Li2O SiO2 Research Articles

Effects of AlN on the thermal and mechanical properties of vitrified bond and vitrified CBN composites

The effects of AlN fillers on the thermal and mechanical properties of vitrified bond and vitrified cubic boron nitride (CBN) composites were systematically investigated. The basic vitrified bond (B2O3–Al2O3–SiO2–Li2O–Na2O–ZnO) and AlN fillers make up the vitrified bond, and then constitute the vitrified CBN composites consists together with CBN grains. When the adding amount of AlN rises from 0wt% to 8wt%, the thermal conductivity of the vitrified bond increases from 0.74W/mK to 1.44W/mK. 6wt% AlN-doped-vitrified bond presents the minimum coefficient of thermal expansion and bending strength of 66.35MPa. The substantial improvement of thermal conductivity may be attributed to the formation of thermal conductive path and new crystalline phase—α-tridymite in the vitrified bond. Microstructure characterization indicates that the main crack deflection and divergency are the primary strengthening mechanisms. Such vitrified CBN composites with good thermal conductivity and mechanical strength might be promising in grinding materials for reducing burn.

The crystallization and microstructure evolution of lithium disilicate-based glass-ceramic

In this work, transformation of crystalline phases and development of microstructure of two kinds of glass-ceramics, SiO2–Li2O–P2O5 and SiO2–Li2O–K2O–Al2O3–ZrO2–P2O5, have been studied by X-ray diffraction and scanning electron microscopy analysis.In the former glass, a small amount of spherical Li2Si2O5 crystals precipitated at the usual nucleation temperature, which changed little in size and packing density with the increase of the temperature. These spherical Li2Si2O5 also have no effect on the volume crystallization of residual glass or the morphology of subsequently formed plate-like Li2Si2O5 crystals. In the latter glass, Li2SiO3 crystallized first and then decomposed at higher temperature, forming two kinds of Li2Si2O5 crystals with different morphologies. The interlocked rod-like shaped Li2Si2O5 crystals were resulted from interaction of Li+ ions and SiO2-rich glassy phase and oriented crystal bundles of Li2Si2O5 were formed from rearrangement of a silicate framework in Li2SiO3.

The influence of the heat treatment temperature in the magnetic characteristics of a SiO2–Li2O–Fe2O3 glass prepared by sol-gel

The glass with the molar composition 88SiO2–2Li2O–10Fe2O3 (mole %) was prepared by the sol-gel route and after heat-treated at temperatures between 250 and 1000°C under oxidizing conditions. Crystalline phases (LiFe5O8, SiO2, Fe2O3 and LiFe(Si2O6)) were observed by X-ray powder diffraction in the glasses treated at temperatures above 800°C. Scanning electron micrographs reveal the presence of particles in the samples heat-treated up to 800°C, which are essentially amorphous or with an incipient crystallization. The increase of the heat-treatment temperature promotes the coarsening of particles.The magnetic characteristics of the prepared glasses and glass ceramics were performed using a vibrating sample magnetometer in function of the magnetic field amplitude (−10T≤B≤10T) and in function of temperature (5K–300K). The samples heat-treated at 900 and 1000°C present a ferrimagnetic behavior accompanied by the appearance of peaks corresponding to the lithium ferrite phase (LiFe5O8). A curious aspect is the saturation magnetization of the sample heat-treated at 800°C is the highest of all samples, indicating the highest amount of LiFe5O8 crystals. The Mossbauer results are in accordance with the magnetization and magnetic susceptibility measurements.

Impedance spectroscopy study of SiO2–Li2O:Nd2O3 glasses

In the present study, neodymium-doped lithium silicate glasses have been prepared by the conventional melt-quenching technique. The dielectric properties, electric modulus, and electrical conductivity of SiO2–Li2O (SiLi–0Nd) and SiO2–Li2O:Nd2O3 (SiLi-1.35Nd) have been studied from 1Hz to 1MHz in the 333–423K temperature range. At a given temperature and frequency, we observe that the conductivity decreases when neodymium ions are added to the glass matrix. The activation energy of two distinct regions was evaluated from the ln(σdc)=f(1/T) plot and was found to be E1 (T<363K)=0.61(0.66)eV and E2 (T>363K)=1.26(1.09)eV for SiLi–0Nd (SiLi–1.35Nd). The dielectric constant (εRe) decreases while the dielectric loss (tan(δ)) increases under Nd2O3 doping. We also observe that for both glasses, εRe and tan(δ) tend to increase with increasing temperature and decrease with increasing frequency.

