A new method for fabricating water repellent silica films having high heat-resistance using the sol–gel method

There has been a great demand in the field of kitchen appliances to develop transparent water repellent films which have high heat-resistance around 300°C. However, those films have not been obtained by conventional sol–gel methods. In this paper, we propose a new method for fabricating transparent water repellent films with high heat-resistance using the sol–gel method, in which silicon or germanium substrates were coated with a solution including tetraethoxysilane (Si(OC₂H₅)₄) and (2-perfluorooctyl)ethyltrimethoxysilane (CF₃(CF₂)₇C₂H₄Si(OCH₃)₃), followed by ‘ammonia-treatment' and annealed at 300°C. The contact angles of water on the ammonia-treated film maintained its initial value, 110° after the heat treatment at 300°C for 250 h while those on the untreated film decreased to 70°, indicating that the ammonia-treatment improves heat-resistance on the film. The mechanism of ammonia-treatment was inferred from FT-IR results; the ammonia-treatment should accelerate hydrolysis and polymerization of FAS and TEOS molecules, resulting in high density of siloxane bonds between FAS and silica glass. These bonds suppress the evaporation of FAS molecules from the film during the heat treatment at 300°C, thus the film has high heat-resistance.     

Electrical properties and deep levels spectra of bulk SiC crystals grown by hybrid physical–chemical vapor transport method

Concentrations of nitrogen shallow donors, boron shallow acceptors, charge carriers, and electron traps were measured as a function of position along the growth axis in a series of undoped 6H–SiC boules grown by sublimation method with and without addition of hydrogen to the growth atmosphere. Elemental analysis by secondary ion mass spectrometry and measurements of electrical properties indicate that the addition of hydrogen suppresses nitrogen incorporation and formation of all electron traps. Concentration of boron is not affected by hydrogen presence. The addition of hydrogen to the growth ambient improves the uniformity of nitrogen incorporation and deep trap distribution along the growth axis. The results are interpreted as due to increased carbon transport and corresponding shift of crystal stoichiometry toward carbon-rich side of the SiC existence range.     

Synthesis and characterization of volatile metal β-diketonate chelates of M(DPM)n (M=Ce, Gd, Y, Zr, n=3,4) used as precursors for MOCVD

High pure Ce(DPM)₄, Gd(DPM)₃, Y(DPM)₃ and Zr(DPM)₄ (DPM=dipivaloylmethanate=2,2,6,6-tetramethyl-3,5-heptanedionato) powders were successfully synthesized from inorganic salts and HDPM in ethanol/aqueous solution followed by recrystallization from toluene. Freshly prepared samples have been characterized by elemental analysis, X-ray diffraction, thermogravimetry-differential thermal analysis, nuclear magnetic resonance spectroscopy and fourier transform infrared spectroscopy. Aged samples, obtained by exposing fresh ones into air for 30 days, were also represented. Various structures, stabilities and volatilities result from different metal atoms and coordination numbers. Those metal β-diketonate chelates are served as precursors of metalorganic chemical vapor deposition for single and multi-component oxide thin films.     

Hydrogenated amorphous silicon with substrate-dependent structure

Hydrogenated amorphous silicon (a-Si:H) thin films grown at 250°C on (1 0 0) crystalline substrate using plasma-enhanced chemical vapor deposition (PECVD) with SiH₄/H₂ gas flow ratio equal to 5/1 (sccm) are investigated by transmission electron microscopy. It is found that the thin film is totally amorphous when grown on a glass substrate. But when the substrate is changed to crystalline silicon, some crystalline grains are found embedded in the amorphous structure in certain regions even if the thickness of the film reaches 600 nm. It is suggested that the amorphous silicon film grown on a crystalline silicon substrate at a temperature of 250°C without heavy H₂ dilution is a mixed network of a small amount of crystalline silicon and the major portion of amorphous silicon.     

Nano-crystallization in LaF3–Na2O–Al2O3–SiO2 glass

Aiming at tailoring optical properties, the precipitation of LaF₃ nano-crystals in LaF₃–Na₂O–Al₂O₃–SiO₂ glass-ceramics is studied thoroughly on the nano-scale using advanced transmission electron microscopic techniques. Nano-sized phase-separation droplets enriched in lanthanum and silicon are formed already in the base glass. Within these less than 20 nm large droplets, LaF₃ crystallizes upon heat treatment. The nano-crystallization mechanism revealed is self-limited since growth is restricted by the size of the droplets. An average crystallite size of around 12 nm is achieved with a narrow size distribution since the phase-separation droplets also contain silicon not incorporated into the growing crystal. Instead, excess silicon relocated to the periphery of the pre-existing phase-separation droplets forms a diffusion barrier around the LaF₃ nano-crystals preventing further crystal growth and/or ripening.     

