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.     

EPR and magnetic susceptibility studies of xCr2O3-(1-x) 3B2O3-PbO glasses

By using X-ray diffraction analysis, EPR and magnetic measurements, the chromium-ion distribution in xCr₂O₃-(1–x) [3B₂O₃-PbO] glasses with 0x35 mol% Cr₂O₃ was studied. EPR investigation evidenced the presence of both Cr³⁺ and Cr⁵⁺ ions, the latter being in small proportion, in agreement with the atomic magnetic moment values. For concentrations x20 mol% Cr₂O₃ the isolated Cr³⁺ ions coexist with those coupled by super-exchange magnetic interactions, the isolated ones prevailing only for x<3 mol% Cr₂O₃. For x>20 mol % Cr₂O₃ microcrystalline precipitates were detected, giving rise to an antiferromagnetic transition with Néel temperature, N _N310 K and paramagnetic Curie temperature, _p=–480 K.     

The crystallization of calcium phosphate glass with the ratio [CaO]/[P2O5] < 1

The phase evolution in a calcium phosphate glass with the molar ratio [CaO]/[P₂O₅] < 1, to which 6.4 mol% TiO₂ and 10 mol% Al₂O₃ were added as nucleation agents, was studied. The results indicate that the primary phase -Ca₂P₂O₇ is formed during surface nucleation and crystallization which are dominant at T < 930°C. No presence of metastable calcium phosphates was registered during the formation of the primary phase. The growth of -Ca₂P₂O₇ crystals occurs on the faceted crystal/glass interface with dendritic morphology at a crystal growth rate that is independent of time. The kinetics of -Ca₂P₂O₇ crystal growth are characterised by a growth activation energy of E _a = 426 ± 15 kJ/mol. The parameters of the unit cell of the -Ca₂P₂O₇ phase decrease with increasing temperature. The secondary TiP₂O₇ and AlPO₄ phases are formed by volume nucleation and crystallization. The temperature of the maximum nucleation rate was determined to be T _n = 690°C, and it is higher than the transformation temperature T _g. In the temperature interval T < 930°C secondary phases appear during long annealing times. At T > 930°C glass volume crystallization is dominant.     

Magnetic and electrical properties of Al3+-substituted MgFe2O4

The structural, magnetic and electrical properties of the Al³⁺-substituted disordered spinel system Mg(Fe_2–x Al _x )O₄ have been investigated by X-ray diffraction, magnetization, a.c. susceptibility and electrical resistivity measurements. The cation distribution derived from the X-ray diffractometry data was found to agree very well with the cation distribution obtained through Mössbauer spectroscopy. The variation of saturation magnetization per formula unit as a function of aluminium context, x, has been satisfactorily explained on the basis of Neel's collinear spin model and the slight discrepancy between the observed and calculated n _B values can be explained in terms of a random canting model. The Néel temperatures calculated theoretically by applying molecular field theory agreed well with the experimentally determined values from thermal variation of susceptibility and electrical resistivity. An unusual metal-like thermo-electric behaviour was found for the compositions with x 0.3 which was attributed to the decrease in the Fe-Fe separation distance arising from aluminium substitution.     

Preparation of B2O3-P2O5-SiO2 coating films by the sol-gel method

ThexB₂O₃ · (20-x) P₂O₅ · 80SiO₂ (in mol%) glass films withx=0, 10 and 20 have been prepared from metal alkoxides by carrying out the coating in a dry atmosphere. These coating films have shown a larger value of load at scratch and a smaller shrinkage during heat-treatment by replacing P₂O₅ in the films with B₂O₃. It has been found that B₂O₃ more effectively reduces the glass transition temperature of SiO₂ glass than P₂O₅. The concentrations of sodium ions, which migrated from soda-lime-silica glass substrates during the film formation, were higher in phosphosilicate and borophosphosilicate films than in borosilicate and pure silica films. This finding should be ascribed to the gettering effects of phosphorus for sodium ions.     

