Transformation of Na2O-CeO2-B2O3 glass into a material with interconnected pores

Substitution of SiO₂ with CeO₂ in the ternary sodium borosilicate system was found to produce phase-separable glasses. Heat treatment of these glasses resulted in separation into two different phases. The one phase enriched in sodium borate was then leached out leaving a CeO₂-rich framework. The structure of the leached material was crystalline (Pt/Rh crucible melt) which changed to a rather net-like appearance if Al₂O₃ resulting from erosion of alumina crucibles was added. B₂O₃ remained partially in the insoluble CeO₂-skeleton. X-ray diffraction analysis of leached material proved the presence of crystalline cubic CeO₂ and cerium borate (metaborate of the aragonite type) in Pt/Rh crucible melts, whereas cubic CeO₂, 2Al₂O₃ · B₂O₃ and traces of sodium borate were detected in Al₂O₃ containing melts. The specific surface areas of the leached materials ranged between 25 and 120m²g^–1 while the main radii of interconnected pores were calculated to be between 0.5 and 17nm. A sintering temperature of about 1500° C was estimated from void volume and bulk density measurements.     

Characterization and sintering of a porous glass-ceramic in the system Na2O-B2O3-Ta2O5

Substitution of SiO₂ in the ternary sodium borosilicate system with Ta₂O₅ was found to produce glasses, which after heat treatment separated into immiscible microphases, one of which was water soluble. The structure of the leached material after heat treatment was a well developed low temperature form of Ta₂O₅. After firing at temperatures between 1100 and 1550°C X-ray diffraction analysis showed the presence of low and high temperature forms of Ta₂O₅ and of orthorhombic Na₂Ta₅O₂₁. The high solubility of up to 40 wt% Ta₂O₅ in the sodium-borate matrix resulting in clear glasses is of practical interest. The specific surface areas of the leached materials ranged between 5.54 and 35.57 m²g⁻¹ while in an additionally Al₂O₃ doped material the value of 307 m²g⁻¹ was measured. Mean pore radii of interconnected pores were calculated to be between 18.63 to 41.12 nm in the Ta₂O₅-rich materials while the additional Al₂O₃ doping decreased the value to 2.71 nm. A sintering temperature between 1500 and 1550°C is estimated from void volume measurements after a series of firing steps at temperatures between 1100 and 1550°C were undertaken.     

Crystallization of CaHf1? x Zr x Ti2O7 (0?≤ x ≤?1) zirconolite in SiO2–Al2O3–CaO–Na2O–TiO2–HfO2–ZrO2–Nd2O3 glasses

Glass-ceramics containing (Hf,Zr)-zirconolite crystals (nominally CaHf_1−x Zr _x Ti₂O₇ with 0 ≤ x ≤ 1) were envisaged to immobilize minor actinides and plutonium. Such materials were prepared in this study by controlled crystallization of glasses belonging to the SiO₂–Al₂O₃–CaO–Na₂O–TiO₂–HfO₂–ZrO₂–Nd₂O₃ system. Neodymium was used as trivalent actinides surrogate. The effect of total or partial substitution of ZrO₂ by HfO₂ (neutron poison for fission reactions) on glass crystallization in the bulk and near the surface is presented. It appeared that Hf/Zr substitution had not significant effect on nature, structure, and composition of crystals formed both on glass surface (titanite + anorthite) and in the bulk (zirconolite). This result can be explained by the close properties of Zr⁴⁺ and Hf⁴⁺ ions and by their similar structural role in glass structure. However, strong differences were observed between the nucleation rate I_Z of zirconolite crystals in glasses containing only HfO₂ and in glasses containing only ZrO₂. Hf-zirconolite (CaHfTi₂O₇) crystals were shown to nucleate only very slowly in comparison with Zr-zirconolite (CaZrTi₂O₇) crystals. Composition changes - by increasing either HfO₂ or Al₂O₃ concentration or by introducing ZrO₂ in parent glass - were performed to increase I_Z in hafnium-rich glasses. The proportion of Nd³⁺ ions incorporated in the zirconolite phase was estimated using ESR.     

Al2O3-Ta2O5-rich glass-ceramic with interconnected pores and its sintering

Substitution of SiO₂ in the ternary sodium borosilicate system with Al₂O₃ plus Ta₂O₅ was found to produce glass which decomposed into microphases and/or crystallized after heat treatment. At least one of the phases present was water soluble. The structure of the material was glassy with the presence of a small crystalline content. Crystalline forms found during powder X-ray diffraction analysis of heat treated, leached and then sintered materials were orthorhombic NaTaO₃ plus AIBO₃, orthorhombic NaTaO₃ and orthorhombic Na₂O · 4Ta₂O₅ plus rhombic 9AI₂O₃ · 2B₂O₃, respectively. The specific surface areas of the leached materials ranged between 96 and 304 m²g^–1, while the mean pore radii of interconnected pores were calculated to be between 2.0 and 8.4 nm. A sintering rate of between 1520 and 1580° C for 5 min were estimated from void volume and bulk density measurements.     

