Crystallization of sol-gel derived glass ceramic powders in the CaO-MgO-Al2O3-SiO2 system

Samples in the cordierite primary crystallization field were prepared in the CaO-MgO-Al₂O₃-SiO₂ quaternary system by sol-gel techniques, using both polymeric and colloidal gels. Dried gels were characterized by infrared spectroscopy, thermal analysis and electron microscopy. The crystallization of gel-like glass from both kinds of gels, as a function of temperature was followed by X-ray diffraction. Cordierite was detected as the only final crystalline phase, although some other crystalline phases, different for each kind of gel, were observed in the pathway reaction. The polymorphic modification of cordierite detected was pseudohexagonal, because no evidence of splitting was observed.     

Primary and secondary crystallization processes of WSe2 films

WSe₂ thin films are obtained by annealing tungsten and selenium constituents in thin-film form, under selenium pressure. Thin films have been investigated by X-ray analysis, scanning and transmission electron microscopy, and by microprobe analysis. Annealing temperature and time were used as parameters. It is shown that stoichiometric thin films, crystallized in the hexagonal structure, are systematically obtained. The grain-size evolution with increasing temperature and/or annealing time is interpreted in terms of primary and second crystallization processes (crystallization and recrystallization).     

Thermally activated crystallization of (GeSe2)70 (Sb2Te3 20(GeTe)10 alloy glass: morphological and calorimetric study

The rate of crystallization of (GeSe₂)₇₀(Sb₂Te₃)₂₀(GeTe)₁₀ vitreous samples was investigated by means of differential scanning calorimetry. Crystallization proceeds in two stages. The activation energy of each stage of crystallization was determined. The evolution of the microstructure during crystallization was investigated by X-ray analysis and optical and scanning electron microscopy. Crystalline nuclei emerge in the bulk and develop in a spherulitic form. A lamellar structure is clearly observed when crystallization is completed.     

Crystal nucleation kinetics in a 40CaO-40P2O5-20B2O3 glass — a study of heterogeneously catalysed crystallization

The crystallization mechanism and nucleation kinetics in a 40CaO-40P₂O₅-20B₂O₃ glass were studied using differential thermal analysis, X-ray diffraction, optical microscopy and scanning electron microscopy/energy dispersive analysis of X-rays. After heat treatments at temperatures around the glass transformation temperature, T _g, the first phase to precipitate was BPO₄ and subsequently the phase 4CaO · P₂O₅ appeared by heterogeneous nucleation on the BPO₄. The kinetics of nucleation were studied as a function of temperature. The maximum nucleation rate occurred at 620°C, which is close to T _g. The crystallization peak height from differential thermal analysis was also used to determine the temperature of the maximum nucleation rate.     

Crystallization behaviour of β-spodumene in the calcination of Li2O-Al2O3-SiO2-ZrO2 gels

This work was carried out in order to prepare precursor powders with a spodumene composition (Li₂O·Al₂O₃·4SiO₂, LAS) and to investigate their crystallization behaviours during calcination. A fine β-spodumene type amorphous powder was obtained through sol-gel techniques using LiOCH₃, Al(OC₂H₅)₃, Si(OC₂H₅)₄ and Zr(OC₂H₅)₄ as the starting metal alkoxides. The process included well controlled hydrolysis polycondensation of the raw alkoxides. Differential thermal analysis (DTA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED) analyses were utilized to study the crystallization behaviour of the gels. The activation energy of β-spodumene crystallization was 192 kJ mol⁻¹ for LAS gels with 4 wt% ZrO₂, being much smaller than those of LAS gel without ZrO₂, 382 kJmol⁻¹. For calcination at 800–1200°C, the crystallized phases comprised a major phase of β-spodumene and a minor phase of zirconia (ZrO₂).     

Liquidus surface in the HfO2-Y2O3-La2O3 system

The liquidus surface in the HfO₂-Y₂O₃-La₂O₃ system was studied by differential thermal analysis in helium at temperatures of up to 2500°C, derivative thermal analysis in air at temperatures of up to 3000°C, x-ray diffraction, optical microscopy, and electron microscopy. The liquidus surface was found to comprise five primary crystallization fields-those of theH-Y₂O_3-,C-Y₂O_3-,F-HfO_2-, andX-La₂O₃-based solid solutions and the pyrochlore phase La₂Hf₂O₇. Three invariant equilibria were identified in the system studied-two peritectics and one eutectic.     

