₅) were obtained simply by varying the starting materials in the hydrothermal synthesis. We also obtained nanostructures of the high temperature tetragonal rutile phase of VO₂ by thermolysis of single-source vanadium (IV) precursors.     

Synthesis,crystal structure,thermal analysis and dielectric properties of two mixed trichlorocadmiates (II)

K_0.57(NH₄) _0.43CdCl₃ and K_0.25(NH₄) _0.75CdCl₃ are orthorhombic, space group Pnma, Z = 4, with a = 8.8760(4) Å, b = 3.9941(2) Å, c = 14.7004(7) Å, and Z = 4, a = 8.9567(9) Å, b = 3.9957(4) Å, c = 14.855 (2) Å, respectively. Final R values are 0.01 and 0.02 for 608 and 834 reflections, respectively. In both the materials, the crystal structure has been determined by X-ray single crystal analysis at room temperature (293 K). The compound structures consist of K⁺ (or NH\(_{{4}}^{{+}})\) cations and double chains of CdCl₆ octahedra sharing one edge extending along b-axis. The mixture of K⁺/NH\(_{{4}}^{{+}}\) cations are located between the double chains ensuring the stability of the structure by ionic and hydrogen bonding contacts N/K–H …Cl. Spectroscopic, dielectric and differential scanning calorimetry (DSC) measurements were performed to discuss the mechanism of the phase transition. These studies show that these materials, K_0.57(NH₄)_0.43CdCl₃and K_0.25(NH₄)_0.75CdCl₃, undergo a phase transition at 438 and 454 K, respectively.     

Structural changes in Sr<Subscript>9</Subscript>In(PO<Subscript>4</Subscript>)<Subscript>7</Subscript> during antiferroelectric phase transition

Structural changes in Sr₉In(PO₄)₇ during the antiferroelectric (AFE) phase transition are studied by X-ray powder diffraction, electron microscopy, second-harmonic-generation, and dielectric measurements. Sr₉In(PO₄)₇ complements a group of Ca₃(VO₄)₂-type ferroelectric (FE) phosphates and vanadates and is the first example of an AFE material in this family. Antiparallel shifts of Sr atoms from their average positions and ordering of the P1O₄ tetrahedra form two contributions in the structural mechanism of the AFE phase transition: a displacive contribution and an order-disorder constituent, respectively. The displacive and order-disorder type of structural changes may account for the obtained value of the Curie–Weiss constant (C ~ 10⁴ K) which is in between the value usually observed for pure displacive (C ~ 10⁵ K) and that for orderdisorder phase transitions (C ~ 10³ K). The structural mechanism of the AFE phase transition in Sr₉In(PO₄)₇ is very similar to that of the FE phase transition in Ca₉R(PO₄)₇ and Ca₉R(VO₄)₇. Both displacive and orderdisorder contributions are responsible for the physical properties of the Ca₃(VO₄)₂-type materials.     

Preparation of vanadium dioxide powders by thermolysis of a precursor at low temperature

A kind of crystal precursor, (NH₄)₅[(VO)₆(CO₃)₄(OH)₉]·10H₂O, less than 12 × 40 m in dimensions was synthesized by reaction of vanadyl dichloride and ammonium hydrocarbonate in solution. The precursor microcrystals were fined to less than 10 m with mean size of about 4 m, by using organic solvents and surfactant in the process of the synthetical technique. Thermolysis of the precursor in a flow of nitrogen gas was investigated by TGA and DTA. The XRD powders patterns for the products from the thermolysis of the precursor show that the VO₂ powder obtained at 350, 365, 380 and 410°C for 30 min were a noncrystalline phase, a monoclinic one named as B phase, a mixed phase of the B and the usual monoclinic A phase, and the pure A phase, respectively. SEM micrographs of VO₂ powders from the fined precursor demonstrated that the particle sizes of these fine powders were less than 2 m. DSC curve of the VO₂ obtained at 450 °C for 30 min from the fined precursor showed that there was an endothermic phase transition at 71 °C.     

Hydrothermal synthesis of W and Mo co-doped VO2(B) nanobelts

W and Mo co-doped VO₂(B) nanobelts which used formic acid as reduction acid, NH₄VO₃ as vanadium source, (NH₄)₆W₇O₂₄?·?6H₂O and (NH₄)₆Mo₇O₂₄?·?4H₂O as doped sources were synthesised by the hydrothermal method. The samples were characterised by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). TEM and HRTEM images showed the samples had a length of 1?µm and a width of 100?nm. XRD, FTIR and XPS spectra revealed that Mo⁶⁺ and W⁶⁺ incorporated into the VO₂(B) lattice and formed solid-solution phases with VO₂(B).     