B NMR investigations in xV2O5–B2O3 and xV2O5–B2O3–PbO glasses

11B (I=3/2) MAS NMR in the binary glass system xV₂O₅–B₂O₃ (x=0.053, 0.43) and the ternary glass system xV₂O₅–B₂O₃–PbO (0.1x1.5) has been investigated at room temperature. In the xV₂O₅–B₂O₃ glasses, one NMR line due to BO₃ unit was observed. Meanwhile in the xV₂O₅–B₂O₃–PbO, two NMR lines which arise from BO₃ and BO₄ units were detected, where the appearance of BO₄ units is produced by the presence of PbO. From the computer-simulation of the ¹¹B NMR central transition line (m=−1/2↔1/2), the quadrupole parameters (e²qQ/h and η) for BO₃ units in xV₂O₅–B₂O₃, and those for BO₃ and BO₄ units in xV₂O₅–B₂O₃–PbO were obtained as a function of x. As the V₂O₅ content increases in xV₂O₅–B₂O₃–PbO, the e²qQ/h and η values of the BO₃⁻ associated resonance are found to slightly decrease and increase, respectively. Meanwhile, the e²qQ/h and η values of BO₄⁻ associated resonance in xV₂O₅–B₂O₃–PbO are found to slightly increase and decrease, respectively. By comparing the intensities of the total transitions (m=−3/2↔−1/2,m=−1/2↔1/2, and 1/2↔3/2) for the ¹¹B NMR line of BO₃ and BO₄ units contained in xV₂O₅–B₂O₃–PbO with those of respective standard samples of 0.053V₂O₅–B₂O₃ and NaBH₄, the quantitative fractions of BO₃ and BO₄ in xV₂O₅–B₂O₃–PbO were obtained as a function of x.     

Te NMR chemical shifts and tellurium coordination environments in crystals and glasses in the Ge-As-Sb-Te system

The local coordination environments of Te atoms have been investigated in crystalline and glassy binary and ternary tellurides in the system Ge-As-Sb-Te using ¹²⁵Te solid-state wideline nuclear magnetic resonance (NMR) spectroscopy. The average ¹²⁵Te NMR chemical shifts in these materials range from 300 to 1050, 90 to 700 and − 2000 to − 4100 ppm for 2, 3 and 6-coordinated environments, respectively. Te atoms are predominantly 2-coordinated in binary Ge-Te, As-Te and ternary Ge-As-Te glasses. The ¹²⁵Te NMR spectrum of the cubic Ge₁Sb₂Te₄ phase with rock salt structure is consistent with a random distribution of Ge/Sb vacancies in the lattice. Besides the coordination number, the ¹²⁵Te chemical shifts in these materials are also found to be sensitive to the chemical identity of the nearest neighbors. ¹²⁵Te NMR spectroscopy shows significant future promise in its application as a technique complementary to diffraction and EXAFS in understanding the short-range structure of amorphous Ge-As-Sb tellurides.     

Multinuclear NMR study of aluminosilicate sol-gel synthesis using the prehydrolysis method

Tetraethoxysilane (TEOS), water and aluminum sec-butoxide (Al(OBu^s)₃) are combined in several stages of prehydrolysis technique, and reaction intermediates at each stage were examined by means of both liquid and solid nuclear magnetic resonance of ¹³C, ²⁹Si, ¹⁷O, and ²⁷Al nuclei. The spectra indicate that when Al(OBu^s)₃ is added to a prehydrolyzed TEOS solution an aluminosilicate precursor is formed in which the aluminum is tetrahedrally coordinated by four silicate ligands. When further water is added, gelation is accompanied by the expansion of aluminum coordination; the spectra indicate that this occurs by the nucleophilic attack of silanol groups. At water contents for which gels of low transparency result, this coordination expansion is accompanied by the formation of new siloxane linkages, but at water contents so high that opaque gels result, the coordination expansion proceeds much more quickly and no new siloxane linkages are observed.     

Homogeneity of a ZrF4-based glass at the nano-scale

A glass of composition 53ZrF₄–20BaF₂–4LaF₃–3AlF₃–20NaF (T_g=260°C) was prepared by careful crucible melting. High-resolution atomic force microscopy of fracture surfaces displayed the presence of nano-pores with diameters of 20–50 nm, being 4–10 nm deep, in all glasses. It was further found that only glasses without annealing and glasses with an annealing step considerably below T_g showed a distinct pattern, i.e. ripples of ≈20 nm in diameter and an rms roughness of ≈0.6 nm. Glasses annealed either near T_g or at the temperatures of maximum nucleation or maximum crystal growth rates showed both regions with the ripple pattern and regions with nano-hillocks, growing in size with increasing annealing temperature and time. Thus these hillocks nearly reach micro-dimensions of ≈270 nm in diameter and ≈65 nm in height following a 90 min annealing step at 343°C, the temperature of maximum crystal growth. These findings give evidence that the glass system, which is thought to be one of the most suitable for fiber drawing, is much less stable against nucleation and crystallization than anticipated.     