Surface tension of PbO-B2O3 and Bi2O3-B2O3 glass melts

The surface tensions of xPbO-(100?x) B₂O₃ (x = 30–80 mol%) and xBi₂O₃-(100?x) B₂O₃ (x = 0–100 mol%) melts were measured using the ring method over the temperature range 973 to 1373 K. The compositional and temperature dependences of surface tension were investigated. Addition of PbO and Bi₂O₃ to B₂O₃ increased the surface tensions of their respective PbO-B₂O₃ and Bi₂O₃-B₂O₃ melts. The surface tension showed a maximum at 60 mol% PbO in the PbO-B₂O₃ melts and at 70–80 mol% Bi₂O₃ in the Bi₂O₃-B₂O₃ melts. The temperature coefficient of surface tension was examined on the basis of its relationship to the structure, and it was suggested that the temperature coefficient of surface tension decreases with an increasing content of four-coordinated boron.     

Thermomechanical properties and stability of mixed strontium-zinc borophosphate glasses

Borophosphate glasses of the SrO-ZnO-B₂O₃-P₂O₅ system were prepared in 3 compositional series of 25ZnO-25SrO-xB₂O₃-(50 – x)P₂O₅, (58.3 – x) ZnO-xSrO-16.6B₂O₃-33.33P₂O₅ and (50 – x)ZnO-20B₂O₃-30P₂O₅ and the changes in their thermomechanical and thermal properties with changes in their composition were investigated. The thermal expansion coefficient decreases with increasing B₂O₃ content, whereas the values of glass transformation temperature T _g and dilatation softening temperature T _d increase. The replacement of ZnO by SrO increases both T _g and T _d. From the DSC results the value of the the Hruby's criterion K _gl was calculated and changes in the glass-forming tendency with changes in glass composition were discussed.     

Investigation of structural transport and magnetic properties of the system Li0.25Cu0.5Fe2.25- x Al x O4

Experimental data on X-ray electrical conductivity, thermoelectric coefficient, magnetic hysteresis and infrared absorption spectra of the system Li_0.25Cu_0.5Fe_2.25–x Al _x O₄ are presented. All the compounds, 0x2.25, showed cubic symmetry. Lattice constant values progressively decreased on increasing the Al³⁺ content. X-ray intensity calculations, magnetic hysteresis and infrared spectroscopy studies indicated the presence of Li⁺, Al³⁺ and Fe³⁺ at tetrahedral and octahedral sites, while Cu²⁺ is present only at the octahedral site. The activation energy and threshold frequency increased with increasing values ofx. The compounds withx1.50 aren-type, and those withx2.0 arep-type semiconductors. Magnetic hysteresis indicated that compounds withx1.50 are ferrimagnetic, and those withx2.0 are antiferrimagnetic. High coercive force,H _c, values and remanence ratios (J _R/J_S) showed that all the compounds exceptx2.0 exhibit single-domain behaviour. The probable ionic configuration for the system is suggested as Li _0.15 ⁺ Al _0.5 ³⁺ Fe _0.35 ³⁺ [Li _0.1 ⁺ Cu _0.5 ²⁺ Fe _0.4 ³⁺ Al³⁺]O ₄ ^2– .     

Pressureless sintering of boron carbide with an addition of polycarbosilane

Boron carbide B _x C (x 4) is pressureless sintered with an addition of both polycarbosilane and a small amount of phenolic resin. After testing many green mixtures, it was found that it was necessary to have a minimal addition of carbon which could denxydize the B _x C and also a second phase (SiC in this work) which could act in inhibiting grain coarsening. The resulting B _x C ceramics have high relative density ( 92% theoretical density) and contain no free carbon and a small amount of SiC (about 5 wt%).     

Physicochemical studies on ZnO-Al2O3 system

Several mixed ZnO-Al₂O₃ systems were prepared by the impregnation method and calcined for 5 h at 300, 600 and 1000 ° C. The crystal structure, surface acidity, surface basicity, surface area, catalytic decomposition of H₂O₂ and the electrical conductivity of the samples prepared were studied. It was found that the decomposition of H₂O₂ is catalysed by the acidic sites formed on the catalyst surface at composition less than 50 mol% ZnO and by basic sites for oxides having composition higher than 50 mol% ZnO. ZnAl₂O₄-spinel was found to be formed at temperatures 600 ° C and it has a catalytic activity and electrical conductivity lower than each of the pure ZnO and the oxide mixtures. The results obtained were correlated together and discussed.     

