Residual stress versus microstructural effects on the strength and toughness of phase-separated PbO–B2O3–Al2O3 glasses

A few authors have reasonably proposed that liquid–liquid phase-separated (LLPS) glasses could show improved fracture strength, S_f, and toughness, K_Ic, as the second phase could provide a barrier to crack propagation via deflection, bowing, trapping, or bridging. Due to the associated tensile or compressive residual stresses, the second phase could also act as a toughening or a weakening mechanism. In this work, we investigated five glasses of the PbO–B₂O₃–Al₂O₃ system spanning across the miscibility gap: Four of them undergo LLPS—three are binodal (two B₂O₃-rich and one PbO-rich) and one is spinodal—and one does not show LLPS (composition outside the miscibility gap). Their compositions were designed in such a way that the amorphous particles are under compressive residual stresses in some and under tensile residual stresses in others. The following mechanical properties were determined: the Vickers hardness, ball on three balls (B3B) strength, and toughness, K_Ic-SEVNB (single-edge V-notch beam [SEVNB]). The microstructures and compositions were analyzed using scanning electron microscopy with energy-dispersive X-ray spectrometry. The spinodal glass showed, by far, the best mechanical properties. Its K_Ic-SEVNB = 1.6 ± 0.1 MPa m^1/2, which embodies an increase of almost 50% over the B₂O₃-rich binodal composition, and 90% considering the PbO-rich binodal composition. Moreover, its fracture strength, S_f = 166 ± 7 MPa, is one of the highest ones ever reported for an LLPS glass. Fracture analyses evidenced that the spinodal composition exhibited the lowest net stress at the fracture point. Moreover, calculations indicate that the internal residual stress level is the lowest in the spinodal glass. The overall results indicate that the microstructural effect of the spinodal glass is the most significant factor for its superior mechanical properties. This work corroborates the idea that LLPS provides a feasible and stimulating solution to improve the mechanical properties of glasses.     

Effects of composition and phase relations on mechanical properties and crystallization of silicate glasses

Crystallization, mechanical properties, and workability are all important for the commercialization and optimization of silicate glass compositions. However, the inter-relations of these properties as a function of glass composition have received little investigation. Soda-lime-silica glasses with Na₂O-MgO-CaO-Al₂O₃-SiO₂ compositions relevant to commercial glass manufacture were experimentally studied and multiple liquidus temperature and viscosity models were used to complement the experimental results. Liquidus temperatures of the fabricated glasses were measured by the temperature gradient technique, and Rietveld refinements were applied to X-Ray powder diffraction (XRD) data for devitrified glasses, enabling quantitative determination of the crystalline and amorphous fractions and the nature of the crystals. Structural properties were investigated by Raman spectroscopy. Acoustic echography, micro-Vicker's indentation, and single-edge-notched bend testing methods were used to measure Young's moduli, hardness, and fracture toughness, respectively. It is shown that it is possible to design lower-melting soda-lime-silica glass compositions without compromising their mechanical and crystallization properties. Unlike Young's modulus, brittleness is highly responsive to the composition in soda-lime-silica glasses, and notably low brittleness values can be obtained in glasses with compositions in the wollastonite primary phase field: an effect that is more pronounced in the silica primary phase field. The measured bulk crystal fractions of the glasses subjected to devitrification at the lowest possible industrial conditioning temperatures indicate that soda-lime-silica glass melts can be conditioned close to their liquidus temperatures within the compositional ranges of the primary phase fields of cristobalite, wollastonite, or their combinations.     