Solid-state reactions in V x O y (NH4VO3)-P2O5 and V x O y (NH4VO3)-(NH4)2HPO4 closed systems

Solid-state reactions in V _x O _y (NH₄VO₃)-P₂O₅ and V _x O _y (NH₄VO₃)-(NH₄)₂HPO₄ closed systems can be used to synthesize vanadyl hydrogen phosphate at 300°C and a variety of ammonium vanadium phosphates at lower and higher temperatures.     

Enhanced hydrophilicity of the Si substrate for deposition of VO2 film by sol–gel method

We have illustrated the role of hydrophilic nature of Si substrate played in the improvement of the contact performance between the vanadium dioxide (VO₂) film and Si substrate. The VO₂ films were fabricated by sol–gel method on single crystal Si substrate, which was pre-treated with hydrophilic solution and obtained a quite improved hydrophilicity. The bonding of Si substrate with precursor V₂O₅ gel was interpreted. The morphology and crystalline structure of the films were investigated by field-emission scanning electron microscopy, atomic force microscopy and X-ray diffraction. It is shown that the surface of the film on Si substrate with enhanced hydrophilicity is quite homogeneous and uniform. The film exhibits the formation of VO₂ phase with (011) preferred orientation. Moreover, the optical pump induced phase transition property of the film was studied by terahertz time-domain spectroscopy, which revealed around 70% reduction of transmission at 0.1–1.5?THz in the VO₂ film across the phase transition.     

Characteristics of CeOx–VO2 composite thin films synthesized by sol–gel process

Pure vanadium dioxide (VO₂) and CeOx–VO₂ (1.5 < x < 2) composite thin films were grown on muscovite substrate by inorganic sol–gel process using vanadium pentaoxide and cerium(III) nitrate hexahydrate powder as precursor. The crystalline structure, morphology and phase transition properties of the thin films were systematically investigated by X-ray diffraction, Raman, X-ray photoelectron spectroscopy, FE-SEM and optical transmission measurements. High quality of the VO₂ and CeOx–VO₂ composite films were obtained, in which the relative fractions of +4 valence state vanadium were above 70 % though the concentrations of cerium reached 9.77 at %. However, much of cerium compounds were formed at the edge of grains and the addition of cerium resulted in more clearly defined grain boundaries as shown in SEM images. Meanwhile, the composite films exhibited excellent phase transition properties and the infrared transmittance decreased from about 70 to 10 % at λ = 4 μm bellow and above the metal–insulator phase transition temperature. The metal–insulator phase transition temperatures were quite similar with about 66 °C of the pure VO₂ and CeOx–VO₂ composite thin films. But hysteresis widths increased with more addition of cerium, due to the limiting effect of grain boundaries on the propagation of the phase transition. Particularly, the CeOx–VO₂ composite film with an addition of 7.82 at % Ce showed a largest hysteresis width with about 20.6 °C. In addition, the thermochromic performance of visible transmittance did not change obviously with more addition of cerium.     

Solid state-hydrothermal synthesis and photoluminescence of LaVO4: Eu nanophosphors

A modified hydrothermal process is engaged in the synthesis of LaVO₄: Eu³⁺ nanophosphor. All kinds of inorganic salts (solid state hydrated rare earth nitrates and NH₄VO₃) and precipitation reagents (ammonia and urea) are mixed to form the solid state precursors instead of general aqueous solution systems, and a little amount water exists in the hydrothermal reaction systems. Both X-ray powder diffraction (XRD) and transmission scanning electronic microscope (TEM) shows that the uniform microstructure with the particle size of around 60 nm and the product from ammonia possesses the higher phase purity than that from urea. LaVO₄: Eu³⁺ shows a strong red emission at 617 nm originating from the ⁵D₀ → ⁷F₂ hypersensitive transition of Eu³⁺ ion. Especially the LaVO₄: Eu³⁺ nanophosphor from ammonia presents the more excellent photoluminescent property (lifetime and quantum efficiency) than that from urea.     

