Structural, thermal and morphological studies of magnesium substituted-lithium manganese oxide spinels

Coexistence of two phases having space groups of Fd3m and P4₃32 in the Mg-doped LiMn₂O₄ spinel is being reported for the first time in this article. Mg-doped LiMn₂O₄ powders have been synthesized by sol-gel method using citric acid as a chelating agent. X-ray powder diffraction (XRD) studies show that the crystal structure of LiMg_xMn_2−xO₄ for x<0.25 is a single-phase cubic spinel, which has space group of Fd3m. The cubic spinel structures having space group of Fd3m and P4₃32 are found to coexist in the compound for x=0.25. The structure becomes single-phase cubic spinel with space group P4₃32 for x>0.25. Field emission scanning electron microscopy (FESEM) shows that particle size of various synthesized powders ranges from 100 to 350 nm. Particle size decreases with increase in Mg content. Differential thermal analysis (DTA) and thermogravimetry (TG) studies show an exponential decay relationship between Mg-doping content and the decomposition temperature to form nonstoichiometry (LiMg_xMn_2−xO_4−δ) in air atmosphere. Fourier transform infrared spectroscopy (FTIR) analysis shows increase in the number of vibrational bands with increase in Mg content, which indicates ordering of the ions in the case of ordered spinel structure, and consequent reduction of the space group symmetry from O_h⁷ to O⁷.     

Upconversion Luminescence of Er3+ Ions in Transparent Germanate Glass Ceramics Containing CaF2 Nanocrystals

The visible upconversion and near-infrared luminescence of Er3＋ ions in germanate glass ceramics containing GaF2 nanocrystals are investigated. The nanocrystals are characterized by x-ray diffraction （XRD） and transmission electron microscopy, showing their mean sizes less than 20hm. High transmittance of the glass ceramics is displayed by absorption spectra. The upconversion luminescence intensity in the glass ceramics increases significantly with increasing temperature. Both the shifts of the XRD peaks and the Stark-split shown in the luminescence spectra indicate the entrance of the Er3＋ ions into the CaF2 nanocrystals, which is confirmed by a Judd-Ofelt analysis. Possible mechanisms of the upconversion luminescence are analyzed.     

Sol-gel-derived organic-inorganic hybrid materials

Optically transparent organic-inorganic hybrid coating materials have been prepared by a sol-gel process. Four different types of the coating material produced by TWI in Cambridge, UK using the patented Vitresyn^® method, all identical in terms of the starting materials, but differing in terms of their relative proportions, have been examined. Tetraethoxysilane was used as the primary inorganic precursor and urethane acrylate was used as the source of the organic component. 3-(Trimethoxysilyl)propyl methacrylate was used as both a secondary inorganic source and a silane coupling agent to improve the compatibility of the organic and inorganic phases. The degree of chemical interaction of the organic and inorganic phases after processing was determined by ²⁹Si and ¹³C nuclear magnetic resonance and Fourier transform infrared spectroscopy. The effect of the relative amount of inorganic starting component in these hybrid materials on their thermal properties was investigated through differential scanning calorimetry and thermogravimetric analysis. Similar degrees of chemical interaction between the organic and inorganic phases were found in all four samples. T³, Q³ and Q⁴ are the main cross-linking network structures in these hybrid systems, the relative proportions of which are determined by the relative proportions of the starting materials.     

Magnetite nanoparticles prepared by the glass crystallization method and their physical properties

Glass melts in the system Fe₂O₃/FeO/CaO/Na₂O/B₂O₃ were prepared from the raw materials, by firstly reducing them by flushing with nitrogen and subsequently roller quenching. The flakes obtained had a thickness of around 150 μm and were thermally treated at temperatures in the range from 550 to 620 °C. X-ray diffraction gave evidence of the occurrence of nanocrystalline magnetite. Magnetization measurements at room temperature show ferromagnetic behaviour and no hysteresis. Temperature-dependent measurements showed a monotonic decrease of the saturation magnetization with temperature, and a Curie temperature of 553 °C. The primary mean particle core diameter is around 10 nm after annealing at 570 °C.     

A combined study of the oxidation mechanism and resistance of AISI D6 steel exposed at high temperature environments

In this work it is thoroughly examined the oxidation performance of D6 tool steel under isochronal and isothermal oxidations. Isochronal oxidation tests, from ambient temperature to 1000 °C, revealed the oxidation rate of the coupons at different temperatures. Four different temperatures were selected for the isothermal oxidation test, which correspond to different oxidation rates. The oxidation and the examination of the samples were accomplished by thermogravimetric analysis (TG) in air with which the mass gain of the samples due to oxidation was simultaneously acquired. The samples were, also, examined by scanning electron microscopy (SEM), in order to observe their surface before and after the oxidation tests. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used for the accurate identification of the as formed oxides. The results revealed that in every case two distinct layers of oxides were formed while their composition was different, depending on the temperature of oxidation. Furthermore, the thickness of the as formed oxides is increased when the oxidation is performed at higher temperatures.     

