Effect of boron on the crystallization of amorphous Si–(B–)C–N polymer-derived ceramics

Amorphous Si–(B–)C–N polymer-derived ceramics (PDCs) with a boron content ranging from 0 to 8.3 at.%, were synthesized by thermolysis of boron-modified poly(methylvinylsilazane). Correlation of the boron content and the thermal stability of these materials in the course of annealing were investigated using high temperature thermal gravimetric analysis (HT-TGA). Furthermore, the initial crystallization of the as-thermolyzed amorphous ceramics was studied by X-ray diffraction (XRD) measurements. The increase of boron content promotes the crystallization of SiC, and inhibits the crystallization of Si₃N₄. Moreover, the ratio of α-Si₃N₄/β-Si₃N₄ in crystalline ceramic decreases with increasing boron content. Thermodynamic modeling proves the influence of boron content on driving force for crystallization. The available thermodynamic model of amorphous Si–C–N domains, nano-crystalline silicon nitride, and nano-crystalline silicon carbide treated as the separated phases has been used to interpret these results.     

HRTEM and ELNES analysis of polycarbosilane-derived Si–C–O bulk ceramics

A series of analyses by high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS) has been carried out to investigate the nanostructure and the chemical bonding of Si–C–O bulk ceramics prepared by pyrolysis of oxygen-controlled polycarbosilane (PCS). HRTEM revealed that the Si–C–O ceramics with higher oxygen content tended to keep an amorphous microstructure. EELS allowed the examination of energy-loss, near-edge-structures (ELNES) that detected clear transition signals of the chemical bonding around Si atoms from Si–C bonding to Si–O bonding with an increase of oxygen content. The microstructure of the PCS-derived Si–C–O bulk ceramics is characterized as a function of oxygen content.     

Tuning of morphology of Ag nanoparticles in the Ag/polyaniline nanocomposites prepared by γ-ray irradiation

Ag/polyaniline (PANI) nanocomposites were prepared by two different methods using γ-ray irradiation. The morphology of the Ag nanoparticles in the nanocomposites was followed by transmission electron microscopy (TEM). In one of the method (Method I), Ag/PANI nanocomposites were prepared by the following sequential steps. PVP-stabilized Ag colloids were prepared by γ-irradiation, aniline was added and oxidatively polymerized. Method II involved the preparation of Ag/PANI nanocomposites by oxidative polymerization of aniline-stabilized Ag colloids prepared by γ-irradiation. The average size of PVP-stabilized Ag sphere-type nanoparticles was 13 nm. The morphology of Ag nanoparticle in Ag/PANI nanocomposites prepared by Method I was sphere-type. On the other hand, the morphology of aniline-stabilized Ag nanoparticle prepared by γ-irradiation was a hexangular-type one. Size of aniline-stabilized Ag nanoparticles was found to depend on the weight ratios of aniline-to-Ag ions used in the preparation. The paper discusses the changes in the morphology of Ag nanoparticles in the Ag/PANI nanocomposites with the method of preparation, source of protection and polymerization.     

Effect of Al addition on the crystallization of a-AlxSi1−x (0.025 x 0.100) by electron-beam irradiation

Si crystals and nano-rods were formed in Al-added amorphous Si films (a-Al_xSi_1−x; 0.025 x 0.100) by the irradiation of a focused electron-beam; the films were in situ heated to be kept at 400 °C and the current density of the electron-beam was 15.7 pA/cm². The size, shape, and concentration of the Si crystallites were varied sensitively with the Al content as well as the irradiation time. Under the electron-beam irradiation, crystallization occurred to produce polycrystalline phases in the a-Al_0.025Si_0.975 film, while rod-shaped Si nanostructures were formed in the a-Al_0.050Si_0.950 and a-Al_0.100Si_0.900 film. It is evident that the removal of Al and as a result the atomic rearrangements and/or local restructuring in the Al/a-Si film are critically affected by the electron-beam irradiation, which lead to the local crystallization and growth of Si nanocrystallites.     