Synthesis and properties of lithium disilicate glass-ceramics in the system SiO2–Al2O3–K2O–Li2O

The purpose of this study was the synthesis of lithium disilicate glass-ceramics in the system SiO₂–Al₂O₃–K₂O–Li₂O. A total of 8 compositions from three series were prepared. The starting glass compositions 1 and 2 were selected in the leucite–lithium disilicate system with leucite/lithium disilicate weight ratio of 50/50 and 25/75, respectively. Then, production of lithium disilicate glass-ceramics was attempted via solid-state reaction between Li₂SiO₃ (which was the main crystalline phase in compositions 1 and 2) and SiO₂. In the second series of compositions, silica was added to fine glass powders of the compositions 1 and 2 (in weight ratio of 20/100 and 30/100) resulting in the modified compositions 1–20, 1–30, 2–20, and 2–30. In the third series of compositions, excess of silica, in the amount of 30 wt.% and 20 wt.% with respect to the parent compositions 1 and 2, was introduced directly into the glass batch. Specimens, sintered at 800 °C, 850 °C and 900 °C, were tested for density (Archimedes’ method), Vickers hardness (H_V), flexural strength (3-point bending tests), and chemical durability. Field emission scanning electron microscopy and X-ray diffraction were employed for crystalline phase analysis of the glass-ceramics. Lithium disilicate precipitated as dominant crystalline phase in the crystallized modified compositions containing colloidal silica as well as in the glass-ceramics 3 and 4 after sintering at 850 °C and 900 °C. Self-glazed effect was observed in the glass-ceramics with compositions 3 and 4, whose 3-point bending strength and microhardness values were 165.3 (25.6) MPa and 201.4 (14.0) MPa, 5.27 (0.48) GPa and 5.34 (0.40) GPa, respectively.     

Preparation of glass–ceramic glazes in the SiO2–Al2O3–CaO–MgO–K2O–Na2O–ZnO system by variable content of ZnO

This paper is focused on glass–ceramic glazes from the SiO₂–Al₂O₃–CaO–MgO–K₂O–Na₂O system with ZnO additions (2.5, 5, 10, 15, 15, 20 and 25 wt%). The compositions were designed based on constant molar ratio of SiO₂/Al₂O₃. In the resulting glazes diopside (CaMg[Si₂O₆]), willemite (Zn₂SiO₄) and vitreous phase were identified by X-ray diffraction. Morphological and structural date of these glazes were supplementary determined by EPMA, FTIR and Raman Spectroscopy. DSC analysis was carried out to characterize thermal properties of the materials.     

EAFD-loaded vitreous and glass–ceramic materials

SiO₂, Na₂O and CaO were mixed and co-melted with electric arc furnace dust waste. The resulting vitreous materials, produced by quenching at ambient atmosphere, were transformed into glass–ceramics by two-stage heat treatment, under thermal conditions that were determined by differential thermal analysis. X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry and transmission electron microscopy were employed to investigate the physical properties of all products. It was found that whilst wollastonite (CaSiO₃) separates from the parent matrix as the dominant crystalline phase in all glass–ceramic products, the crystallization mode depends on the batch composition. Leaching tests evidenced that vitreous products were chemically durable. Devitrification did not significantly affect leach resistance so glass–ceramic materials retain the leach resistance that was achieved by vitrification.     

Influence of crystalline complexes on electrical properties of PEO:LiTFSI electrolyte

Polymer electrolytes of poly(ethylene oxide) matrix with lithium imide salt LiN(CF₃SO₂)₂ were prepared by casting from solution. Thin films with compositions corresponding to molar ratios 6:1, 3:1 and 2:1 EO:Li were investigated by impedance spectroscopy, impedance spectroscopy simultaneous with polarizing microscope observation, X-ray diffraction and differential scanning calorimetry. The presence of PEO:LiTFSI stoichiometric complexes was found to significantly decrease conductivity at temperature of crystallization, which indicates that those complexes should be regarded as poorly conductive. Changes of properties of amorphous phase related to crystallization were also observed. Crystallization induced phase segregation, which in some cases caused considerable shift of the glass transition temperature of amorphous phase remaining in a semicrystalline system. For PEO:LiTFSI electrolyte with molar ratio of 3:1 EO:Li this effect was found to be responsible for enhancement of conductivity of semicrystalline sample in respect to the amorphous one, which was observed at low temperatures. Phase separation involving precipitation of LiTFSI salt was also found to be a likely explanation for significant enhancement of conductivity for PEO:LiTFSI 2:1 electrolyte subjected to rapid cooling below the glass transition temperature.     

