The effect of heat treatment on the paracrystallinity of an opal-CT found in a bentonite

To investigate temperature dependence of paracrystallinity for opal-CT, a bentonite containing approximately 34% by mass opal-CT have been used as material. Since opal-CT can not be separated entirely, the bentonite samples have been heated at different temperatures in the interval from 200 °C to 1300 °C for 2 h, and at 1050 °C for different time intervals changing from 2 h to 24 h. The X-ray diffraction (XRD) patterns of the original and heated samples have been obtained. The increase in the paracrystallinity has been discussed according to the thermal behavior of the relative intensity (I/I₀), relative full width at half-maximum peak height (FWHM/FWHM₀ ≡ W) and d-value of the most characteristics XRD peak for opal-CT between 0.405 nm and 0.410 nm region. The increase in I/I₀ from 1 to 3, and in d(l 0 1) spacing from 0.4050 to 0.4095 and decrease in W from 1 to 0.6 show that there is an increase in paracrystallinity for opal-CT by rising the temperatures between 800 °C and 1300 °C. The increase, of I/I₀ value from 1 to 5 by heating at 1050 °C while time increases from 2 h to 24 h shows that the paracrystallinity of opal-CT increase by time and reaches steady state condition approximately 1300 °C.     

Effect of vacuum annealing on charge transport and trapping in a-Si1 − xCx:H/c-Si heterostructures

The processes of charge transport and trapping in amorphous Si_1 ? xC_x:H films deposited on crystalline p-type Si wafers and annealed in vacuum in the temperature range 300–650 °C have been evaluated. Current–voltage (I–V), capacitance–voltage (C–V) and admittance–temperature (G–T) characteristics were measured in the temperature range 100–350 K. The spectrum of thermal effusion of hydrogen was measured from room temperature up to 1000 °C.C–V characteristics indicate a slight increase of the dielectric constant k and a large hysteresis after annealing at 450 °C. The hysteresis is believed to be associated with mobile hydrogen effusion from the a-SiC:H film, and it is not seen after a 650 °C anneal. From I–V data the maximum rectification ratio is observed after annealing at 450 °C. Variable-range hopping (VRH) conduction at the Fermi level is found to dominate the forward current of the as-deposited structure. After annealing at 450 °C the forward current can be described by space-charge limited (SCL) mechanisms with trapping at shallow levels with energy of about 0.12 eV. After annealing at 650 °C the process of VRH conduction appears again, but the density of hopping sites is much higher than in the as-grown sample. From admittance spectra, the energy position of respective traps in a-SiC:H is at (E_V + 0.45) eV for as-deposited material and it decreases slightly after vacuum annealing. On the basis of these results, an energy band diagram of the a-Si_1 ? xC_x:H/p-Si structure annealed at 450 °C is proposed.     

Effect of isothermal annealing and visible-light illumination on the AC-impedance behavior of undoped selenium thin films

The effect of post-deposition isothermal annealing (30 °C ? T_A ? 70 °C) and visible-light illumination on the complex AC-impedance of undoped selenium thin films deposited at the substrate temperatures T_S = 30, 50, 70 °C has been studied in the frequency range 0.2–12 kHz. The AC-impedance of amorphous selenium (a-Se) films (T_S, T_A < 50 °C) was mainly capacitive, with no loss peaks being observed in their Z″(ω)–ω curves, irrespective of illumination. This behavior was ascribed to a dominant charge-carrier trapping effect of bulk/surface charged defects usually present in a-Se. On the other hand, the measured Z″(ω)–Z′(ω) diagrams of illuminated polycrystalline Se samples (50 °C ? T_S, T_A ? 70 °C) exhibited almost full semicircles, whereas their Z″(ω)–ω curves revealed prominent loss peaks at well-defined frequencies. As the annealing temperature or light intensity is increased the loci of the points determined by intersections of these semicircles with the Z′-axis at the low-frequency side shift greatly towards the origin, while the loss-peak positions shift to higher frequencies. These experimental findings were explained in terms of a significant increase in electrical conductivity of selenium films due to thermally-induced crystallization at temperatures beyond glass-transformation region of undoped selenium and to creation of electron–hole pairs by visible-light illumination.     