A magnetic resonance study on the structure of amorphous networks in the Si–B–N(–C) system

The amorphous networks Si₃B₃N₇ and ‘SiBN₃C' are studied by solid-state nuclear magnetic resonance (NMR), continuous-wave and pulse electron paramagnetic resonance (EPR), and by one- and two-dimensional electron nuclear double resonance spectroscopy. In both compounds, boron is found to be coordinated exclusively by nitrogen with close to trigonal planar geometry and close to equal bond lengths. Silicon is four-coordinated by nitrogen with the coordination tetrahedra being distorted to accommodate the coordination preferences of boron. REDOR measurements demonstrate that boron resides in the second coordination sphere of silicon. Carbon incorporation into the Si–B–N network does not lead to any observable changes in NMR parameters including the average dipolar coupling between ¹¹B nuclei which depends on the average distance of the boron atoms. Only spin–lattice relaxation of the nuclei is accelerated due to the generation of paramagnetic centers. The unpaired electrons appear to be delocalized over several carbon atoms and exhibit significant hyperfine couplings to boron, silicon, nitrogen, and some residual protons. In contrast to electron spectroscopic imaging experiments, the magnetic resonance results suggest formation of carbon clusters.     

Effect of different fuels on the alumina–zirconia nanopowder synthesized by sol–gel autocombustion method

In this research, a sol–gel autocombustion route has been proposed to synthesize alumina–zirconia nanopowder, using aluminium nitrate, zirconium oxychloride and various fuels such as citric acid, acetylacetone, oxalic acid and urea. The phase analysis and particle size in the presence of different fuel were compared. The results showed 100% tetragonal phase as well as particle size of 60 nm in the presence of citric acid. FTIR confirms the formation of -Al₂O₃ in corroboration with X-ray studies.     

Synthesis of Nb2O5·nH2O nanoparticles by water-in-oil microemulsion

Hydrous niobium oxide (Nb₂O₅·nH₂O) nanoparticles had been successfully prepared by water-in-oil microemulsion. They were characterized by X-ray diffraction (XRD), thermal analysis (TG/DTG), Fourier transform infrared spectroscopy (FTIR), BET surface area measurement, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed that the nanoparticle was exactly Nb₂O₅·nH₂O with spherical shape. Their BET surface area was 60 m² g⁻¹. XRD results showed that Nb₂O₅·nH₂O nanoparticles with crystallite size in nanometer scale were formed. The crystallinity and crystallity size increased with increasing annealing temperature. TT-phase of Nb₂O₅ was obtained when the sample is annealed at 550 °C.     

Crystallization kinetics of the Zr61Al7.5Cu17.5Ni10Si4 alloy using isothermal DSC and TEM observation

The crystallization behavior and microstructure development of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy during annealing were investigated by isothermal differential scanning calorimetry, X-ray diffractometry and transmission electron microscopy. During isothermal annealing of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy at 703 K, Zr₂Cu crystals with an average size of about 5 nm were first observed during the early stages (30% crystallization) of crystallization by TEM. The Zr₂Cu crystal size increased with annealing time and attained an average size of 20 nm corresponding to the stage of 80% crystallization. In addition, the change in particle size with increasing annealing time exhibited a linear relationship between grain growth time and the cube of the particle size for the Zr₂Cu type crystalline phase. This indicates that the crystal growth of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy belongs to a thermal activated process of the Arrhenius type. The activation energy for the grain growth of Zr₂Cu is 155 ± 20 kJ/mol in the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ amorphous alloy. The lower activation energy for grain growth in compared to that for crystallization in Zr₆₅Cu₃₅ 440 kJ/mol crystal corresponds to the rearrangement of smaller atoms in the metallic glass, Al or Si (compare to Zr).     