Volume nucleation of crystals in glass based on blast-furnace slag

The nucleation of crystals in glass obtained by blending metallurgical slag with silicon dioxide has been studied. The type of crystallization (homogeneous or heterogeneous, volume or surface) is revealed for each of nine compositions of synthesized glass. It is shown that the first crystalline phase in a volume-crystallizing glass is perovskite (CaO · SiO₂); in this phase a nucleation of the main phase occurs: melilite (solid solution of gehlinite 2CaO · Al₂O₃ · SiO₂ in akermanite 2CaO · MgO · 2SiO₂). The fundamental characteristics of homogeneous (for a catalizing phase, perovskite) and heterogeneous (for a catalyzed phase, melilite) of crystallization are determined: the steady-state nucleation rate I _st, time of unsteady-state nucleation τ, crystal growth rate U, and activation energy of frictional flow. The temperature dependences of I _st, τ, and U are obtained. Practical recommendations are presented for the use of blast-furnace slag as a raw material for the synthesis of glass and their further utilization.     

Microstructure and firing of casts of high-zirconia refractories

Conclusions The main crystal phase of all the high-zirconia compositions is baddeleyite. Those refractory compositions in which this phase is virtually the only one with SiO₂/Al₂O₃1.2 make it possible to produce casts without cracks even with a very low concentration (6–7%) of glass phase with firing in a hot well. The compositions which have other crystal phases as well as baddeleyite do not guarantee production of casts without cracks, even with a high concentration of glass phase and with controlled firing. This is clearly due to the structural features of these refractories.Translated from Ogneupory, No. 2, pp. 52–55, February, 1981.     

Thermal properties of ecological phosphate and silicate glasses

The present work deals with ecological phosphate and silicate glasses that belong to the oxide systems: Li₂O-MgO-P₂O₅, Li₂O-CaO-P₂O₅, Li₂O-MgO-P₂O₅-Fe₂O₃; Li₂O-CaO-P₂O₅-Fe₂O₃ and SiO₂-R₂O-R′O (R = Na, K; R′ = Mg, Ca), the last system contains certain amounts of ZrO₂, ZnO, TiO₂. These ecological glasses do not contain toxic substances as BaO, PbO, As₂O₃, As₂O₅, fluorine, CdS, CdSe and they have applications as regards the retention and counteracting action of the harmful compounds resulted from the nuclear plants. The replacement of MgO by CaO leads to an insignificant increasing of the thermal expansion index and a slight decreasing of the characteristic temperatures, except the softening point where the effect is opposite. Adding of iron oxide in the phosphate glass composition causes the increasing of characteristic temperatures and the decreasing of thermal expansion index, both in MgO and CaO-containing phosphate glasses. The ecological silicate glasses are used as opal glasses free of fluorine as well as for lead-free crystal glass (CFP) where BaO and PbO are replaced by non-toxic oxides as K₂O, MgO, ZrO₂, and TiO₂. The paper presents different glass compositions and the technological parameters to prepare the ecological glass samples. Both ecological phosphate and silicate glasses have been characterized as regards the characteristic temperatures (vitreous transition point, low and high annealing points, softening point) and the thermal expansion coefficient. This study presents the changes of the thermal parameters when CaO replaces MgO in phosphate glass samples and the role of iron oxide in the vitreous network. In the case of silicate glasses, the viscosity and wetting angle dependency of temperature are presented. The elemental analysis of the ecological glasses was made by XPS (X-ray photoelectron spectroscopy) which also put in evidence the iron species from the vitreous network.     