Nuclear magnetic resonance studies of alkaline earth phosphosilicate and aluminoborosilicate glasses

The ¹¹B, ²⁷Al, ²⁹Si and ³¹P magic angle spinning (MAS) NMR spectra of MO–P₂O₅, MO–SiO₂–P₂O₅ and MO(M^′₂O)–SiO₂–Al₂O₃–B₂O₃ (M=Mg, Ca, Sr and Ba, M^′=Na) glasses were examined. In binary MO–P₂O₅ (M=Ca and Mg) glasses, the distributions of the phosphate sites, P(Q_n), can be expressed by a theoretical prediction that P₂O₅ reacts quantitatively with MO. In the ternary 0.30MO–0.05SiO₂–0.65P₂O₅ glasses, the 6-coordinated silicon sites were detected, whose population increases in the order of MgOxCaO–0.05SiO₂–(0.95−x)P₂O₅ glasses, its population increases with an increase in f (=([P₂O₅]−[MO]−[B₂O₃]−[Na₂O])/[SiO₂]) and has maximum at f=9. The signal due to the 5-coordinated silicon atoms is also observed when x is smaller than 0.45. When three network-forming oxides such as SiO₂, Al₂O₃ and B₂O₃ coexist, Al₂O₃ reacts preferably with MO. The populations of 4-coordinated boron atoms, N₄, are expressed well with r/(1−r), where r=([Na₂O]−[Al₂O₃])/([Na₂O]−[Al₂O₃]+[B₂O₃]). The correlation of the Raman signal at 1210 and 1350 cm⁻¹ with the NMR signal of Si(Q₆) at −215 ppm is also seen.     

Silicon nanowire growth by electron beam evaporation: Kinetic and energetic contributions to the growth morphology

The vertical and epitaxial growth of long (up to a few microns) silicon nanowires on Si(1 1 1) substrates by electron beam evaporation (EBE) (10⁻⁶–10⁻⁷ mbar) is demonstrated at temperatures between 600 and 700 °C following the vapour–liquid–solid (VLS) growth mechanism from gold nanoparticles. The silicon atoms are provided by evaporating silicon at varying evaporation currents (I_E) between 35 and 80 mA, which results in growth rates between 1 and 100 nm/min. The growth peculiarities in the interaction triangle, evaporation current (I_E), growth temperature (T_S) and gold layer thickness (d_Au) will be reported. Kinetic and energetic contributions to the morphology of silicon nanowires will be discussed.     

Oscillator strength of the infrared absorption band near 1080 cm in SiO2 films

It has been well known that the absorption maximum of the peak near 1080 cm⁻¹ in amorphous SiO₂ films shifts continuously with variation of thickness and properties such as stress. This is a first report on the oscillator strength of the absorption against frequency at the absorption maximum. SiO₂ films on silicon wafers were prepared by thermal growth in either dry O₂ or an O₂/H₂ mixture or liquid-phase deposition in HF saturated with silica gel. The oscillator strength continuously decreased from 1×10⁻⁴ down to 1×10⁻⁵ with the frequency shift from 1099 to 1063 cm⁻¹.     

The impact of the nitridation process on the properties of the Si–SiO2 interface

A newly developed technique for the simultaneous measurement of the oxide–silicon interface properties and of minority carrier lifetime in the silicon volume was used for a systematic study of the nitridation process of oxide films.This technique is based on the surface recombination velocity measurements, and does not require the formation of a capacitor structure, so it is suitable for the measurement of as-grown interface properties. Oxides grown both in dry and in wet environments were prepared, and nitridation processes in N₂O and in NO were compared to N₂ annealing processes. The effect of nitridation temperature and duration were also studied, and processes of rapid thermal oxidation (RTO) and nitridation (RTN) were compared to conventional furnace nitridation processes. Surface recombination velocity was correlated with nitrogen concentration at the oxide–silicon interface obtained by secondary ion mass spectroscopy (SIMS) measurements. Surface recombination velocity (hence surface state density) decreases with increasing nitrogen pile-up at the oxide–silicon interface, indicating that in nitrided interfaces surface state density is limited by nitridation. NO treatments are much more effective than N₂O treatments in the formation of a nitrogen–rich interface layer and, as a consequence, in interface state reduction. X-ray photoelectron spectrometry (XPS) analyses were used to extend our correlation to very thin oxides (3 nm).     