A low-temperature solution route to hollow NH4VO3 microspheres with controllable shells

NH₄VO₃ hollow microspheres with controllable shells have been synthesised by the interaction between AgNO₃ and NH₄VO₃ via a facile one-step low-temperature solution-based method. The diameters and surface roughness of NH₄VO₃ hollow spheres can be readily controlled by altering the molar ratios of NH₄VO₃ to AgNO₃. When the molar ratios of NH₄VO₃ to AgNO₃ increase from 6?:?1 to 8?:?1, the diameters of NH₄VO₃ hollow spheres increase from 500–600 to 800–900?nm and the building blocks of the shells are assembled by nanoparticles and nanorods. The introduction of AgNO₃ and H₂O₂ plays an important role in the formation of NH₄VO₃ hollow microspheres.     

DTA measurements of normal-incommensurate phase transition in (NH4)2ZnCl4 and K2ZnCl4 crystals

Differential thermal analysis (DTA) was applied to determine the changes in enthalpy and entropy of (NH₄)₂ZnCl₄ and K₂ZnCl₄ crystals at their phase transition from the orthorhombic normal phase to the incommensurate phase. The temperature of this transition, T _i , is 406 K for (NH₄)₂ZnCl₄ and 555 K for K₂ZnCl₄ and the entropy changes (S/R) are 0.053 and 0.035, respectively. The low value obtained for S/R is characteristic of incommensurate phase transitions. The results were compared with the data reported for other crystals of the A₂BX₄ family. Thermal properties of the crystals of the A₂ZnCl₄ subgroup were found to the correlated with the length of A-Cl bonds.     

Growth of VO2 films with metal-insulator transition on silicon substrates in inductively coupled plasma-assisted sputtering

Single-phase monoclinic vanadium dioxide (VO₂) films were grown on a Si(100) substrate using inductively coupled plasma (ICP)-assisted sputtering with an internal coil. The VO₂ film exhibited metal-insulator (M-I) transition at around 65 °C with three orders of change in resistivity, with a minimum hysteresis width of 2.2 °C. X-ray diffraction showed structural phase transition (SPT) from monoclinic to tetragonal rutile VO₂. For conventional reactive magnetron sputtering, vanadium oxides with excess oxygen (V₂O₅ and V₃O₇) could not be eliminated from stoichiometric VO₂. Single-phase monoclinic VO₂ growths that are densely filled with smaller crystal grains are important for achieving M-I transition with abrupt resistivity change.     

The large modification of phase transition characteristics of VO2 films on SiO2/Si substrates

The vanadium oxide (VO₂) films have been prepared on SiO₂/Si substrates by using a modified Ion Beam Enhanced Deposition (IBED) method. During the film deposition, high doses of Ar⁺ and H⁺ ions have been implanted into the deposited films from the implanted beam. The resistance change of the VO₂ films with temperature has been measured and the phase transition process has been observed by using the X-ray Diffraction technique. The phase transition of the IBED VO₂ films starts at a low temperature of 48 °C and ends at a high temperature of 78 °C. It is found that the phase transition characteristics can be adjusted by changing the annealing temperature or the time and the phase transition characteristics of the IBED VO₂ films depend on the quantity and location of argon atoms in the film matrix.     

均匀沉淀法可控制备氯化镉氨配合物

以金属氯化物为反应原料,以尿素为沉淀剂,以醇为溶剂,采用均匀沉淀法制备了多种过渡金属盐氨配合物。以氯化镉氨配合物为典型的研究对象,考察了尿素含量、反应时间和温度等参数对所得产物尺寸及形貌的影响,通过X射线粉末衍射(XRD)、热重-差示扫描量热法(TG-DSC)、扫描电子显微镜(SEM)和激光粒度分析仪(LPSA)对所得产物物相组成、热稳定性、形貌及粒子尺寸分布等进行了表征。实验结果表明所得产物为二氯化二氨合镉Cd(NH₃)₂Cl₂。产物粒子的尺寸随着尿素含量的增加而增大,反应时间较短有利于得到单分散且呈多面体的Cd(NH₃)₂Cl₂,随着反应时间的延长,所得Cd(NH₃)₂Cl₂颗粒团聚严重;随着反应温度的升高,所得产物粒子形貌变化不大且几乎没有团聚现象,产物粒子的尺寸均一性更好。     

Preparation and superconductivity of NbN powder by a vapour phase reaction

The formation of superconducting NbN powders by the vapour phase reaction of the NbCl₄-NH₃-H₂ system has been investigated. The properties of NbN powders changed with the reaction temperature, mixing temperature of NbCl₄ and NH₃, and gas composition ratio ([NH₃]/[NbCl₄]). The reaction system gave NbN powders consisting of particles with sizes less than 50 nm. The process of particle formation is discussed. The NbN powders produced had vacancies at both the Nb and N sublattices. The vacancies influenced the superconducting transition temperature (T _c) of NbN, the highestT _c observed being 14.1 K.     