X-ray study of lithium disilicate glass: High pressure densification and polyamorphism

We have investigated the structural changes induced by high-pressure in lithium disilicate glass with stoichiometric composition Li2O.2SiO2 (LS2). Using toroidal type high-pressure chambers, glass samples were processed at 2.5GPa, 4GPa, 6GPa and 7.7GPa at room temperature. Synchrotron X-ray diffraction measurements were used to obtain the radial distribution functions (RDF) in order to follow the structural changes induced after processing at high pressure. Compared to a pristine sample, the main change observed for the samples processed up to 6GPa was associated to the distortion of the SiO4 tetrahedral structure. However, for the sample processed at 7.7GPa, a drastic change in the RDF indicated the formation of a distinct amorphous phase. The observed polyamorphism is important for a better understanding of the nucleation kinetics of crystalline phases under high pressure.

Nucleation and Crystallization Kinetics of a Multicomponent Lithium Disilicate Glass by in Situ and Real-Time Synchrotron X-ray Diffraction

In this study, the high-temperature phase transformation of a multicomponent lithium disilicate glass was investigated by in situ and real-time synchrotron X-ray diffraction in the SiO2–Li2O–P2O5–Al2O3–ZrO2 glass system. Quantitative phase analysis via the Rietveld method was performed on the high-resolution data aiming to reveal the crystallization sequence, crystallization kinetics, and the role of P2O5 on nucleation. It is found that the nucleation of lithium metasilicate (LS) and lithium disilicate (LS2) in this complex glass is triggered by the steep compositional gradients associated with the disordered lithium phosphate (LP) precursors in the glass matrix. The LS2 crystals grow at the expense of the LS, cristobalite, and quartz phases in the glass during the isothermal crystallization process at 770 °C. The nucleation kinetics is temperature dependent, and the induction period of nucleation is longer at a lower temperature.

Enhancement of ionic conductivity in Li2O–SiO2 glass in nanodimensions grown within pellets of ZnO nanorods and magnetodielectric properties of these nanocomposites

23Li2O·77SiO2 glass with nanodimensions was grown within the pores of a pellet formed by ZnO nanorods of a diameter of 70nm and lengths varying from 80nm to 270nm. The resistivity of the nanoglass was less than that of its bulk counterpart by more than three orders of magnitude. The activation energy was found to be 35% lower than that of the bulk glass. This was ascribed to an increase in free volume in the nanodimensional glass. The ac conductivity variation at high frequency showed an exponent of 0.5 indicating two dimensional movements of lithium ions in the nanoglass. Nanocomposites of lithia–silica nanoglass and CuO nanoparticles showed magnetodielectric property with dielectric permittivity at 500kHz decreasing by 5.3% at 2T. This was explained on the basis of space charge polarization and Hall Effect. Nanocomposites of lithia–silica nanoglass and ZnO nanorods exhibited a giant magnetodielectric effect with the dielectric permittivity increasing by 20% to 99% at 2T depending on the measuring frequency. This was also ascribed to space charge polarization with the ZnO nanophase showing a negative magnetoresistance.

Structure–property relationships and densification-crystallization behaviours of simplified lithium disilicate glass compositions

The role of each oxide component on the structure of molten glass and on their densification and crystallization behaviours is expected to be more easily assessed for compositions comprising a small number of components. In the present work, binary (Li2O–SiO2), ternary (Li2O–Al2O3–SiO2), and quaternary (Li2O–K2O–Al2O3–SiO2) glass compositions were selected. The aim was to investigate the relationships between the compositions and structure of the glasses, namely, the effects of each component on the extent of liquid-in-liquid phase separation, their thermal behaviour and the phase assemblage in the disilicate glass–ceramics. The distribution of structural units in the experimental glasses was assessed using 29Si MAS-NMR spectroscopy. Adding Al2O3 played a dual role as network former and modifier when added to the binary system by enhancing Q2 and diminishing Q3 structural units. The processing window for sintering, the difference between the onset of crystallisation and glass-transition temperature (Tc−Tg), was too narrow for the less polymerised network structures (binary and ternary systems), hindering the densification of the corresponding glass powder compacts. Oppositly, compositions in the quaternary system featured excellent densification behaviour coupled with high mechanical strength. These features derive from a more rigid glass network comprising four coordinated (AlO4/2)− units and K+ cations in its vicinity.