Evaluation of Agriglass as an Environment Friendly Slow Release Fertilizer

Six different agriglass compositions of the (P₂O₅-SiO₂-K₂O) system were prepared with the addition of some oxides; Fe₂O₃, MnO₂, ZnO, CuO. Glasses were synthesized by a melting quenching technique at 1150 ^°C. The chemical activity of glass grains of different diameters (0.5 and 1 mm) was estimated by measuring available K₂O % by a flame photometer, and P₂O₅ % by a spectrophotometer, in conditions simulating the plant root zone by shaking in DTPA solution (NH₄HCO₃-DTPA: ammonium bicarbonate diethylenetriaminepentaacetic acid) for 30 min. The results showed that PS1 glass with main composition (55 P₂O₅, 5 SiO₂, 30 K₂O, 5 Fe₂O₃, 5CuO) wt.%, produced high release nutrient values. The other chemical activity tests were established to evaluate the PS1 glass by shaking in distilled water for 0.5, 1, 2 and 3 hours, and immersion in DTPA solution and 2 wt% citric acid solutions for 1, 2, 3 and 4 months. A greenhouse experiment was performed to evaluate two rates of PS1 agriglass (PS1R1 and PS1R2) compared with the recommended rate of ordinary mineral fertilizers (MF). Although, the effect of MF is higher than both vitreous fertilizer rates in the measurement of 100 grains weight, grains and Stover weight, the application of agriglass with high rate (PS1R2) gave higher results than that of MF in the measurements of ears weight, plant height, and crop weight. It has been found that PS1 glass can be used in practice for maize as an environmentally safe fertilizer.     

Formation and stability of glasses with ultra-optical properties in TiO<Subscript>2</Subscript>–Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–PbO system

To obtain ultra-optical property in glasses, as the basis for photonic applications, the glass forming region of TiO₂–Bi₂O₃–PbO system was investigated and determined by melting series of compositions in the system. The glass-forming boundary region was defined. The best compositions for glass formation were found to be around the eutectic and peritectic regions in the corresponding phase diagram. Generally, stability increased with the addition of TiO₂, acting as a conditional glass former, to a maximum of 15TiO₂ mol %. Replacing PbO with Bi₂O₃ in the glass worsened the stability, due to the increase of heavy cation Bi^3– in the glass structure. Finally, the refractive index and dispersion of some stable glasses were measured, which were as high as 2.435 and 10.2, respectively.     

Algebraic Structure Count of Cyclobutadienyl Bridged Polyacenes

Abstract

By substitution of the benzene rings in [n]phenylenes with polyacene units (all being of the same length) a new class of compounds is obtained. By analogy we call this class [n]acenylenes. Beside linear (X_n), we also examine angularly linked acenylenes which might be realized in two extreme ways: by connecting the polyacene units in a strictly spiral way (Y_n) or in the so called zig-zag manner (Z_n).

The recursion formulae for the algebraic structure count (ASC) of X_n, Y_n and Z_n are derived here, and they read as:

ASC(X_n) = (h + 1). ASC(X_n-1)—ASC(X_n-2)

ASC(Y_n) = h. ASC(Y_n-1) + ASC(Y_n-2)

ASC(Z_n) = h. ASC(Z_n-1) + ASC(Z_n-2)

where n denotes the number of polyacene units and h the number of hexagons in each polyacene unit.

It is proved that Y_n and Z_n have equal ASCs.     

Raman imaging as a useful tool to describe crystallization of aluminum/iron-containing polyphosphate glasses

Polyphosphate glasses are materials of a wide spectrum of applications in many fields. The subject of the work is polyphosphate glasses containing aluminum and iron. Three compositions of the glasses were obtained and the materials have been characterized in terms of their crystallization. The differences in crystallization behavior between powder and bulk materials were compared. The crystallized materials were analyzed by Raman scattering spectroscopy and X-ray diffraction method. It was evidenced that depending on the glass composition the main crystalline phases were Al(PO₃)₃, AlPO₄, FePO₄, Fe₃(P₂O₇), Fe₄(P₂O₇)₃, FePO₄. The glass crystallization leads to enrichment of the residual glassy phase in P₂O₅ and increases its polymerization. Thus, it was observed the glass inhomogeneities are being increased due to crystallization. The two dimensions spectral maps of the bulk crystallized samples were executed to describe the mechanism and type of crystallization. The depth profiling proves the differences between surface and interior phase composition.     