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.     

Synthesis and characterization of SnO-containing phosphorous oxynitride glasses

SnO-containing oxynitride phosphate glasses have been obtained by ammonolysis and their structure studied by nuclear magnetic resonance. The nitrided glasses are characterized by tetrahedral units P(O,N)₄ in which nitrogen atoms have substituted both bridging and non-bridging oxygen atoms. The N/O substitution in the anionic network induces important changes in the glass properties such as an increase in the glass transition temperature and a decrease in the coefficient of thermal expansion. ³¹P MAS NMR shows that PO₄, PO₃N and PO₂N₂ units coexist within the vitreous network and their relative proportions are a function of the nitrogen content as well as of the base glass composition. The influence of the different modifiers on the nitridation process is explained through a comparative study of the LiNaSnPON and LiNaPbPON systems. Unlike lead in oxynitride glasses, tin affects the nitridation mechanism by limiting the nitrogen/oxygen substitution in the anionic network, so that the substitution model is assumed to be closer to the one taking place in alkali phosphate glasses LiNaPON.     

Intermediate-range order in phosphorus-selenium glasses. Constraints from P and Se NMR spectroscopy

³¹P-⁷⁷Se spin echo double resonance and in situ high-temperature ⁷⁷Se nuclear magnetic resonance (NMR) experiments allow the quantification of the ratio of phosphorus-bonded to non-phosphorus-bonded Se atoms in phosphorus-selenium glasses. When interpreted in conjunction with the phosphorus speciations known from ³¹P NMR, these data allow deductions about the intermediate-range order in these glasses. The results indicate that and P-P bonded microstructures are mostly randomly linked in these glasses, with a slight preference for the formation of P-Se-P bridges. The experimental selenium speciations strongly disagree with various hypothesized structures based on isolated PSe_n units, molecular P₄Se_n clusters, and extended phosphorus-enriched domains with P-Se-P links.     

Effect of CaO on the surface morphology and strength of water soaked Na2O–B2O3–Al2O3–SiO2 vitrified bond

The surface morphology of Na₂O–B₂O₃–Al₂O₃–SiO₂ vitrified bond with and without calcium oxide was studied by soaking vitrified bonded microcrystalline alumina composites in water. The content of water introduced to the vitrified bond was determined by thermal gravity analysis, and the effects of water and calcium on the phase separation and nucleation of the vitrified bond were investigated using scanning electron microscope and energy-dispersive X-ray spectrometer. Soaked in water for 72 h, the Na₂O–B₂O₃–Al₂O₃–SiO₂ vitrified bond presented a porous surface, and its bending strength declined with increasing sintering temperature. However, the Na₂O–CaO–B₂O₃–Al₂O₃–SiO₂ vitrified bond was more durable against aqueous coolant even needle-shape crystals were found clustered on the surface of the vitrified bond. The crystals were enriched with aluminosilicate tested by energy-dispersive X-ray spectrums. The appearance of crystals lessened the dissolution of the vitrified bond and made the bending strength increase in the sintering temperature region between 870 °C and 930 °C.     

Ionic transport crossover in mixed conducting chalcogenide glasses detected by Te-Mössbauer spectroscopy

¹²⁵Te-Mössbauer spectroscopy has been used to verify whether the observed ionic transport crossover for the Ag–As–Se–Te glasses with a silver content of 15 at.% at approximately equimolar Se/Te ratio is accompanied by variations in the hyperfine interaction parameters produced by corresponding changes in the microstructural organization of the glass network. We found that an unusual decrease of the quadrupole splitting with decreasing tellurium fraction, r=Te/(Se+Te)<0.5, is caused by the combined effect of increasing Te–Ag(As) interatomic distances and a non-random substitution of multiple Se sites by Te with a preferential occupation of those with at least one Ag nearest neighbour. The latter process leads also to a step-like decrease in the isomer shift at r0.35. The assumed expansion of the glass network at smaller r and the contrasting network contraction for the Te-rich glasses are consistent with the ionic transport crossover.     

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.     

Structure and dynamics of oxide melts and glasses: A view from multinuclear and high temperature NMR

From a presentation of the various nuclear magnetic resonance (NMR) experiments that allows to characterize the local structure and dynamics of oxide glasses and melts, we show that it becomes possible to evidence not only the details of the coordination state of the constituting atoms but also the nature of polyatomic molecular motifs extending over several chemical bonds.