Effect of OH-content on thermal and chemical properties of SnOP2O5 glasses

Glasses with 55–60 mol% SnO and 40–45 mol% P₂O₅ have shown extremely large differences in the chemical and thermal properties depending on the temperature at which they were melted. Glasses prepared at low melting temperature, 450–550 °C, had low T_g, 150–200 °C, and low chemical stability. Glasses prepared at high melting temperature, 800–1200 °C, had much higher T_g, 250–300 °C, and much higher chemical stability. No significant differences were found by ¹¹⁹Sn Mössbauer and ³¹P Nuclear Magnetic Resonance spectroscopy. Large differences in the OH-content could be detected as the reason by infrared absorption spectroscopy, thermal analyses, and ¹H Nuclear Magnetic Resonance spectroscopy. In samples with low T_g, a broad OH – vibration band around 3000 nm with an absorption intensity >20 cm^?1, bands at 2140 nm with intensity ～5 cm^?1, at 2038 nm with intensity ～2.7 cm^?1, and at 1564 nm with intensity ～0.4 cm^?1 were measured. These samples have shown a mass loss of 3–4 wt% by thermal gravimetric analyses under argon in the temperature range 400–1000 °C. No mass loss and only one broad OH-band with a maximum at 3150 nm and low absorption intensity <4 cm^?1 could be detected in samples melted at high temperature, 1000–1200 °C, which have much higher T_g, ～300 °C, and much higher chemical stability.     

Thermal expansion of glass–ceramics in the system BaO/Al2O3/B2O3

Glasses in the system BaO/Al₂O₃/B₂O₃ with and without the addition of platinum were melted. In one sample series, the BaO-concentration was varied while the ratio [Al₂O₃]/[B₂O₃] was kept constant. In another sample series, the [BaO]/[Al₂O₃]-ratio (= 0.9) was kept constant and the B₂O₃ concentration was varied. The samples were thermally treated at 720 °C for 24 h and subsequently at 780 °C for 4 h. In most thermally treated samples, the crystalline phase BaO · Al₂O₃ · B₂O₃ occurred. At some compositions, the platinum-doped samples showed larger concentrations of the crystalline phases. The most remarkable property of the obtained glass–ceramics is their zero or negative thermal expansion coefficient. Here, notable differences were observed: samples with fine grained microstructures showed thermal expansion coefficients approximately zero up to temperatures of around 80 °C. By contrast, samples with coarser microstructures and large spheroidal crystals exhibit negative expansion coefficients up to temperatures of around 280–375 °C. The thermal expansions of these samples were close to those of the mean thermal expansion of the unit cell of the BaO · Al₂O₃ · B₂O₃ phase. The thermal expansion of the fine grained samples was approximately equal to that of the crystallographic a-axis of the BaO · Al₂O₃ · B₂O₃ phase.     

GeO2 based high k dielectric material synthesized by sol–gel process

Recently, there has been lot of research on new high dielectric constant (high k) materials for use in future generations of ultra-large scale integrated circuits (ULSI). There are number of requirements for the new high k materials, such as high dielectric constant, thermal stability (400 °C or higher), high mechanical strength, and good adhesion to neighboring layers. Keeping in view the properties required for the replacement of existing SiO₂ dielectrics, new high k dielectric material based on GeO₂ has been synthesized. Polycrystalline GeO₂ thin films have been deposited by simple, and cost effective sol–gel spin coating process. The obtained xerogel films of germanium oxide have been annealed at 400 °C, 600 °C and 800 °C for 3 h in argon atmosphere. Elemental composition, morphology, and phase analysis have been measured by employing X-ray photoelectron spectroscopy, scanning electron microscopy, and X-ray diffraction techniques, respectively. The formation of the hexagonal GeO₂ phase at and above 400 °C has been reported. The composition of the annealed films have been measured and found to be 68 at.% of O, 32 at.% of Ge for GeO₂, which are close to the stoichiometry of the GeO₂.     

Systematic study of spectroscopic properties and thermal stability of lead germanate glass doped with rare-earth ions

Glasses with composition 50GeO₂–(50?x)PbO–5PbF₂–xLnF₃ (Ln = Pr³⁺–Yb³⁺) were prepared from commercial raw materials. The content of PbF₂ was constant and amounted to 5 mol% whereas the concentration of luminescent ions was diverse (0.2 and 2 mol%). Thermal stability of the glasses were monitored by differential thermal analysis (DTA). It has been found that increase of rare-earth fluoride content from 0.2 mol% to 2 mol% brings about a shift of the glass crystallization onset from 450 °C to 487 °C for Nd-doped samples and from 466 °C to 482 °C for Tm-doped samples. Optical absorption and emission spectra of Ln active ions in oxyfluoride glass have been investigated at room temperature in the ultraviolet (UV), visible (VIS) and near-infrared (NIR) region. Oscillator strengths, phenomenological Judd–Ofelt intensity parameters Ω_2,4,6, radiative transition probabilities, branching ratios and radiative lifetimes of luminescent levels have been estimated. Analysis of decay curves of luminescence revealed that the increase of rare-earth fluoride content from 0.2 mol% to 2 mol% shortens the lifetime of the ⁴F_3/2 level of Nd³⁺ from 224 μs to 90 μs.     