Kinetic vs. thermodynamic control of crystal nucleation and growth in molten silicates

A series of calcium aluminosilicate liquids have been experimentally heat-treated at high but variable states of undercooling, from just above the glass transition to the vicinity of the solidus. The mineralogy and chemistry of crystalline phases which appear in these experiments have been quantified using a combination of Transmission Electron Microscopy and Raman spectroscopy. The results show that mineral compositions are highly variable as a function of temperature, but that changes are governed by the contrasting and strongly temperature-dependent mobilities of network-modifying and network-forming cations. Whereas equilibrium crystals form near the liquidus, disordered and non-stoichiometric phases precipitate near the glass transition. Despite this apparently complex situation, the relative importance of thermodynamic and kinetic factors is found to be a single function of T − T_g (where T is temperature and T_g the glass transition temperature), regardless of the silicate composition. The existence of this mastercurve may be used to control the composition of novel composite materials such as glass ceramics.     

Pore formation in glass-ceramics: Influence of the stress energy distribution

In glass-ceramics, the density difference between the new, semi-crystalline system, and the ambient phase requires a deformation of the grains. However, the first stage of surface induced crystallization is the creation of rigid shell, opposing the shrinkage. Therefore, an important stress appears inside the grain. If the average density of the new system is higher than that of the ambient phase, a tensile stress is generated. In the opposite case, a compressive stress is developed. As soon as the system is neither pure elastic body nor pure plastic one, the concentration of the stress energy depends on the distance from the interface. We describe theoretically the distribution profile of the stress energy. Depending on the stress attenuation parameter and the grain size, there are two solutions. The first one predicts a maximum in the middle of the grain. According to the second, there are two maxima close to the crystal/glass interface. This explains the appearance of cabbage like crystals or of crystalline grains with a pore in the center.     

Local structural environment and intra-4f transition of rare-earth ion in chalcogenide glass: Comparison between Dy-doped Ge–As–S and Ge–Ga–S glasses

We have employed Dy L₃-edge extended X-ray absorption fine structure measurements to investigate the local structures of Dy doped in Ge–As–S and Ge–Ga–S glasses. It turned out that Dy–S distance on average is conspicuously longer in Ge–Ga–S glass despite the number of the nearest neighboring S atoms being nearly identical with each other. The enhanced rare-earth solubility of Ge–Ga–S glass is then explained in connection with decrease in covalence of Ga–S bonds compared with Ge–S or As–S bonds, and structural correlation between GaS₄ tetrahedra and Dy. The discrepancy in the local structural environments of trivalent Dy, however, results in no significant changes in the spectral lineshape and the mean energy of its intra-4f-configurational transitions.     

Features in the photoluminescence line-shape of heavily Er-doped Ge–S–Ga glasses

Erbium doped chalcogenide glasses are of great interest in the integrated optoelectronic technology due to their Er³⁺ intra-4f emission at the standard telecommunication wavelength of 1.54 μm. In this paper, the photoluminescence (PL) of a series of (GeS₂)_x(Ga₂S₃)_100−x (x = 75 and 67) glasses doped with high amounts of Er₂S₃ (1.8, 2.1, 2.4 and 2.7 mol%) under excitation with 1064 nm light has been studied. A quenching PL effect at 1.22 аt.% Er-doped (GeS₂)₇₅(Ga₂S₃)₂₅ and 1.39 аt.% Er-doped (GeS₂)₆₇(Ga₂S₃)₃₃ glasses has been established. The relative changes in PL line-shape at around 1540 nm have been estimated by deconvoluting the spectra to Gaussian sub-bands centered at 1519 ± 1, 1537 ± 1, 1546 ± 1, 1555 ± 1 and 1566 ± 4 nm, which correspond to F₂₁, F₁₁, F₂₂, F₁₂ and F₁₃ transitions in the ⁴I_13/2 and ⁴I_15/2 energy levels and have intensity and manifestation that are strongly depend on the Er-doping level. The influence of gallium on the PL efficiency has been evaluated with a view to enhanced emission cross-section.     