Effect of heat treatment and irradiation on electrical conductivity and crystallization in the BaO-B2O3-Fe2O3 glass system

A glass system was prepared according to the formula 75 mol % B₂O₃-(25 –x) mol % BaO –x mol % Fe₂O₃, wherex = 0, 1, 2.5, 5, 7.5 and 10. The glasses were subjected to heat treatment at 550° C for 2, 6, 12, 18 and 24 h. The glasses were also irradiated using-rays at a dose of 4.805 × 10⁴ rad h^–1 for 12, 18 and 24 h. An X-ray diffraction technique was used to identify the separated crystalline phases. The electrical conductivity and activation energy of untreated, heat-treated and irradiated samples were measured and calculated. The rate and the dimensions of crystallization were also calculated by using the Avrami equation. It was found that-Fe₂O₃ is the separated phase when a sample containing 7.5 mol% Fe₂O₃ is heat treated for 24h;-Fe₂O₃ and Fe₂O₃ are the separated phases when the sample containing 10 mol% Fe₂O₃ is heat treated for 6, 12 and 18 h, with the addition of BaO when the sample is heat treated for 24 h. A miminum value for the electrical conductivity of glass samples was found to occur around an Fe₂O₃/BaO ratio of 0.425. The rate of crystallization in the sample containing 10 mol% Fe₂O₃ is 1.30607 × 10^–3 and the geometry of crystallizationn is 1.2238, which indicates that the crystallization was in one dimension.     

Influence of dissolution rate on crack growth and fatigue of Na2O-Al2O3-B2O3 SiO2 glasses

Fracture toughness, macroscopic crack growth and dynamic fatigue of glasses of the system 15Na₂O-4Al₂O₃-xB₂O₃-(81-x)SiO₂ are studied. The fracture toughness as a function of B₂O₃ content correlates to the dependence of elastic modulus which has a maximum between 20 and 30 mol%. The shape of the crack growth curve changes characteristically. Region 1 of the curves normalized toK _Ic is shifted to higher crack growth velocities (smallerK _I/K _Ic values, respectively) for increasing B203 content. The rise of velocity correlates approximately to the dissolution rate of the glasses in water. The determination of the slope in region 1 is problematic, particularly for poorly resistant glasses. The slopen of the crack growth velocity fitted by the power law decreases above 20 mol% B₂O₃ in correspondence with the results of the dynamic fatigue (23 >n > 7). A clear fatigue limit at one fifth of the inert strength occurs in glasses with B₂O₃ content above 20 mol%.     

Si3N4-ZrO2 composites with small Al2O3 and Y2O3 additions prepared by HIP

Si₃N₄-ZrO₂ composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO₂ and ZrO₂ stabilized with 3 mol% Y₂O₃. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al₂O₃ and 0–6 wt% Y₂O₃. Composites prepared at 1550 °C contained substantial amounts of unreacted -Si₃N₄, and full density was achieved only when 1 wt% Al₂O₃ or 4 wt % Y₂O₃ had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO₂ starting powder was stabilized by Y₂O₃, fully dense Si₃N₄-ZrO₂ composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm^–2 and 4.5 MPam^1/2, respectively. Examples of such materials were either Si₃N₄ formed with an addition of 2–6 wt% Y₂O₃ or Si₃N₄-ZrO₂ composites with a simultaneous addition of 2–6 wt%Y₂O₃ and 2–4 wt% Al₂O₃.     

Effect of gadolinium content on the thermal volume change of Ba4Na2Nb10O30 − Ba3NaGdNb10O30 solid solutions at ferroelectric phase transition temperature

For sintered samples of Ba₃₊ x Na_1+xGd_1–xNb₁₀O₃₀ (1 x 0), the effect of the addition of gadolinium to barium sodium niobate (BNN,x = 1, tungsten bronze type) was examined by differential scanning calorimetry (DSC) and X-ray powder diffractometry. The volume change (i.e. especially the change ofc-axis length) at the ferroelectric phase transition temperature (T _c< 590 dg C) decreased with the increasing gadolinium content. This result suggests that the addition of gadolinium or other lanthanide elements to BNN is possibly effective to obtain an uncracked single crystal with the Czochralski technique. The DSC indicator method is useful to determine the relative magnitude of volume change atT _c for the same type of sample series, a method which is much easier than the dilatometer method. The relation between the DSC peak area and the volume change of unit cell atT _c is discussed from the thermodynamic viewpoint.     