Crystallization of CaO−Al2O3−SiO2 and CaO−MO−Al2O3−SiO2 (M=Mg,Zn) glasses

A range of CaO−Al₂O₃−SiO₂ glasses have been prepared by fusion of pure starting materials in platinum crucibles. Compositions containing large amounts on network formers, Al₂O₃ and SiO₂, are difficult to crystallize. If the amount of network former is reduced, glasses will self-nucleate and crystallize more readily, but the products of crystallization tend to react with water. This conflict has been partly resolved by adding MgO and ZnO and tailoring compositions so as to produce a phase, variously designated “Q” or “pleochroite”, ideally Ca₂₀Al_32-2v MgvSivO₆₈, withv close to 4. Pleochroite crystallizes with a typically fibrous morphology. Preliminary experiments on fragments and melt-cast glass rods indicate that these compositions can be heat treated without deformation to yield highly crystalline, transparent ceramics.     

Effect of Al2O3 concentration on zirconolite (Ca(Zr,Hf)Ti2O7) crystallization in (TiO2,ZrO2,HfO2)-rich SiO2–Al2O3–CaO–Na2O glasses

Glass-ceramic matrices containing zirconolite (nominally Ca(Zr,Hf)Ti₂O₇) crystals in their bulk that would incorporate high proportions of minor actinides (Np, Am, Cm) or plutonium could be envisaged for their immobilization. Zirconolite-based glass-ceramics can be prepared by controlled crystallization of zirconolite in glasses belonging to SiO₂–Al₂O₃–CaO–Na₂O–TiO₂–ZrO₂–HfO₂ system. In this study, neodymium was used as trivalent actinides surrogate. Increasing Al₂O₃ concentration in glass composition had a strong effect on the nucleation rate I _z of zirconolite crystals in the bulk, on the amount of neodymium incorporated in zirconolite phase and on the crystal growth rate of silicate phases (titanite + anorthite) from glass surface. These results could be explained by the existence of competition—in favor of aluminum—between Al³⁺ and (Ti⁴⁺, Zr⁴⁺, Hf⁴⁺) ions for their association with charge compensators cations to facilitate their incorporation in the glassy network. Differential thermal analysis (DTA) was used to study exothermal effects associated with bulk and surface crystallization. ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra showed that aluminum enters glasses network predominantly in 4-fold coordination. Neodymium optical absorption and fluorescence spectroscopies showed that the Al₂O₃ concentration changes performed in this study had not significant effect on Nd³⁺ ions environment in glasses.     

The effect of additives on the crystallization and sintering of 2MgO-2Al2O3-5SiO2 glass-ceramics

Crystallization and sintering behaviour of three cordierite (2MgO-2Al₂O₃-5SiO₂) glasses containing different amount of additives were investigated and compared by using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and the Archimedes method. The stoichiometric 2MgO-2Al₂O₃-5SiO₂ (MAS) glass and the 2MgO-2Al₂O₃-5SiO₂ glass containing 3 wt% of B₂O₃ and 3 wt% of P₂O₅ (MASBP) showed two exotherms (one for -cordierite formation from a glass and the other for -cordierite formation from the -cordierite phase), whereas the 2MgO-2Al₂O₃-5SiO₂ glass containing 2 wt % of B₂O₃, 2 wt% of P₂O₅, and 2 wt % of TiO₂ (MASBPT) showed only a single exotherm representing -cordierite formation. By using Kissinger, Augis-Bennett, Ozawa, and modified Kissinger methods, the activation energy values for -cordierite formation in the MASBP and MASBPT glasses were determined as 310±6 and 326±13 kJ mol^–1, respectively, whereas that in the MAS glass was determined as 868±5 kJ mol^–1. The MASBPT glass showed the lowest peak temperature value for -cordierite formation (980 °C) amongst the three glasses. Both the MASBP and MASBPT glasses showed excellent sintering behaviour (> 99.7% of theoretical density).     