Pressure and temperature stability range of crystalline lithium metasilicate in a binary Li2O·2SiO2 glass

The aim of this work was to investigate the formation of lithium metasilicate (Li2SiO3) under high pressure and high temperature starting from lithium disilicate glass with the stoichiometric composition Li2O·2SiO2 (LS2). Monolithic LS2 glass samples were processed at 4.25GPa, 4.5GPa, 5GPa, 5.5GPa, 6GPa and 6.5GPa and submitted, simultaneously, to heat treatment for nucleation and growth of the crystalline phases. The crystallization of the samples was investigated by X-ray diffraction, Raman spectroscopy and optical microscopy. In the pressure and temperature range investigated, crystalline Li2Si2O5 remained stable up to 6GPa and the formation of the metasilicate phase Li2SiO3 started at 4.25GPa coexisting with quartz and coesite up to 6.5GPa. The segregation of SiO2 phases during the crystallization of Li2SiO3 was a consequence of the lithium deficiency of the starting LS2 composition.

Fabrication of a high-strength lithium disilicate glass-ceramic in a complex glass system

The aim of this work was to develop a high-strength lithium disilicate glass-ceramic for dental restorative applications. A complex lithium disilicate glass composition was designed in the SiO2–Li2O–CaO–P2O5–ZrO2 glass system. Three sets of samples were employed to investigate the effects of heating profile, crystallization annealing temperature and holding time on the phase evolution, microstructure and mechanical properties. The results show that no lithium metasilicate was detected during the crystallization process. Semi-quantitative analysis reveals that the lithium disilicate glass-ceramics consist of 57–62 wt% of rod-like lithium disilicate crystallites. A high flexural strength of 439 ± 93 MPa was observed in the glass-ceramic when it was subjected to two-stage annealing at 674 °C. Additional higher temperature annealing stages were of little help for the improvement of bending strength, but did enhance the Vickers indentation fracture toughness (VIF) from 0.93 to 1.29 MPa·m1/2. It is found that both the crystallization temperature and the holding time affect the phase transformation, morphology, and crystallite size of these glass-ceramics, but the crystallization temperature has a more profound effect than the holding time. The glass-ceramic developed in this study could be a good candidate for dental restorative applications, especially for posterior three-unit bridges.

Densification of Lithium Disilicate under High Pressure Investigated by XPS

The aim of this work was to investigate by X-ray photoelectron spectroscopy the effect of high pressure on the chemical environments of Si 2p, O 1s and Li 1s in lithium disilicate glass ceramic with stoichiometric composition Li2O·2SiO2 (LS2). A group of samples was processed at 2.5 GPa, 4 GPa and 7.7 GPa at room temperature and a second group was crystallized under high pressure and high temperature. Large shifts of the binding energy toward higher energies were observed in the X-ray photoelectron spectroscopy spectra for samples of the first group after densification at 2.5 and 4 GPa. For samples processed at 7.7 Gpa, the major component of the binding energy for the Si 2p environment remained practically unchanged compared to the pristine sample but new components, with smaller intensities, appeared in the spectra, indicating the existence of distinct Q-species induced by high pressure. This behavior may be related to changes in the number of bridged and non-bridged oxygen atoms in the glass structure. The results for the second group of samples, crystallized under high pressure, showed evidences of three binding energies for the O atoms, one of them related to non-bridged and two of them to bridged O atoms.

Effect of sintering time on the microstructure, flexural strength and translucency of lithium disilicate glass-ceramics

Glass-ceramics derived from the SiO2–Li2O–K2O–Al2O3–ZrO2–P2O5 system were produced by a two-stage sintering cycle with a primary thermal treatment at 700°C followed by a secondary thermal treatment at 900°C. The periods during which the materials were subjected to each temperature level (700°C/900°C) were the following: A, 1h/1h; B, 2h/2h; C, 3h/3h; D, 4h/4h; E, 5h/5h; F, 6h/6h; and G, 8h/8h. The X-ray diffraction results revealed that all groups had a similar crystalline structure though the sintering times were different. The main crystalline phase corresponds to Li2Si2O5 and the minor phases correspond to Li2SiO3 and Li3PO4. Scanning electron microscopy analysis showed an interlocking microstructure of rod-shaped Li2Si2O5 crystals in all specimens. However, the glass-ceramics produced with different sintering times showed different crystal sizes and spatial distribution. The glass-ceramic sintered during 5h in each temperature level (700°C/900°C) showed the highest flexural strength (308±25MPa). The translucency parameters (TP) of the glass-ceramics studied were not affected by sintering time since similar TP values were observed in all specimens.