Crystallization behavior in a highly driven marginal glass forming alloy

Al₉₀Sm₁₀, a marginal glass former, was rapidly solidified using Cu-block single roller melt spinning at wheel speeds of 30 and 40 m/s. The product phases of rapid solidification were identified and analyzed using high energy synchrotron X-ray diffraction (HEXRD), high resolution transmission electron microscopy, and atom probe tomography. The as-quenched structure consists of a saturated amorphous phase and nanocrystalline Al with typical length scale of about 5 nm. The appearance of a pre-peak on HEXRD diffraction patterns and a low activation energy for first crystallization as determined using the Kissinger and Ozawa methods indicate some local ordering in the amorphous phase. The devitrification phase transformation path was determined using in situ high energy synchrotron radiation. Three phases, MS1, H1, and Al₄Sm, were identified during decomposition of the amorphous phase. MS1, H1 and Al₄Sm are cubic, hexagonal and orthorhombic metastable phases, respectively.     

The effect of aluminum sources on synthesis of low expansion glass–ceramics in lithia–alumina–silica system by sol–gel route

This paper investigated the effect of different aluminum sources on the crystallization behaviors, thermal expansion and morphology of lithium aluminosilicate glass–ceramics sintered at different temperatures. Specimens were prepared by sol–gel technology. The crystalline phase in the specimen with Al(NO₃)₃9H₂O aluminum source sintered at 1300 °C has shown a major phase of Li₂Al₂Si₃O₁₀ accompanied by minor phase of LiAl₅O₈ and trace amount of SiO₂. The β-eucryptite appeared as the only phase in the specimens with aluminum isopropoxide or aluminum isopropoxide mixed with Al(NO₃)₃9H₂O aluminum sources sintered at 1300 °C. The sintering temperature has significant effect on the thermal expansion behavior of all the specimens. SEM indicated that microstructures were spherulitic crystals and no distinct differences were observed in the specimens with different aluminum sources.     

A study of the chemical reactions involved in Li–Ca–N system

The chemical reactions and phases involved in the potential flux system of Li–Ca–N for the growth of bulk GaN crystals have been investigated under varying conditions. It is found that no preferential nitrification of Li or Ca by N₂ in Li–Ca melts at 500 °C. Only the ternary compound LiCaN is identified in the Li–Ca–N system under the present experimental conditions. Static N₂ pressures are found to enhance the formation of LiCaN compared to an N₂ stream. LiCaN forms from two possible pathways: one is a modified metathesis chemical reaction represented by Li₃N+Ca→LiCaN+2Li, and the other is a combination chemical reaction represented by Li₃N+Ca₃N₂→3LiCaN. The formation of LiCaN by the metathesis reaction is thermodynamically favored over the other pathway. In addition, the formation of LiCaN might benefit from a slightly larger initial amount of Li₃N compared with Ca or Ca₃N₂.     

Rapid quenching and glass formation in chalcogenide glasses: Preparation and properties of Ge–As–Te glasses over an extended composition ranges

In Ge–As–Te system, the glass forming region determined by normal melt quenching method has two regions (GFR I and GFR II) separated by few compositions gap. With a simple laboratory built twin roller apparatus, we have succeeded in preparing Ge_7.5As_xTe_92.5−x glasses over extended composition ranges. A distinct change in T_g is observed at x = 40, exactly at which the separation of the glass forming regions occur indicating the changes in the connectivity and the rigidity of the structural network. The maximum observed in glass transition (T_g) at x = 55 corresponding to the average coordination number (Z_av) = 2.70 is an evidence for the shift of the rigidity percolation threshold (RPT) from Z_av = 2.40 as predicted by the recent theories. The glass forming tendency (K_gl) and ΔT (=T_c−T_g) is low for the glasses in the GFR I and high for the glasses in the GFR II.     