Preparation and properties of glass-ceramic materials obtained by recycling goethite industrial waste

The recycling of toxic goethite waste, originated in the hydrometallurgy of zinc ores, in glass-ceramic matrices has been studied. Oxide compositions suitable to form glasses were prepared by mixing the goethite waste with granite scraps and glass cullet, yielding the following oxide composition (wt%): SiO₂, 44.6; Al₂O₃, 3.3; Fe₂O₃, 25.5; MgO, 1.6; CaO, 4.5; Na₂O, 5.9; PbO, 3.1; ZnO, 6.5; K₂O, 1.0; TiO₂, 2.0; other 2.0. By proper addition of carbon powder, the initial Fe³⁺/Fe²⁺ ratio (12) of glasses melted in air at 1450 °C was approximated to the stoichiometric value of magnetite (2) to obtain high nucleation and crystallization rates. The heat treatment of iron supersaturated goethite glasses above 630 °C led to the formation of magnetite nuclei with a high tendency to grow and coalesce with time. The crystallization of pyroxene, occurring on the magnetite crystals above 800 °C, was found to be influenced by the nucleation period, so that the highest crystalline volume fraction, V _f (0.80–0.85), was obtained for 90–120 min nucleation time at 670 °C and 120 min crystallization at 860 °C. This revised version was published online in November 2006 with corrections to the Cover Date.     

Doping of iron phosphate glasses with Al2O3, SiO2 or B2O3 for improved thermal stability

The effects of doping 60 P₂O₅–40 Fe₂O₃ (mol%) glasses with 5–10 mol% SiO₂, Al₂O₃ or B₂O₃ on their thermal stability, iron environments and redox were investigated. Thermal stability improved markedly with 5% dopant addition in the order Al₂O₃ > SiO₂ > B₂O₃ ≫ base glass. Solubility of pro rata additions when melted at 1150 °C was 5–10% SiO₂, <5% Al₂O₃, and >10% B₂O₃. It was possible to dissolve 5% Al₂O₃ by replacing Fe₂O₃. These additions generally had little effect on dilatometric measurements and iron environments, however the Fe²⁺/ΣFe redox ratio increased in the order base glass < Al₂O₃ < SiO₂ < B₂O₃. This behaviour was broadly consistent with the effects of glass basicity. The increased thermal stability of these glasses may improve their suitability for applications such as waste immobilisation or sealing.     

Glasses and glass-ceramics in the SrO–TiO2–Al2O3–SiO2–B2O3 system and the effect of P2O5 additions

The glass formation abilities of various compositions in SrO–TiO₂–Al₂O₃–SiO₂, SrO–TiO₂–B₂O₃–SiO₂, SrO–TiO₂–Al₂O₃–B₂O₃, and SrO–TiO₂–Al₂O₃–SiO₂–B₂O₃ systems were studied. Many new compositions were found to be suitable for the casting of crack-free, optically clear glasses of different color and with glass transition temperatures ranging from 595 to 775 °C. The crystallization behavior, structure, and thermal expansion behavior of selected glasses were analyzed by DTA, XRD, dilatometry, and heat treatment. The effect of P₂O₅ on the glass structure and crystallization behavior was also studied. P₂O₅ played a dual role depending on composition. In some glasses it acted as a nucleating agent while in others it suppressed crystallization. Heat treatment of borate and borosilicate glasses transformed them into glass-ceramics while comparable SrO–TiO₂–Al₂O₃–SiO₂ glasses showed a lower tendency to crystallize and form glass-ceramics under the same conditions.     

Combined spectroscopic ellipsometry and attenuated total reflection analyses of Al2O3/HfO2 nanolaminates

The microstructure of thin HfO₂-Al₂O₃ nanolaminate high κ dielectric stacks grown by atomic vapor deposition has been studied by attenuated total reflection spectroscopy (ATR) and 8 eV spectroscopic ellipsometry (SE). The presence of Al₂O₃ below HfO₂ prevents the crystallisation of HfO₂ if an appropriate thickness is used, which depends on the HfO₂ thickness. A thicker Al₂O₃ is required for thicker HfO₂ layers. If crystallisation does occur, we show that the HfO₂ signature in both ATR and 8 eV SE spectra allows the detection of monoclinic crystallites embedded in an amorphous phase.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> on the crystallization behavior of SiO<Subscript>2</Subscript>–MgO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

A series of glass comprising of SiO₂–MgO–B₂O₃–Y₂O₃–Al₂O₃ in different mole ratio has been synthesized. The crystallization kinetics of these glasses was investigated using various characterization techniques such as differential thermal analysis (DTA), thermo gravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Crystallization behavior of these glasses was markedly influenced by the addition of Y₂O₃ instead of Al₂O₃. Addition of Y₂O₃ increases the transition temperature, T _g, crystallization temperature, T _c and stability of the glasses. Also, it suppresses the formation of cordierite phase, which is very prominent and detrimental in MgO-based glasses. The results are discussed on the basis of the structural and chemical role of Y³⁺ and Al³⁺ ions in the present glasses.     