Structural and magnetic properties of nanocrystalline particles in an amorphous Fe73.5Nb3CuSi13.5B9 matrix

We report structural and magnetic properties of fine particles embedded in an amorphous magnetic matrix. As-quenched amorphous Fe_73.5Nb₃CuSi_13.5B₉ ribbons (FINEMET) were submitted to the thermal treatments of several times (1 ? t ? 240 min) at 570 °C using a conventional furnace. The analyses of the X-ray diffraction patterns at room temperature reveal that our samples consist of single phase Fe₃Si nanocrystals embedded in a residual amorphous phase. Magnetic measurements show that the saturation moment at T = 450 °C increases as a function of annealing time. This behavior is attributed to an increase of the fraction of nanocrystallites in the residual amorphous phase.     

An alumina mat with a nano microstructure prepared by centrifugal spinning method

Ceramic fiber products specially alumina mat because of low thermal conductivity and high melting point are used as high temperature insulating materials. Alumina has so high melting point (T_m > 2040 °C) that its mat can be produced through sol–gel method. In this research alumina mat has been manufactured by sol–gel spinning method using our laboratory-designed centrifugal spinneret. The desired viscosity of sol for spinning is 150 P. Phase transformation of the product begins at 600 °C and there is not any amorphous phase at 800 °C and theta alumina (θ-Al₂O₃) is the main phase. In this work, transformation of transitional phase to alpha alumina (α-Al₂O₃) takes place from 1000 °C to 1200 °C. The optimum percent of silica in alumina mat is 4 wt%. Fibers constitute network structure that their average diameter is about 10 μm and contains very fine grains (100 nm). The silica percent concerning the limits of this study (<10 wt%) does not effect on fiber diameter, but grain size decreases from about 200 nm to less than 100 nm while increasing silica percent.     

Structural investigations of nitrided Nb2O5 and Nb2O5–SiO2 sol–gel derived films

Sol–gel derived xNb₂O₅–(100 ? x)SiO₂ films (where x = 100, 80, 60, 50, 40, 20, 0 mol%) were nitrided at various temperatures (800 °C, 900 °C, 1000 °C, 1100 °C and 1200 °C). The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The XRD results have shown that the temperatures below 1100 °C were too low to obtain a pure NbN phase in the samples. The AFM observations indicate that the formation of the NbN phase and the size of NbN grains are related to the silica content in the layer. NbN grains become more regular and larger as the niobium content increases. The maximum grain size of about 100 nm was observed for x = 100. Preparation of the Nb₂O₅–SiO₂ sol–gel derived layers and the subsequent nitridation is a promising method of inducing crystalline NbN in amorphous matrices. It follows from the XPS results that a small amount of Nb₂O₅ remains in the films after nitridation at 1200 °C and that nitrogen reacted not only with Nb₂O₅ but also with SiO₂.     

Dilithium dialuminium trisilicate phase obtained using coal bottom ash

The Li₂Al₂Si₃O₁₀ glass-ceramics well crystallized and with a regular morphology was produced starting from a mixture of Li₂CO₃, TiO₂, Al₂O₃ and coal bottom ash, after reducing the magnetite phase content. Its measured thermal expansion coefficient in the temperatures range from 25 °C to 300 °C is α_(25–300) = −23.4 × 10⁻⁷ °C⁻¹. This value is ≈18% smaller than that for the commercial lithium glass-ceramics (−23.4 × 10⁻⁷ °C⁻¹ to 50 × 10⁻⁷ °C⁻¹).     

Ag-doped antibacterial porous materials with slow release of silver ions

The inorganic–organic hybrids of polyethyleneglycol (PEG), tetraethoxysilane (TEOS) and triethylphosphate (TEP) doped by silver ions were prepared by sol–gel method. After molding and heating at 600 °C to remove organic components, porous Ag–P₂O₅–SiO₂ monoliths were obtained. Thermogravimetry (TG), differential thermal analysis (DTA), infrared spectra, ultraviolet–visible (UV–vis) spectra and pore structure of the samples were measured to show that organic components and residual water could be removed by a heat-treatment up to 600 °C and the mesopores with 6 nm pore diameter were formed. Specific surface area and pore volume of the samples were adjusted with different contents of TEP in the starting composition. Ag⁺ ions could be stably released into water at 30 °C up to 28 days. Antibacterial experiment showed that such materials treated at 600 °C could restrain Escherichia coli effectively.     