The effect of MgAl2O4 on the formation kinetics of Al2TiO5 from Al2O3 and TiO2 fine powders

The formation of Al_2(1–x)Mg _x Ti_(1+x)O₅ solid solutions from Al₂O₃-TiO₂-MgAl₂O₄ powder mixtures of 1 m particle size and moderate purity has been studied at 1300°C for different final composition values: x=0 (pure Al₂TiO₅), 10^–3, 10^–2 and 10^–1. Analysis of the kinetic data and microstructural observation indicates that MgAl₂O₄ affects the mechanism of Al₂TiO₅ formation by providing active nuclei for the growth of the new phase. These nuclei are probably constituted by Mg_0.5AlTi_1.5O₅, i.e. the equimolar Al₂TiO₅-MgTi₂O₅ solid solution, and are formed by reaction between MgAl₂O₄ and TiO₂ at temperatures above 1150 °C. As the value of x increases, the number of titanate particles per unit volume accordingly increases and the conversion of the original oxides is faster. At values of x10^–2, the prevailing mechanism is the nucleation and growth of Al₂TiO₅ nodules for fractional conversion up to 0.8. Further conversion of the residual Al₂O₃ and TiO₂ particles dispersed into the titanate nodules is slower and controlled by solid-state diffusion through Al₂TiO₅. At x=0.1, a large number of nucleation sites is present, and solid-state diffusion through Al₂TiO₅ becomes important even in the initial stage of reaction, as the diffusion distances are strongly reduced. The study of Al₂TiO₅ formation under non-isothermal conditions in the temperature range 1250–1550°C shows that reaction proceeds between 1300 and 1350 °C for x=0.01 and between 1250 and 1300 °C for x=0.1. Densification of the titanate becomes important at temperatures above 1300°C for x=0.1, but only above 1450 °C for x=0.01.     

Low-temperature specific heat of YBa2Cu3O7, YBa2Cu3O6, Y2BaCuO5, YBa3Cu2O7, BaCuO2, and CuO

We have measured the low-temperature specific heat (1.3T20 K) and the dc magnetic susceptibility (100T250 K) of eight samples of the high-T _c superconductor Y _x Ba_3–x Cu₃O_7– (x=0.9, 1.0, 1.1) and of two samples of nonsuperconducting YBa₂Cu₃O₆₊. We have also performed specific heat measurements on the possible impurity phases: YBa₃Cu₂O₇, Y₂BaCuO₅, CuO, and BaCuO_2+x . The superconducting samples all have a nonzero, sample-dependent linear term * and an upturn inC/T at very low temperature. We show that this anomalous behavior is at least partly due to the presence of a small amount (1%) of BaCuO_2+x impurity phase in the measured samples. This is evidenced by the correlation between * and the Curie component of the susceptibility, which is proportional to the amount of paramagnetic impurities.     

EPR study of Cu2+ in Na2O-NaF-B2O3-Bi2O3 glasses

Electron paramagnetic resonance (EPR) studies have been performed on the glass system (30–x) NaF-xNa₂O-50B₂O₃-20Bi₂O₃ doped with CuO as a paramagnetic probe. The calculated g values indicate that Cu²⁺ is in a tetragonally elongated octahedral co-ordination, and the Jahn-Teller character gives rise to anisotropic hyperfine structure. The spin-Hamiltonian parameters and -bonding parameter, ², are calculated. Varying the fluoride ion concentration and its effect on these parameters is also discussed.     