Preparation and properties evaluation of zirconia-based/Al2O3 composites as electrolytes for solid oxide fuel cell systems

Yttria-doped zirconia powders containing 3 to 8 mol% Y₂O₃ and 0 to 20 wt% Al₂O₃ were prepared by both mixing commercial oxides and a coprecipitation method, and the mechanical and electrical properties have been examined as a function of the Al₂O₃ content. The bending strength of the composite at room temperature increased with increasing Al₂O₃ content. In the temperature range 500–1000 °C the bending strength increased with Al₂O₃ content up to 10 wt% and then decreased, the measured value at 1000 °C (200 MPa) being higher than those at lower temperatures for cubic zirconia materials. Fracture toughness (K_IC) decreased with increasing Y₂O₃ content in the Al₂O₃-free zirconia materials. Al₂O₃ additions enhanced the fracture toughness and this was maximum (7 MPa m^1/2) for the composite ZrO₂-3 mol% Y₂O₃/10 wt% Al₂O₃. The electrical conductivity of cubic ZrO₂/Al₂O₃ composites decreased monotonically with Al₂O₃ content, but in tetragonal ZrO₂/Al₂O₃ composites hardly varied or apparently increased up to 10 wt% Al₂O₃. At 1000 °C the highest electrical conductivity was 0.30 S cm^–1 for ZrO₂-8 mol% Y₂O₃, and this decreased up to 0.10 S cm^–1 for the composite ZrO₂-8mol% Y₂O₃/20 wt% Al₂O₃.     

Enhanced leakage current properties of HfO2/GaN gate dielectric stack by introducing an ultrathin buffer layer

The band alignments of HfO₂/GaN, HfO₂/SiO₂/GaN and HfO₂/Al₂O₃/GaN gate dielectric stacks were comparatively investigated by using X-ray photoelectron spectroscopy. It was observed that the introduction of an ultrathin buffer layer film (SiO₂ or Al₂O₃) in HfO₂/GaN stack can make the band alignments more symmetrical with larger barrier height as identified by the valence band offsets and electron energy loss spectrum measurements. At room temperature, the leakage current density as function of temperature is 4.1 × 10^?6, 3 × 10^?7 and 9.8 × 10^?8 A cm^?2 at the bias of 1 V for the HfO₂/GaN, HfO₂/Al₂O₃/GaN and HfO₂/SiO₂/GaN gate dielectric stacks, correspondingly. With temperature increase from room temperature to 300 °C, the HfO₂/SiO₂/GaN gate dielectric stack exhibits lowest lower leakage current density than that of others. The HfO₂/GaN high-k gate dielectric stack with an ultrathin SiO₂ buffer layer appears to be a promising candidate for future GaN based high temperature metal-oxide-semiconductor (MOS) devices applications.     

Effect of crucible material on optical bandgap and activation energy of Na<Subscript>2</Subscript>O-CdO-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

In the sodium cadmium phosphate glasses, the effect of diffused alumina from alumina crucible has been assessed by measuring various properties such as mass density, refractive index, optical bandgap and dc conductivity. The results of measurements corresponding to glasses prepared in alumina crucible have been compared with those of glasses prepared in platinum crucible with and without adding Al₂O₃. The Optical bandgap and direct current (dc) electrical conductivity of the Na₂O-CdO-P₂O₅ glasses prepared in alumina and platinum crucibles have been determined at room temperature. These glasses have also been electrically characterised in the temperature range 293–423 K. Activation energy E_a of the samples prepared in alumina crucible lies in the range 0.60–0.96 eV whereas it stands in the range of 0.57–0.94 eV for the samples prepared in platinum crucible.     

The effect of rare-earth oxides on the crystallization of CaO–Al2O3–SiO2 glasses

CaO–Al₂O₃–SiO₂ –La₂O₃–Nd₂O₃ glass was prepared and their physical properties, such as density, glass transition temperature and crystallization temperature, were measured. The heat treatment of these glasses precipitated Ca₂La₈(SiO₄)₆O₂ oxyapatite (CLS) crystal for 20CaO–10Al₂O₃–60SiO₂–10La₂O₃ (mol%) glass and Ca₂Nd₈(SiO₄)₆O₂ oxyapatite (CNS) crystal was precipitated with Nd₂O₃ substitution. Crystallization of these glasses was observed at the surface and internally within the samples. The spherical and stick-like crystals were observed throughout the bulk of the glasses and the surface crystal layer of oxyapatite crystals were strongly oriented along the c-axis. The apparent activation energies of crystal growth were estimated as 360 kJ mol^–1.     