Phase boundaries and domain boundaries in crystals

Domain boundaries, i.e. interfaces between different orientation variants of the same crystal species, and phase boundaries, i.e. interfaces between two different modifications of the same compound, exhibit rather similar features. This has been investigated by means of polarized light and X-ray topography for a series of structurally related sulphates which were grown as large single crystals from aqueous solution. The major results are as follows: (i) Domain interfaces frequently adopted only a few orientations which are low-energy boundaries of best structural fit. These preferred orientations may be parallel to low-indexed lattice planes or to non-crystallographic planes (W and W^t walls according to the classification of Sapriel,Phys. Rev. B12, 1975, 5128). Illustrations of such (transition-induced) domain boundaries in KLiSO₄, NH₄LiSO₄, RbLiSO₄, CsLiSO₄,(NH₄)₃H(SO₄)₂ will be presented. (ii) For many first-order transitions the phase boundaries prefer planes of minimum strains, i.e. low energy, which again may be low-index lattice planes or non-crystallographic planes. These preferred orientations can be calculated from the strain tensor of the transition with the relative lattice-parameter changes as tensor components). If the transition isotherm deviates from the minimum strain orientation, characteristic zigzag boundaries with segments parallel to the (symmetrically equivalent) preferred planes may result. Zigzag phase boundaries have been observed in RbLiSO₄ and {N(CH_{3 4}}₂ZnCl₄. (iii) The shape and the density of transition-induced domains is influenced by the orientation of the phase boundaries and its velocity of motion. For the 708 K. transition of KLiSO₄ and the 413 K transition of (NH_{4 3}H(SO_{4 2} (in both cases loss of the trigonal axis), among the minimum-strain domain boundaries those normal to the phase boundary are preferred. In N(CH₃)_{4 2}ZnCl₄, the domain density increases with the phase boundary velocity.     

Electrical and thermal properties of ammonium and potassium oxalates

The d.c. electrical conductivity () and the relative permittivity () were accurately measured as a function of temperature in the range 300 < T < 450 K for ammonium oxalate monohydrate (AOX), (NH₄)₂ C₂O₄H₂O and potassium oxalate monohydrate (POX), K₂C₂O₄H₂O. Differential thermal analysis (DTA), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) from 300 K up to 520 K were also performed for each compound. The data of the electrical parameters combined with the DTA and/or DSC thermograms suggest the existence of a possible change in the electrical conduction at 350 K for AOX and at 355 K for POX. Like most hydrogen-bonded molecules, the charge carriers may be protons and the electrical conduction is therefore protonic. At somewhat higher temperatures, dehydration and/or decomposition takes place in each compound and is reflected as a sudden loss in weight in the TGA plot.     

Lithium disorder in the vicinity of the superionic phase transition in monoclinic and rhombohedral Li3In2(PO4)3

Rhombohedral and monoclinic modifications of the Li₃In₂(PO₄)₃ compound have been studied by X-ray diffraction, infrared (i.r.) analysis, Raman light scattering, differential thermal analysis (DTA), differential scanning calorimetry (DSC) and impedance spectroscopy, in a wide temperature range of 290–600 K. Analysis of the data allows the suggestion that the superionic phase transition, observed in these materials in the temperature interval 370–385 K, belongs to Faraday type phase transformations. PO₄ tetrahedra, which are a partition of the (In₂P₃O₁₂) ₃₀₀ ^3– rigid skeleton, have been found deformed. This seems to be due to an ordered lithium ion-vacancy distribution in the low temperature, non-superionic, polymorphs. With increasing temperature these deformations disappear. This disappearance can be caused by a decrease of the correlation length of the lithium ion-vacancy order.     

Temperature anomalies of DEA-CuCl4 and TEA-CoCl2Br2/PMMA nanocomposites

Phase transitions were studied for the new synthesized ferroic (NH₂(C₂H₅)₂CuCl₄ (DEA-CuCl₄) and (NH₂(C₂H₅)₄CoCl₂Br₂ (TEA-CoCl₂Br₂) nanocrystallites (NC) incorporated into polymer matrices. Comparison with the bulk crystals is performed. A giant temperature shift (from 305.1°C to 360.3°C) of onset temperature for TEA-CoCl₂Br₂ nanoparticles after their incorporation into the PMMA matrix was found and is substantially larger compared to the DEA-CuCl₄. The DSC analysis shows that the temperature of phase transition is crucially dependent on the thermocycling and temperature rate. The principal role of the nano-sized effects is shown.