Synthesis and characterization of hafnium oxide and hafnium aluminate ultra-thin films by a sol–gel spin coating process for microelectronic applications

Currently there are intense industry-wide efforts in searching for new high dielectric constant (high-k) materials for use in future generations of ultra-large scale integrated circuits (ULSI). There are number of requirements for the new high-k materials, such as high dielectric constant, thermal stability (400 °C or higher), high mechanical strength, and good adhesion to neighboring layers. Oxide spinels comprise a very large group of structurally related compounds many of which are of considerable technological significance. Spinels exhibit a wide range of electronic and magnetic properties in particular nickel, hafnium, cobalt, containing spinels. In the present investigation, crack free, dense polycrystalline monoclinic structure of pure HfO₂, and Al₂HfO₅ ultra-thin films have been prepared by a simple and cost effective sol–gel spin coating method. The formation of the monoclinic HfO₂ phase at 600 °C and complete formation of the single phase Al₂HfO₅ at 800 °C has been reported. The composition of the annealed films has been measured and found to be 70 at.% of O, 30 at.% of Hf for HfO₂ and 22 at.% of Al, 12 at.% of Hf and 66 at.% of O for Al₂HfO₅ films, which are close to the stoichiometry of the HfO₂ and Al₂HfO₅ thin films.     

Glasses in the SbPO4–WO3 system

Glasses in the binary system (100 − x)SbPO₄–xWO₃ (20 ⩽ x ⩽ 60, x in mol%) have been prepared and characterized. Differential thermal analysis (DTA) shows that the glass transition temperature, T_g, increases from 412 °C, for samples containing 20 mol% of WO₃, to 481 °C observed for glass containing 60 mol%. Sample containing 40 mol% in WO₃ were observed to be the most stable against devitrification. The structural organization of the glasses has been studied by using Fourier transform infra-red (FTIR), Raman, ³¹P Magic angle spinning (MAS) and spin echo nuclear magnetic resonance (NMR) spectroscopies. Results suggest two distinct networks comprising the glass structure, one with high SbPO₄ content and the other characteristic of the highest WO₃ content samples. The glasses present photochromic properties. Colour changes are observed for samples after exposure to ultraviolet or visible laser light. XANES, at L₁ absorption edge of tungsten, suggests partial reduction from W⁶⁺ to W⁵⁺ species during the laser irradiation. The photochromic effects and the colour changes, promoted by laser excitation, are reversible and easily removed by heat for during 1 h at 150 °C. Subsequent ‘write/erase’ cycles can be done without degradation of the glasses.     

Phase evolution on heat treatment of sodium silicate water glass

Heat treatment of sodium silicate water glass of the nominal composition Na₂O/SiO₂ = 1:3 was carried out from 100 °C up to 800 °C and the advancement of the resulting phases was followed up by powder X-ray diffraction, scanning electron microscopy and thermogravimetry along with differential thermal analysis. The water glass, initially being an amorphous solid, starts to form crystals of β-Na₂Si₂O₅ at about 400 °C and crystallizes the SiO₂ modification cristobalite at about 600 °C that coexists along with β-Na₂Si₂O₅ up to 700 °C. At 750 °C Na₆Si₈O₁₉ appears as a separate phase and beyond 800 °C, the system turns into a liquid.     

Growth and characterization of Hexakis(thiourea)nickel(II) nitrate crystals

Single crystals of hexakis(thiourea)nickel(II) nitrate [Ni(SC(NH₂)₂)₆](NO₃)₂ are grown by slow evaporation of methanolic solution at room temperature. Structural analysis by single crystal X-ray diffraction analysis reveals that the crystal belongs to monoclinic system with space group C2/c and the cell parameters are a=22.046(2) Å, b=9.3325(4) Å, c=16.221(2) Å, Z=8. The metal is coordinated by six thiourea groups with Ni–S–C bond angles ranging from 114.81° to 116.85° and Ni–S bond lengths lying in the range 2.35 to 2.61 Å in a distorted octahedral geometry. The interesting feature observed in this study is that although it crystallizes in centrosymmetric structure, contrary to expectations, it exhibits a positive second harmonic generation (SHG) result, quite likely due to the change in stereochemical arrangement. An Nd:YAG laser with a modulated radiation of 1064 nm directed on the powdered sample leads to local noncentrosymmetry and this could be due to the loss of thiourea ligands resulting in tetrakis(thiourea)nickel(II) complex causing green light emission. The powder X-ray diffraction study reveals the crystallinity of the grown material. The vibrational patterns in FT-IR clearly evidence the complex formation. Thermogravimetric analysis (TG) reveals the purity of the sample and no decomposition is observed up to the melting point. The crystal is further characterized by diffused reflectance spectroscopy, dielectric studies and microhardness analysis.     