Specific heat of around the glass transition

Dynamic calorimetric measurements are performed for the quaternary metallic glass Zr₆₅Al_7.5Cu_17.5Ni₁₀ in order to analyse the dependence on different heating rates for the glass transition temperature T_g. We compare two different temperature programs used for sample relaxation, to estimate the influence of the thermal history on T_g. A lower limit for the glass transition temperature T_g was calculated according to two different models based on the fact, that width and temperature of the glass transition depend on the experimental time scale set by the heating rate: One model assumes a Vogel-Fulcher-Tammann type behaviour, as used to describe more or less “fragile” glass formers and the other assumes an Arrhenius-like behaviour, which is related to “strong” glass formers. The values obtained from both models differ by about 80K. From additional absolute specific heat capacity measurements we calculate the Kauzmann temperature T_K, as a lower limit for the temperature of the glass transition from thermodynamic aspects. Comparing T_K with the temperature values obtained from the two evaluation models we can classify the quaternary metallic glass Zr₆₅Al_7.5Cu_17.5Ni₁₀, to behave more like a “strong” glass former. Received: 23 January 1998 / Received in final form and Accepted: 31 August 1998     

A thin-film transistor with a very thin a-Si:H layer and its application for an LC panel

One of the disadvantages of applying an a-Si:H thin-film transistor (TFT) to an active matrix-addressed liquid crystal (LC) panel is that a TFT with an a-Si:H has a very large photo-leakage current because of the high photo-conductivity of an a-Si:H itself.We have tried decreasing the photo-leakage current by varying the thickness of an a-Si:H layer (L) in TFTs and investigated the characteristics of TFTs, mainly drain voltage versus drain current containing photo-leakage current (I _ph).As a result, it is shown that lnI _ph is proportional to InL, and its gradient is 1.5–2.0. We assume that the thinner an a-Si:H layer is, the more effective the recombination of carriers at the interface states is forI _ph.We have applied TFT with a very thin a-Si:H layer (30nm) to a full-color active matrix-addressed LC panel for a moving picture display and realized a display of good quality under illuminated condition of 5×10⁴lx without a shading layer in it.     

Montmorillonite modified with liquid crystalline diol hydrochlorides: Preparation and characterization

Intercalation of liquid crystalline species into the interlayer space of montmorillonite was performed by the ion-exchange method. Liquid crystalline diol hydrochlorides that differ in the spacer lengths (6, 8 or 10 methylene units) and the type of end group bound to mesogen (nitro or methoxy) was used. Parent clay, purified clay (montmorillonite) and organically modified montmorillonite were characterized by X-ray diffraction analysis (XRD) and simultaneous differential scanning calorimetry-thermogravimetric analysis (DSC-TGA). The basal spacing of unmodified montmorillonite, determined by XRD, was 1.25 nm and after modifications with α-[Bis(2-hydroxyethyl)amino]-ω-(4-nitroazobenzene-4′-oxy)alkane hydrochloride it increased up to 1.96 nm. XRD peak broadening and a loss of peak intensity were very pronounced in the montmorillonite modified with α-[Bis(2-hydroxyethyl)amino]-ω-(4-methoxyazobenzene-4′-oxy)alkane hydrochloride, with 10 methylene units, indicating a disordered structure. TGA results showed that the modification reduced the adsorption of water from 8% for unmodified montmorillonite to 2-3% for organically modified montmorillonite and that the cation exchange between inorganic and organic ions was between 40 and 50%.     

Examination of the oxidation resistance of high-alloyed tool steels at elevated temperatures

In the present work the structure of two different tool steels is examined before and after oxidation up to 1000 °C in air. The materials under examination have different chromium contents. Also, the first contains vanadium (S1 tool steel) and the second tungsten (S2 tool steel) as alloying element, while the rest are common. The examination took place by thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy and X-ray diffraction. From this study it is deduced that the structure of the two steels, before oxidation, has several distinguishing differences mostly in the chromium distribution in the iron matrix. The oxidation tests revealed that S2 oxidizes at higher temperatures than S1, but finally, at 1000 °C, S2 tool steel has greater mass gain, because it oxidizes at a higher rate.     

Computation of the string tension in four-dimensional Yang–Mills theory using large N reduction

Continuum reduction and Monte Carlo simulation are used to calculate the heavy quark potential and the string tension in large N Yang–Mills theory in four dimensions. The potential is calculated out to a separation of nine lattice units on a lattice with extent six in each direction.