Crystallization and structure of a MgO-Al2O3-SiO2-TiO2 glass-ceramic

The structure and changes occurring during the volume crystallization of a MgO-Al₂O₃-SiO₂-TiO₂ glass induced by heat-treatment at temperatures in the range 740 to 1300° C were investigated by means of DTA, scanning electron microscopy, X-ray diffraction, electron microprobe analysis, density and other measurements. Crystallization proceeds by liquid phase separation and coalescence of small particles forming larger microcrystalline regions dispersed in the glassy phase, these regions fill the bulk of the glass during the heat-treatment. Such a mechanism of crystallization leads to the formation of a fine-grained homogeneous structure whose chemical composition is identical to that of the glassy matrix (as is shown by the electron microprobe). The scanning electron microscope revealed two crystalline phases in the initial stage (850 to 890° C) of crystallization while X-ray diffraction identified only a -cordierite type structure, which at temperatures above 1100° C is transformed into the -form. The X-ray diffraction patterns showed further that a cordierite 2MgO.2Al₂O₃.nSiO₂ solid solution may be formed dependent on the heattreatment temperature. Stereoscan micrographs of various stages of crystallization show the development of the glass-ceramic. In addition, the influence of the technique employed to prepare the electron microscope specimens, on their surface topography is also shown.     

Crystallization behaviour of Ni-Co-base metallic glasses by isochronal and isothermal annealing

The crystallization behaviour of three amorphous alloys, Co₅₀Ni₂₅Si₁₅B₁₀, Ni₅₀Co₂₅Si₁₅B₁₀ and Ni₅₀Co₂₅P₁₅B₁₀, was studied by means of differential thermal analysis in conjunction with scanning transmission electron microscopy. Isochronal annealing showed a strong dependence of crystallization on scan rate over the range of 1.99 to 20.70 K min^–1. At high Co/Ni ratios, a sequential two-stage crystallization process involving primary MS-I phase followed by MS-II phase precipitation was observed. At low Co/Ni ratios MS-I and MS-II crystallization were concurrent and inseparable. Replacement of the metalloid Si with P as the glass-former dramatically reduced the activation energy for crystallization as well as the crystallization temperature. A mechanistic understanding of these findings was pursued in light of TEM/STEM microanalysis     

Nanocrystallization of Ge-Al-Cr-Ce-Sm alloy

Ge₆₈Cr₁₄Al₁₀Ce₄Sm₄ amorphous alloy was found to have a multistage crystallization behaviour. First stage of crystallization has been studied by differential scanning calorimetry, X-ray diffraction and transmission electron microscopy including high-resolution imaging. This phase transformation can not be described just by one of the following mechanisms: polymorphous, eutectic, primary and spinodal decomposition and is rather a combination of the polymorphous and primary crystallization. An activation energy E (an energy barrier opposing a reaction) obtained using Kissinger analysis from the shift of the peak temperature in the DSC curve was 360 kJ/mol. Crystalline structure of an unknown phase formed under heating was studied.     

Glass formation and crystallization in the GeSe2-Sb2Te3 system

The glass formation and crystallization of liquid-quenched (GeSe₂)_100-y /(Sb₂Te₃) _y alloys was investigated by means of differential scanning calorimetry, X-ray diffraction and optical and scanning electron microscopy. By water quenching glasses are obtained from compositions in the range 5?y?30. Qualitative parametrization of glass-forming tendency gives, as best glass formers, alloys with y?20. Crystallization on heating proceeds in one stage for glasses withy?20 and in two stages for those with greater Sb₂Te₃ content. For compositions lying in the GeSe₂ primary crystallization region crystals appear preferentially at the surface of the sample, but for the other compositions (24?y?30) the crystals emerge in the bulk and often develop in spherulitic or axialitic form.     

Synthesis of GaN nanorods by ammoniating Ga2O3 films on TiO2 (rutile) layer deposited on Si(111) substrates

GaN nanorods have been synthesized by ammoniating Ga₂O₃ films on a TiO₂ middle layer deposited on Si(111) substrates. The products were characterized by X-Ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transformed infrared spectra (FTIR) and high-resolution transmission electron microscopy (HRTEM). The XRD analysis indicates that the crystallization of GaN film fabricated on TiO₂ middle layer is rather excellent. The FTIR, SEM and HRTEM demonstrate that these nanorods are hexagonal GaN and possess a rough morphology with a diameter ranging from 200 nm to 500 nm and a length less than 10 μm, the growth mechanism of crystalline GaN nanorods is discussed briefly.     