Seeded Stage III glass dissolution behavior of a statistically designed glass matrix

The dissolution rate of some glasses accelerates after prolonged time spent at a slow, residual dissolution rate. This phenomenon is referred to as Stage III behavior. The acceleration in glass dissolution rate linked to Stage III behavior is significant and may be the most impactful behavior to long-term performance of glass in a repository. This work is aimed at understanding the effect of glass composition on Stage III behavior to add a level of technical defensibility to glass disposal. To this end, a set of 24 glass compositions were statistically designed, where eight glass components (SiO₂, B₂O₃, Al₂O₃, CaO, Na₂O, SnO₂, ZrO₂, and Others) have been independently varied in order to study the individual effects of each glass component. These glasses have been subjected to static dissolution tests at 90°C in deionized water and then seeded with zeolite Na-P2 28 days into the testing to induce Stage III behavior. The response of the glasses to the zeolite seeds fell into four primary types: (1) no response to seeds; (2) an immediate linear sustained acceleration in the rate; (3) an immediate linear acceleration in the rate followed by a decrease; and (4) a progressive acceleration in the rate that is concurrent with the addition of the seeds. The main glass components observed to influence these behaviors were CaO, Al₂O₃, B₂O₃, and ZrO₂, where (1) CaO influenced which glasses showed a Stage III response to seeds (high CaO: types 2, 3, and 4) or did not respond to seeds (low CaO: type 1), (2) Al₂O₃ and B₂O₃ influenced which glasses showed a sustainable Stage III response (high Al₂O₃: types 2 and 4) versus transitory response (low Al₂O₃ and high B₂O₃: type 3), and (3) ZrO₂ concentration influenced whether glasses showed a linear (high ZrO₂: type 2) versus progressive (low ZrO₂: type 4) response to seeds.     

Derivation of the Structural Integrity of Residual (SIR) glass model for the enhancement of waste loading

A new model based on glass structure to allow for enhanced waste loading in nuclear waste glass while maintaining chemical durability is proposed. The model is derived by splitting the molar concentrations of the targeted starting glass composition into theoretical crystalline phases anticipated to be observed during devitrification and a residual glass. An empirically derived relationship based on maintaining the residual glass structure, determined from a calculated non-bridging oxygen content, was demonstrated to successfully screen glasses for acceptable durability. The proposed model can successfully identify durable glass compositions containing 20–35 wt% Al₂O₃, a concentration that would significantly increase the projected waste loading in glasses processed at the Hanford Tank Waste Treatment and Immobilization Plant.     

Microstructure and mechanical properties of reaction-hot-pressed zirconium diboride based ceramics

ZrB₂ ceramics were prepared by in-situ reaction hot pressing of ZrH₂ and B. Additions of carbon and excess boron were used to react with and remove the residual oxygen present in the starting powders. Additions of tungsten were utilized to make a ZrB₂-4 mol%W ceramic, while a change in the B/C ratio was used to produce a ZrB₂-10 vol% ZrC ceramic. All three compositions reached near full density. The baseline ZrB₂ and ZrB₂–ZrC composition contained a residual oxide phase and ZrC inclusions, while the W-doped composition contained residual carbon and a phase that contained tungsten and boron. All three compositions exhibited similar values for flexure strength (~520 MPa), Vickers hardness (~15 GPa), and elastic modulus (~500 to 540 GPa). Fracture toughness was about 2.6 MPa m^1/2 for the W-doped ZrB₂ compared to about 3.8 MPa m^½ for the ZrB₂ and ZrB₂–ZrC ceramics. This decrease in fracture toughness was accompanied by an observed absence of crack deflection in the W-doped ZrB₂ compared with the other compositions. The study demonstrated that reaction-hot-pressing can be used to fabricate ZrB₂ based ceramics containing solid solution additives or second phases with comparable mechanical properties.     