BPO4@B2O3 and (BPO4@B2O3):Eu3+: The novel single-emitting-component phosphors for near UV-white LEDs

Nowadays much effort has been devoted to exploring novel luminescent materials with low-cost, high stability and excellent luminescent properties. In this paper, a new kind of luminescent material BPO₄@B₂O₃ was prepared by using a facile method. The as-obtained samples contain numerous BPO₄ nanoparticles enclosed by amorphous and crystalline B₂O₃ homogeneously, which exhibits a broad emission band ranging from 380 to 700 nm under near-UV irradiation. More importantly, it is worth noting that the BPO₄@B₂O₃ phosphor exhibits the excellent thermal quenching property, which endows it with a promising prospect as phosphors for high power white LEDs. To further promote its application as white light phosphors, Eu³⁺ ions were doped into the BPO₄@B₂O₃ samples and prepared the (BPO₄@B₂O₃):Eu³⁺ phosphors with chromaticity coordinates (0.3022, 0.3122). The corresponding packaging of LEDs indicates that both BPO₄@B₂O₃ and (BPO₄@B₂O₃):Eu³⁺ can be considered as the promising phosphors for WLEDs.     

Synthesis,physical, ultrasonic waves,mechanical, FTIR,and dielectric characteristics of B2O3/Li2O/ZnO glasses doped with Y3+ ions

Six specimens of glasses with formula (70???x)B₂O₃/15Li₂O/15ZnO/xY₂O₃: x?=?0.0, 0.5, 1.0, 1.5, 2.0, and 2.5 mol%) have been synthesized via a conventional melt quenching technique. The produced specimens were named as BLZY0.0–BLZY2.5 according to x values. The physical, ultrasonic longitudinal (V_L) and shear (V_S) velocities, FTIR, and dielectric (50 Hz to 5 MHz) characteristics of the prepared glasses have been examined. With increasing content of Y₂O₃ from 0.0 to 2.5 mol%, the density (ρ) of the system increases linearly from 2512?±?11 to 2695?±?14 kg/m³, while the molar volume (V_M) decreases linearly from 2.6?±?0.011 to 2.57?±?0.013?×?10^?5 m³/mol. The oxygen packing density (OPD) as a number of the oxygen per unit composition in the glass sample is describing the packing tightness of the oxide network and thoroughly the compactness of the glass matrix. Values of the average boron–boron separation (d_B–B) decrease from 4.162 to 4.035?×?10^?10 (m) with 0 to 2.5 mol% Y₂O₃. Increasing formation of Y³⁺ ionic bonds with [BO_4/2]^1? may have an effect of lowering bond strength of B–O and thus shifting the absorption IR peak position. By increasing Y₂O₃ content in the investigated samples, the (V_L) and (V_S) increase linearly for the full-studied compositional range. The increasing number of strengthened bonds due to change coordination of B ions from 3 to 4 due to the increasing field strength of inserted accumulated Y³⁺ ions has the incentive impact to higher mechanical properties. The dielectric constant was decreased for Y₂O₃ content up to 1.5 mol% referring to cross-linkage formation with other elements, while the reduction in porosity at high content of Y₂O₃ is the main responsible for gradual enhancement in dielectric constant.

     

Anomalous temperature dependence ofD andT 2 for dilute solutions of3He in solid4He

Results of measurements of the spin diffusion coefficientD and NMR relaxation timesT ₁,T ₂, and T_1p are presented for a range of fractional³He concentrations 1 × 10^–4x ₃2.5 × 10^–3 in solid⁴He at molar volumes 19.85V _m21.0 cm³ and temperatures 0.4<T<2 K in both hcp and bcc phases. We observe a minimumD(T) atx ₃=5×10^–4, which is interpreted in terms of a transition from coherent impuriton motion to thermally activated diffusion. ForT<0.8 K, (lnD)/(lnV _m)=60±8. TheT ₂ measurements show a minimum as a function of temperature forx ₃10^–3. TheT ₂ (T) andT ₁ (T) results yield values for activation energy and tunneling frequency of vacancies in these dilute solutions. Forx ₃=5×10^–4 andT 0.5 K,T ₂ (V _m) is anomalous.T ₁ measurements at the same concentration indicate there is an important contribution to the spectral density of dipole field fluctuations in the kHz region.Financial support provided for apparatus, materials, and a research studentship (ARA) by the Science Research Council.