Crystal growth of Bi2Sr2CuO6 from flux system KCl-KF and its superconductivity

Single crystals of Bi₂Sr₂CuO₆ (BSCO) were grown from high-temperature solution using KCl-KF as flux. Differential thermal analysis (DTA) of the starting materials enabled an appropriate temperature programme to be introduced for crystal growth by spontaneous nucleation. By means of the improved thermogravimetric (TG) method, Bi₂Sr₂CuO₆ crystals were grown after determination of the crystallization temperature. A single crystal in the form of a thin rectangular platelet up to 6×4 mm² was obtained and was identified by X-ray powder pattern and by scanning electron microscopy (SEM). Analyses were performed by using high-resolution electron probe microanalysis (EPMA), which indicated that it is the nonstoichiometric compound Bi_2+xSr_2–xCuO₆. The superconducting properties are strongly dependent on the composition of Bi and Sr in the crystals and on heat treatment. The transition temperature of 7 K with 0.5 K width for as-grown crystals, has been obtained.     

The effect of different platinum concentrations as nucleation agent in the BaO/SrO/ZnO/SiO<Subscript>2</Subscript> glass system

The recently reported solid solution of Ba_1?xSr _x Zn₂Si₂O₇ has a similar structure to the high-temperature phase of BaZn₂Si₂O₇ and possesses a coefficient of thermal expansion close to zero or even negative. Without nucleating agents, glasses in this system show solely surface crystallization. In order to stimulate volume crystallization, different quantities of platinum were added to the glass. The characteristic temperatures of the glasses were obtained by differential scanning calorimetry. In order to achieve volume crystallization, a two-step thermal treatment (nucleation and crystal growth) above the glass transition temperature was carried out. The phase identification was performed by X-ray diffraction, and the microstructure was studied by scanning electron microscopy including energy-dispersive X-ray spectroscopy and electron backscatter diffraction.     

Popcorn like morphology and absence of room temperature ferromagnetism in Ni doped SnO<Subscript>2</Subscript> nanoparticles

Ni-doped SnO₂ nanoparticles were synthesized by the microwave oven assisted solvothermal method. The structural characterization was done by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy. The outcomes confirmed that Ni-doped SnO₂ nanoparticles have a pure rutile-type tetragonal phase of SnO₂ structures with a high degree of crystallization and a crystallite size of 10–14 nm. Popcorn like SEM morphology of the nickel doped sample is shown. Optical characterization was done by UV–Vis spectrometer, fluorescence spectroscopy and electron paramagnetic resonance spectroscopy. Magnetic characterization was done by vibrating sample magnetometer (VSM). The VSM measurements revealed that the Ni doped SnO₂ powder samples were diamagnetic at room temperature. This diamagnetic result is in contradiction to earlier published results.     

Crystallization and optical properties of a transparent mullite glass ceramic

Transparent mullite - based glass ceramics in the system SiO₂-Al₂O₃-B₂O₃-ZnO-K₂O were prepared. The activation energy of crystallization, the Avrami constants and the mechanism of crystallization of mullite were determined by the Matusita method through differential thermal analysis. X-ray diffraction and scanning electron microscopy methods were also used to determine the type and morphology of the precipitated crystalline phase. In addition, the degree of crystallinity was determined by the internal standard method. The results showed that bulk crystallization was the main mechanism for crystallization of the glasses. Optical absorption and photoluminescence properties of the obtained transparent glass-ceramics and their initial glasses were also studied. The glass-ceramics exhibited two emissions at 680 and 700 nm.     

Chemische Zusammensetzung und Mikrostruktur polymerabgeleiteter Gläser und Keramiken im System Si–C–O. Teil 2: Charakterisierung der Gefügeausbildung mittels hochauflösender Durchstrahlungselektronenmikroskopie und Feinbereichsbeugung

Chemical Composition and Microstructure of Polymer‐Derived Glasses and Ceramics in the Si–C–O System. Part 2: Characterization of microstructure formation by means of high‐resolution transmission electron microscopy and selected area diffraction Liquid or solid silicone resins represent the economically most interesting class of organic precursors for the pyrolytic production of glass and ceramics materials on silicon basis. As dense, dimensionally stable components can be cost‐effectively achieved by admixing reactive filler powders, chemical composition and microstructure development of the polymer‐derived residues must be exactly known during thermal decomposition. Thus, in the present work, glasses and ceramics produced by pyrolysis of the model precursor polymethylsiloxane at temperatures from 525 to 1550 °C are investigated. In part 1, by means of analytical electron microscopy, the bonding state of silicon was determined on a nanometre scale and the phase separation of the metastable Si–C–O matrix into SiO₂, C and SiC was proved. The in‐situ crystallization could be considerably accelerated by adding fine‐grained powder of inert fillers, such as Al₂O₃ or SiC, which permits effective process control. In part 2, the microstructure is characterized by high‐resolution transmission electron microscopy and selected area diffraction. Turbostratic carbon and cubic β‐SiC precipitate as crystallization products. Theses phases are embedded in an amorphous matrix. Inert fillers reduce the crystallization temperature by several hundred °C. In this case, the polymer‐derived Si–C–O material acts as a binding agent between the powder particles. Reaction layer formation does not occur. On the investigated pyrolysis conditions, no crystallization of SiO₂ was observed.     