Al2O3/K2O-containing non-stoichiometric lithium disilicate-based glasses

The crystallisation kinetics of experimental glasses in 3 different systems: (A) Li2O–SiO2, (B) Li2O–Al2O3–SiO2 and (C) Li2O–K2O–Al2O3–SiO2 were studied under non-isothermal conditions. The DTA results revealed a stronger tendency to crystallisation of binary compositions in comparison to the ternary and quaternary compositions comprising Al2O3 and K2O which present the lower crystallisation, i.e. the crystallisation propensity follows the trend A > B > C. The devitrification process in the Li2O–SiO2 and Li2O–Al2O3–SiO2 systems began earlier and the rate was higher in comparison to that of glasses in the quaternary Li2O–K2O–Al2O3–SiO2 system. Thus, addition of Al2O3 and K2O to glasses of Li2O–SiO2 system was demonstrated to promote glass stability against crystallisation. However, the activation energy for crystallisation was shown to depend also on the SiO2/Li2O ratio with the binary system showing a decreasing trend with increasing SiO2/Li2O ratio, while the opposite tendency was being observed for compositions with added Al2O3 and K2O.

Boron removal from metallurgical grade silicon using a CaO–Li2O–SiO2 molten slag refining technique

The distribution of boron between CaO–SiO2 slag phase and silicon phase was examined in order to establish the effects of slag composition and basicity on boron removal from metallurgical grade silicon (MG-Si). Though the addition of Li2O to CaO–SiO2 slag showed a little obvious advantage to boron removal, the boron concentration in MG-Si can be reduced from 18ppmw to 1.3ppmw with the ratio of CaO–Li2O–SiO2 slag to MG-Si for 4:1 (mass). The reactive mechanism between boron in MG-Si and CaO–Li2O–SiO2 slag is described using an ionization theory of molten slag and an electrochemical model was established, which depicted the oxidation and transfer of boron between silicon and slag phases. The experiment of slag refining for rich-B Si-B alloy showed that the generated (BO3)3− combines into Li2O·2B2O3 with slag instead of CaO·B2O3 on account of the more negative standard Gibbs free energy of formation for Li2O·2B2O3 than that of CaO·B2O3.

Crystalline phase formation, microstructure and mechanical properties of a lithium disilicate glass–ceramic

In this work, crystalline phase formation, microstructure and mechanical properties of a lithium disilicate (LS2, Li2Si2O5) glass–ceramic in the SiO2–Li2O–Al2O3–P2O5 system were investigated. A four-stage heat treatment process was used for crystallization of the glass. The effects of holding time and temperature at the final stage on the crystalline morphology of the glass–ceramic were studied. The experimental results revealed the two crystallization peaks at 672 and 839 °C. At a temperature lower than 770 °C and holding time of 20 min, the lithium metasilicate (Li2SiO3) phase dominates. On the other hand, when the glass was heated to a higher temperature or held for a longer time, the LS2 phase dominates and some other minor phases such as cristobalite and lithium phosphate emerge. Scanning electron microscopy and energy dispersive spectrometry revealed a large number of nanosized ZrO2 particles when the crystallization temperature was above 790 °C. Vickers hardness of the LS2 glass–ceramic was about 8.1–8.4 GPa and flexural strength was in the range of 282–307 MPa. Crack deflection was observed along the LS2 cluster boundaries. The crystallization sequence was proposed to explain the observed microstructure and phases.