Investigation of the growth processes from vapor phase of silicon carbide epitaxial layers: Thermodynamic analysis of the high temperature processes in the system Si‐C‐H‐Cl

In the present work are presented the results of the thermodynamic analysis of the interaction processes in the system Si‐C‐H‐Cl in the temperature interval 1000‐3000 K. The equilibrium pressures of the components in the system Si‐C‐H‐Cl with taking account the formation of the condensed phases C, Si and SiC have been determined. The optimal conditions giving the maximum yield of silicon carbide by pyrolysis of mixture of volatile compounds of carbon and silicon have been defined. The thermodynamic analysis of the examined system showed that the increasing of the content of hydrogen in the initial mixture allows to decrease the optimal temperature for obtaining of silicon carbide by the method of pyrolysis and essentially to increase its maximum possible yield. (© 2008 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron microscopy investigations on structures of ZrO2‐rich phases in the quasibinary system ZrO2‐Zr3N4

Slowly cooled nitrided zirconia samples were investigated and characterized by means of transmission electron microscopy (TEM). Besides the well‐known different modifications of ZrO₂ some ZrO₂‐rich oxynitride phases could be further found, called β‐type phases. An overview regarding the formation of such different structural modifications could be gained, supported by previously performed measurements using powder X‐ray diffractometry (XRD). Similar to fast cooled nitrided zirconia samples Zr(N,O), the slowly cooled ones also contain different kinds of ZrO₂ precipitates, which can be obviously emphasized by using the methods of diffraction contrast. A super cell was successively built, derived from the unit cells of the β and β′ phase, in order to explain the structure of the observed modulated β″ phase. Agreements from the comparisons between experimental high resolution images and the simulated ones of such modulated structures confirm the suggested starting points. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structure,thermal analysis and IR spectroscopy investigations of N,N‐dimethyl‐1,3‐propanediammonium monohydrogenmonophosphate trihydrate

A new organic monohydrogenmonophosphate (C₅H₁₆N₂)HPO₄.3H₂O (abbreviate as MPAP) is prepared by reacting H3PO4 with N, N‐dimethyl‐1,3‐propanediamine. This compound crystallizes in the orthorhombic crystal system, space group Pca2₁. Unit cell parameters are a % 8.1445(1) Å, b % 11.7734(2) Å, c % 12.9021(2) Å, with, Z % 4 and ρ_m % 1.31 g cm⁻³. The structure was solved, using the direct methods and refined against F² to a reliability R factor of 0.0257. Three types of hydrogen bonds participate to the structural cohesion: O(P)—H…O, O(W)—H…O and N—H…O. The first one connects HPO₄ groups in infinite chains. This organization of the phosphoric groups creates voids in which are located the water molecules which are themselves connected by the second type of hydrogen bonds to the adjacent phosphoric groups that lead to a typical layer organization of a polyanion [HPO₄.(H₂O)₃ ]²ⁿ⁻_n. The third hydrogen bond type is responsible for the cohesion between the two‐dimensional polyanions. Thus, a framework in a threedimensional way is then created. The thermal decomposition of MPAP shows a large endothermic effect corresponding to the elimination of the water molecules and a set of endotherms which are probably due to the evolution of ammonia from the structure and the decomposition. The title compound was also characterized by IR spectroscopy, the interpretation of the spectra is based on theoretical analyses and literature data. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Absorption and photoluminescence of PbS QDs in glasses

Optical properties of PbS quantum dots (QDs) precipitated inside the oxide glass matrix were investigated. Photoluminescence (PL) from the PbS QDs showed peak wavelengths located at 1170–1680 nm with widths of 150–550 nm. Radii of QDs in glasses were 2.3–4.7 nm depending upon the thermal treatment. Peak wavelengths of PL bands shifted as much as 70 nm as the temperatures and excitation irradiances increased. Calculated effective local temperatures indicated that these shifts of PL spectra were associated with local heating induced by the temperatures and laser beam.