Effects of annealing in O2 and N2 on the microstructure of metal organic chemical vapor deposition Ta2O5 film and the interfacial SiO2 layer

Ta₂O₅ films were made by metal organic chemical vapor deposition (MOCVD) and annealed at various temperatures under N₂ and O₂ ambients in a conventional furnace and a rapid thermal reactor (RTR). The microstructure and composition of the Ta₂O₅ film and the interfacial SiO₂ layer before and after various annealing treatments were studied using X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), and Auger electron spectroscopy (AES). The as-deposited Ta₂O₅ film has an amorphous structure. The surface topology of the as-deposited Ta₂O₅ film is smooth without any apparent features. Annealing of the as-deposited film results in crystallization to an orthorhombic structure with (1 0 0) preferred orientation, and an increase in surface roughness, with the appearance of grain boundaries under AFM. The crystallization temperature varies in the various annealing treatments. An interfacial SiO₂ layer was found between the as-deposited/annealed Ta₂O₅ films and silicon substrate. The annealing treatments result in an increase in thickness of the SiO₂ layer and roughness changes of the Ta₂O₅/SiO₂/Si interfaces, which are discussed in terms of element diffusion and thermodynamic stability.     

Crystallization and microstructure of a glass seal for rapid laser sealing in the system CaO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript>

A glass with the composition 61 CaO·30 Al₂O₃·9 SiO₂ was studied with respect to its crystallization behavior and its suitability as a rapidly crystallizing material for laser sealing. The glass was studied by differential scanning calorimetry; from the profiles recorded, Avrami activation energies and Avrami coefficients were calculated. The latter are in the range between 0.99 and 1.55 which is supposedly attributed to sole surface crystallization. During thermal treatment as well as during laser sealing, Ca₁₂Al₁₄O₃₃, CaAl₂O₄ and Ca₃Al₂O₆ are formed. These phases were also observed in SEM micrographs as evidenced by electron backscatter diffraction from the attributed Kikuchi patterns. Transmission electron microscopy showed a crystallized CaO- and SiO₂-enriched interface which strongly adhered to the Al₂O₃ ceramic. The porosity of the crystallized seal was in the order of few percent. The studied glass proved suitable as crystallizing seal for rapid laser sealing.     

CO2响应性PS-b-PDMAEMA接枝十二烷基硫酸钠的研究

采用原子转移自由基可控聚合法(ATRP)成功合成了二氧化碳响应的聚苯乙烯嵌段聚甲基丙烯酸二甲氨基乙酯(PS-b-PDMAEMA),通过电导率的变化证明了其响应性的存在。同时,发现在常温常压下 PS-b-PD-MAEMA 因其独特的 CO2刺激响应性可以与十二烷基硫酸钠(SDS)发生接枝反应,从而获得了结晶性显著增强的新型聚合物。采用核磁共振(NMR)、红外光谱(IR)、透射电镜(TEM)、热重分析(TGA)和差示扫描量热分析(DSC)等手段对产物的结构和性能进行了表征,结果表明 SDS 与 PDMAEMA 中质子化的叔胺基团发生接枝反应,从而延长了增侧链的长度并引发规整排列,实现了结晶性的显著增强。     

Catalytic reduction of CO2 to alcohol with Cu2Se-combined graphene binary nanocomposites

Photocatalytic reduction of CO₂ over a Cu₂Se–graphene nanocatalyst has been investigated. The nanocomposite material was successfully prepared via a modified hydrothermal method. The structure and properties of the prepared composite were characterized by X-ray diffraction, scanning electron microscopy with energy dispersive X-ray microscopy, transmission electron microscopy, Raman spectroscopic analysis, and X-ray photoelectron spectroscopy. A synergetic effect of the combination of Cu₂Se and graphene appeared in the form of excellent photocatalytic reduction capability of CO₂. The photocatalytic efficiencies of the samples were characterized by testing for the photoreduction of CO₂ to alcohol under visible light irradiation, which produced results such as to suggest that there is a significant potential for graphene-based semiconductor hybrid materials to be used as photocatalysts for reduction of CO₂.