Boron removal from metallurgical-grade silicon using lithium containing slag

The possibility of extracting boron impurity from metallurgical-grade silicon by lithium containing slag refining for solar cell application was investigated. The distribution coefficients of boron between CaO–SiO2–Li2O and CaO–SiO2–LiF slags were examined. The boron content in the refined silicon was studied under different conditions of mass ratio of slag to silicon and refining time. The results showed that a small amount of Li2O or LiF had significant effect on the distribution coefficients of boron. The removal of boron impurity using CaO–SiO2–LiF system was more effective than using CaO–SiO2–Li2O system. Values of boron content in the refined silicon did not decrease significantly when the mass ratio exceeded 4, while it had been obviously falling with the increase in refining time. When the mass ratio of CaO–SiO2–Li2O slag to metallurgical-grade silicon was 4, the boron content in silicon was successfully reduced from 2.2×10−5 to 1.3×10−6 after slag refining for 2h at 1823K.

Synthesis and structure of Na–Li–Si–Al–P–O–N glasses prepared by melt nitridation using NH3

Na–Li–Si–Al–P–O–N glasses have been prepared by nitridation of a pre-synthesized Na2O–Li2O–SiO2–P2O5–Al2O3 glass under anhydrous ammonia. Nitrogen for oxygen substitution increases the network connectivity leading to increases in microhardness and glass transition temperature. Raman and 31P MAS–NMR spectroscopy indicate sequential nitridation reactions forming PO3N and PO2N2 species. The data collected so far show no evidence of N/O substitutions in the silicate sub-network.

Behavior of the refractive index of lithium disilicate glass ceramic processed at high pressure and high temperature

The aim of this work was to investigate the effect of high pressure and high temperature on the refractive index of lithium disilicate glass ceramic with the stoichiometric composition Li2O·2SiO2 (LS2). A first group of monolithic LS2 glass samples were processed at 2.5GPa, 4GPa and 7.7GPa at room temperature and a second group was submitted to high pressure and, simultaneously, to heat treatments for nucleation and growth of the crystalline phases. For comparison, samples submitted to the same heat treatments at 1atm were also investigated. The refractive index of the samples was obtained by spectral ellipsometry and the results were clearly dependent on the particular pressure and temperature conditions. The crystallization of the samples was investigated by X-ray diffraction. For the samples processed under high temperature at 1atm and at 2.5GPa a fraction of the originally amorphous glass was transformed to a monoclinic phase of lithium disilicate. For the samples processed under high temperature and at 4GPa, a large fraction of the originally amorphous glass transformed to an orthorhombic phase while, at 7.7GPa, it was observed the formation of lithium metasilicate.

Effect of K2O on structure–property relationships and phase transformations in Li2O–SiO2 glasses

Glass compositions with formula (71.78 − x)SiO2–2.63Al2O3–(2.63 + x)K2O–23.7Li2O (mol.%, x = 0–10) and SiO2/Li2O molar ratios far beyond that of stoichiometric lithium disilicate (Li2Si2O5) were prepared by conventional melt-quenching technique to investigate the influence of K2O content on structural transformations and devitrification behaviour of glasses in the Li2O–SiO2 system. The scanning electron microscopy (SEM) examination of as cast non-annealed glasses revealed the presence of nanosized droplets in glassy matrices suggesting occurrence of liquid–liquid phase separation. An overall trend towards depolymerization of the silicate glass network with increasing K2O content was demonstrated by employing magic angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopy. The distribution of structural units in the experimental glasses was estimated using 29Si MAS-NMR spectroscopy suggesting the appearance of Q2, enhancement of Q3 and diminishing of Q4 groups with increasing K2O contents. X-ray diffraction (XRD) and differential thermal analysis (DTA) were used to assess the influence of K2O on devitrification process and formation of lithium disilicate (Li2Si2O5) and/or lithium metasilicate (Li2SiO3) crystalline phases.

Lithium niobate bulk crystallization promoted by CO2 laser radiation

The crystallization induced by laser radiation is a very promising technique to promote glass/ceramic transformation, being already used to produce crystalline patterns on glass surfaces. In this work, a SiO2–Li2O–Nb2O5 glass, prepared by the sol–gel route, was submitted to CO2 laser radiation and conventional heat-treatments in order to induce the LiNbO3 crystallization. The structure and morphology of the samples prepared by both routes was analyzed as a function of exposure time, radiation power and heat-treatment temperatures by XRD, Raman spectroscopy and SEM. The results reveal a correlation between the crystallization degree of LiNbO3 particles and glass matrix with the heat treatment type and experimental parameters. An heat-treatment at 650°C/4h was necessary to induce crystallization in heat treatments samples while 4W/500s was enough for laser radiation ones, corresponding a reduction time processing of ∼14000s.