Crystal growth kinetics in cordierite and diopside glasses in wide temperature ranges

We measured and collected literature data for the crystal growth rate, u(T), of μ-cordierite (2MgO · 2Al₂O₃ · 5SiO₂) and diopside (CaO · MgO · 2SiO₂) in their isochemical glass forming melts. The data cover exceptionally wide temperature ranges, i.e. 800–1350 °C for cordierite and 750–1378 °C for diopside. The maximum of u(T) occurs at about 1250 °C for both systems. A smooth shoulder is observed around 970 °C for μ-cordierite. Based on measured and collected viscosity data, we fitted u(T) using standard crystal growth models. For diopside, the experimental u(T) fits well to the 2D surface nucleation model and also to the screw dislocation growth mechanism. However, the screw dislocation model yields parameters of more significant physical meaning. For cordierite, these two models also describe the experimental growth rates. However, the best fittings of u(T) including the observed shoulder, were attained for a combined mechanism, assuming that the melt/crystal interface growing from screw dislocations is additionally roughened by superimposed 2D surface nucleation at large undercoolings, starting at a temperature around the shoulder. The good fittings indicate that viscosity can be used to assess the transport mechanism that determines crystal growth in these two systems, from the melting point T_m down to about T_g, with no sign of a breakdown of the Stokes–Einstein/Eyring equation.     

Preparation and second-order optical nonlinearity of inorganic–organic hybrid films

The sol–gel technique has been used to prepare a thin film for second-order nonlinear optics (NLO). An azo-dye molecule 2,5-dimethyl-4-(4′-nitrophenylazo)phenol (DMNPAP) was covalently bonded to the silicon oxide network through hydrolysis and co-condensation between the alkoxysilane dye and tetraethyl silicate (TEOS) in relatively high loading densities (43 wt%). The resulting film showed a thermal stability up to 302 °C in thermogravimetric analysis (TGA) thermograms and a flat surface morphology in atomic force microscope (AFM) image. The orientation behavior of the poled film was studied by UV–visible spectroscopy. The NLO activity (d₃₃), which was estimated to be 6.5 pm/v at 1064 nm by in situ second harmonic generation (SHG) measurement, the thermal stability of the second-order nonlinearity was also reported.     

Crystallization of polymer-derived SiC/BN/C composites investigated by TEM

The crystallization behavior of two polymer-derived Si/B/C/N ceramics with similar compositions lying close to the three-phase field BN + SiC + C was investigated by (high-resolution) transmission electron microscopy. The materials were high-temperature mass stable up to T = 2000 °C. During thermolysis at 1050 °C a homogeneous amorphous solid formed. SiC crystallization started at about 1400 °C. Further annealing to higher temperatures up to 2000 °C led to formation of microstructures composed of SiC crystals embedded into a structured BNC_x matrix phase. With increasing temperature, both the size of the crystallites and the ordering of the matrix phase increased.     

Pyrolytic conversion studies of blended polycarbosilane and polyaluminasilazane

Multinuclear solid-state NMR spectroscopy, FTIR and Raman experiments are employed to investigate the pyrolytic conversion of blended polycarbosilane and polyaluminasilazane (denoted CA) up to 800 °C, with the aim of studying structural evolutions and interactions between polycarbosilane and polyaluminasilazane during the pyrolysis process. Vinyl and SiCH₃ units can react with Si–H, SiCH₃ and Si–CH₂–Si groups below 400 °C. These crosslinking reactions can increase the ceramic yield of the blended precursors. At 500 °C aromatic carbon is formed, and N–H and Si–H groups vanish at 600 °C and 700 °C, respectively. At 600 °C, SiCH₃ and Si–H units can further react with SiCN₃, SiC₂N₂, N–H and C–H units. An amount of amorphous carbon and CSi₄ and CSi₃H groups are detectable at 800 °C. Even at this temperature there are still many aromatic protons. In addition, there are also SiC₄, SiC₃N, SiCN₃ and SiN₄ units. Silicon forms SiN₄ more readily than SiC₄. Many AlN₅ groups transform into AlN₆ groups. The D and G bands of graphite are observed in CA pyrolyzed at 1400 °C. According to the XRD patterns, the reflection of crystalline β-Si₃N₄ vanishes at 1700 °C, and the residue pyrolyzed at 1800 °C mainly contains a large number of 2H-SiC/AlN solid solution crystals and a few β-SiC crystals.