Phase separation and crystallization in glasses of the lithium silicate system xLi2O · (100 ? x)SiO2 (x = 23.4, 26.0, 33.5)

The phase separation in ultimately homogenized glasses of the lithium silicate system xLi₂O · (100 − x)SiO₂ (where x = 23.4, 26.0, and 33.5 mol % Li₂O) has been investigated. The glasses of these compositions have been homogenized using the previously established special temperature-time conditions, which provide the maximum dehydration and the removal of bubbles from the glass melt. The parameters of nucleation and growth of phase_separated inhomogeneities and homogeneous crystal nucleation have been determined. The absolute values of the stationary nucleation rates I _st of lithium disilicate crystals in the 23.4Li₂O · 76.6SiO₂ and 26Li₂O · 74SiO₂ glasses with the compositions lying in the metastable phase separation region have been compared with the corresponding rates I _st for the glass of the stoichiometric lithium disilicate composition. It has been established that the crystal growth rate have a tendency toward a monotonic increase with an increase in the temperature, whereas the dependences of the crystal growth rate on the time of low-temperature heat treatment exhibit an oscillatory behavior with a monotonic decrease in the absolute value of oscillations. The character of crystallization in glasses with the compositions lying in the phase separation region of the Li₂O-SiO₂ system is compared with that in the glass of the stoichiometric lithium disilicate composition. The inference has been made that the phase separation weakly affects the nucleation parameters of lithium disilicate and has a strong effect on the crystal growth.     

Crystallisation and characterisation of dielectric glass ceramics in Li2O–Al2O3–SiO2 system

Abstract

Glass ceramics in the Li₂O–Al₂O₃–SiO₂ system have been synthesised to produce bulk materials grown in a glass phase via quenching followed by controlled crystallisation. The crystallisation and microstructure of Li₂O–Al₂O₃–SiO₂ (LAS) glass–ceramic with nucleating agents (B₂O₃ and/or P₂O₅) are investigated by differential thermal analysis, X-ray diffraction and scanning electron microscopy and the effects of B₂O₃ and P₂O₅ on the crystallisation of LAS glass are also analysed. The introduction of both B₂O₃ and P₂O₅ promotes the crystallisation of LAS glass by decreasing the crystallisation temperature and adjusting the crystallisation kinetic parameters, allows a direct formation of β spodumene phase and as a result, increases the crystallinity of the LAS glass ceramic. Microstructural observations show that the randomly oriented, nanometre sized crystalline is found with residual glass concentrated at crystallite boundaries. Furthermore, it is interesting that codoping of B₂O₃ and P₂O₅ creates not much effect on the crystallisation temperature. The dielectric properties of the glass–ceramics formed through controlled crystallisation have a strong dependence on the phases that are developed during heat treatment. The dielectric constant is continuously increased and the dielectric loss is decreased with addition of additives where mobile alkali metal ions (e.g. Li⁺) are incorporated in a crystal phase and minimise the residual glass phase.     

Structural Evolution and Properties of 0.3Pb(In1/2Nb1/2)O3–0.38Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 Ferroelectric Ceramics with Different Sintering Times

The structural evolution and properties of 0.3Pb(In_1/2Nb_1/2)O₃–0.38Pb(Mg_1/3Nb_2/3)O₃–0.32PbTiO₃ (0.3PIN‐0.38PMN‐0.32PT) ferroelectric ceramics with different sintering times have been investigated. The content of the tetragonal phase is increased in samples sintered for more than 6 h, despite that the composition falls in the rhombohedral region of the previously established phase diagram. The results show that the metastable tetragonal phase at room temperature is induced and stabilized by the tensile residual stresses. Excessively long sintering time generally leads to grain coarsening, loss of lead, and deterioration of properties, while the increasing amount of the tetragonal phase, and the large residual tensile stress appear to improve the dielectric and electromechanical properties. This study offers new insights into the sintering of Pb‐based ferroelectric ceramics with complex compositions.     

The influence of the preparation technique and thickness of As2Se3 · AgBr glass layers on the electrical conductivity

A glass of the composition 0.5As₂Se₃ · 0.5AgBr is prepared in three forms: a monolithic sample, a film 1 μm thick, and layers ～20 nm thick separated by vitreous arsenic selenide layers of the same thickness. The temperature dependences of the electrical conductivity are investigated for all three objects. The results obtained are discussed taking into account the specific features of the technique used for evaporating the films and the mutual influence of alternating vitreous nanolayers.