A lanthanum-tantalum (Al) oxide porous glass ceramic

The substitution of the end-member oxides in the ternary glass forming sodium borosilicate system was studied. The replacement of SiO₂ with combinations of La₂O₃ and Ta₂O₅₂ was found to produce glasses which, after heat treatment decomposed to form a leachable sodium borate phase and an insoluble ceramic phase. The mode of phase decomposition is sensitive to impurities: the importance of crucible selection, e.g. Al₂O₃ or Pt/Rh is demonstrated by resulting differences in pore size, surface area, chemical resistance and structure of the insoluble phase for a series of La₂O₃-Ta₂O₅ and La₂O₃-Ta₂O₃-Al₂O₃ glass ceramic. These new materials have a specific surface area of up to 145 m² g^?1 and average pore radii ranging between 0.7 and 28.6 nm and show a varying degree of chemical resistance.     

Influence of TiO2 incorporation in HfO2 and Al2O3 based capacitor dielectrics

Atomic layer deposition was applied to fabricate metal oxide films on planar substrates and also in deep trenches with appreciable step coverage. Atomic layer deposition of Ru electrodes was realized on planar substrates. Electrical and structural behaviour of HfO₂-TiO₂ and Al₂O₃-TiO₂ nanolaminates and mixtures as well as Al₂O₃ films were evaluated. The lowest leakage current densities with the lowest equivalent oxide thickness were achieved in mixed Al₂O₃-TiO₂ films annealed at 700 °C, compared to all other films in as-deposited state as well as annealed at 900 °C. The highest permittivities in this study were measured on HfO₂-TiO₂ nanolaminates.     

Preparation and characterization of sol–gel derived glasses in the ternary Na2O–Al2O3–P2O5 system

A new sol–gel synthesis route to organic free Na₂O–Al₂O₃–P₂O₅ xerogels was developed. These xerogels contain P₂O₅ up to 67.5 mole % and Al₂O₃ up to 20 mole % and could be melted to homogeneous glasses in air at significantly lower temperatures than those applied using the conventionally melting method. Aluminum isopropoxide dissolved in isopropanol, orthophosphoric acid dissolved in isopropanol and sodium nitrate were used as precursors for Al₂O₃, P₂O₅ and Na₂O respectively. Clear stable alcoholic solutions were prepared in the presence of HNO₃. Addition of HNO₃ in the concentration used, enabled homogeneous mixing of the precursors, controlled over the gelation step and prevented the fast precipitation of the AlPO₄ crystalline phase which breaks down the glass formation ability. The sol–gel route developed was investigated using different analytical tools. Differential thermal analysis (DTA) coupled with thermo-gravimetric (TG) analysis indicated that neither organic residues nor sodium nitrate residues were present in the dried gels after gelation. Fourier transform infrared spectroscopy (FTIR) showed the presence of all characteristic phosphate groups and bonds in the dried gels, especially the P=O and the P–O–P bonds. Solid state ³¹P Magic angle spinning nuclear magnetic resonance (MAS NMR) revealed the formation of Q² chain-like polyphosphates of variable polymerization degrees in the dried gels. Solid state ²⁷Al MAS NMR revealed that octahedrally coordinated Al is the preferred moiety in the dried gels. However, powder X-ray diffraction (XRD) investigation indicated the presence of crystalline Q¹ oligomeric pyrophosphate species in the dried gels. Isotropic chemical shifts belonging to these pyrophosphate species were also recorded in the ³¹P MAS NMR spectra. Characterization of the prepared glasses showed that their properties are in very good agreement with those glasses prepared by the conventionally melting method.     

Corrosion by a Heavy Metal Oxide Glass

Melts of lead bismuth gallate compositions are highly corrosive and attack on crucibles of different materials. In the present study, corrosion by a base glass (50PbO-30Bi2O3-20Ga2O3 in mole fraction) melted using different crucibles and the effect on UV-VIS and IR edges were studied. By melting the base glass in platinum/2% rhodium, gold zirconia and alumina crucibles showed less effect on the IR edge and therefore shifted the infrared edge to longer wavelength, whereas silica crucible contaminated the glass, causing a severe deterioration in the infrared and hence shifted infrared edge to much shorter wavelength. In the UV-VIS region, base glass melted in platinum/2% rhodium crucible shifted the edge to the longest wavelength whereas silica crucible shifted the edge to shorter wavelength. The contaminants from gold, zirconia and alumina crucibles caused the UV-VIS edge of the base glass to lie between the two extremes of Pt/2% Rh and SiO2 crucibles. The glasses melted in above mentioned crucibles were