Li2O Al2O3 SiO2 Glass Ceramics Research Articles

High efficiency near infrared emission from Pb1-xSrxSe Quantum dots in lithium aluminosilicate glass-ceramics

Pb1-xSrxSe quantum dots (QDs) were prepared in Li2O-Al2O3-SiO2 (LAS) glass-ceramics with 0–2.0 mol% SrO by conventional melt-quenching method. The near-infrared photoluminescence and photodarkening properties of Pb1-xSrxSe QDs in LAS glass-ceramics were studied. Upon the heat treatments of 490–550 °C for 10 h, there are LiAlO2 nanocrystals and PbSe QDs co-existed in the glassy matrix without SrO, and 1024–1745 nm near infrared emission from PbSe QDs were realized. With the increasing of SrO concentration from 0 to 2 mol%, the absorption wavelengths blue-shifted from 1738 to 1380 nm at 540 °C, the PL peak wavelengths blue-shifted from 1788 to 1538 nm at 540 °C. The blue-shifts are attributed to the increasing bandgap by incorporation of Sr2+ ions into PbSe crystalline lattice. Pb1-xSrxSe QDs display better photoinduced anti-darkening properties than pure PbSe QDs with the increasing of pumping power, and the Pb1-xSrxSe QDs embedded in the LAS glass with 2.0 mol% SrO shows the highest photoluminescence quantum yield of ∼36.9%.

Effect of ZnO on the structural, physio-mechanical properties and thermal shock resistance of Li2O–Al2O3–SiO2 glass-ceramics

The composition of lithium aluminosilicate (LAS) with different zinc oxide-magnesium oxide (ZnO–MgO) contents that ranged from 0 to 1.45 wt percent (wt%) was investigated to determine the thermal shock resistance properties of the glass-ceramics. The LAS glasses were melted in an alumina crucible at 1550 °C for 5 h, and the green compact samples were then heat-treated at 1100 °C for 3.5 h. The presence of zinc oxide (ZnO) in the compositions did not change the major crystal phase of β-spodumene. However, the addition of ZnO shifted the pronounced peak to a lower angle and increased the percentage of crystallinity from 55% to 59%. Additionally, the function of ZnO in LAS glass-ceramics as the network modifier was confirmed through Fourier Transform Infrared Spectroscopy (FTIR) analysis. The physio-mechanical properties were improved when 1.45 wt% ZnO was added to the LAS glass-ceramics. The results showed increased density (2.42 g/cm3), low porosity (0.85%), high flexural strength (125.23 MPa), and low coefficient of thermal expansion (25–800 °C) (CTE(25–800 °C)) value of 1.73 × 10−6 °C−1. Meanwhile, the thermal shock resistance properties evaluation of the LAS glass-ceramics at different ZnO contents were conducted at different thermal shock temperatures of 200 °C, 500 °C, and 800 °C. The critical temperature of the LAS specimens with 1.45 wt% ZnO demonstrated the ability to withstand a thermal shock at 800 °C while preserving 87% of their initial strength of 108.40 MPa, exemplifying the best LAS glass-ceramics properties for rapid high-temperature change applications.

Mechanical behavior of the joints between carbon/carbon composites and Li2O–Al2O3–SiO2 glass ceramics with YAS interlayer at high temperature

YAS (Y2O3–Al2O3–SiO2) glass ceramics were selected to join carbon/carbon (C/C) composites and Li2O–Al2O3–SiO2 (LAS) glass ceramics through vacuum hot-pressing sintering, due to the corresponding excellent compositional and structural stability at high temperature. Microstructures, mechanical properties and fracture features of the C/C-LAS joints were investigated. The joints with YAS interlayer exhibited more favorable results than the joints with MAS (MgO–Al2O3–SiO2) interlayer subsequently to shear testing at 300 and 600 °C. Strength retention of 70% for YAS joints was achieved at 600 °C, while for MAS joints the strength retention was only 28%. The joining properties amount decrease of YAS joints was due to degradation of the intrinsic properties of YAS and LAS glass ceramics. Regarding MAS joints, the poor structural stability of MAS interlayer at high temperature was the reason for the corresponding decrease. As a result, the mechanical behavior of joints at elevated temperatures was not only related to wettability and interfacial bonding, but it was also related to the intrinsic properties as well as to high-temperature stability of the interlayer and the joined substrates.

Effect of Cr2O3 addition on crystallization, microstructure and properties of Li2O–Al2O3–SiO2 glass-ceramics

A new glass-ceramic with high flexural strength and low thermal expansion based on Li2O–Al2O3–SiO2 (LAS) was prepared in this study. The effects of Cr2O3 addition on the crystallization, microstructure, flexural strength, thermal expansion, and electrical properties of LAS system were investigated. The crystallization kinetics based on DSC analysis was calculated using Kissinger and Ozawa methods, which showed that the activation energy E decreases from 158.5 to 149.3 kJ/mol, indicating that Cr2O3 is beneficial to the crystallization of LAS; the crystallization index n varies between 4.46 and 5.09, indicating that the crystallization manner is the volumetric crystallization. XRD analysis was estimated by the whole pattern fitting method, demonstrating that Cr2O3 addition could change the phase contents and promote the crystallinity. The crystallization of CaMgSi2O6 and Cr2O3 with higher CTE not only properly adjusted CTE for matching Si, but also dramatically improved the flexural strength for LAS glass-ceramic. Moreover, we provided a modified formula to calculate CTE of glass-ceramic in the acceptable range. LAS glass-ceramic with 3 wt% Cr2O3 sintered at 800 °C exhibited good properties: σ = 208 MPa, α = 2.64 × 10−6/°C, ε = 8.3, tanδ = 3.6 × 10−3, ρ = 8.82 × 1012 Ω cm, indicating its suitability for LTCC application.

Effects of TiO2 on microstructures and properties of Li2O–Al2O3–SiO2 glass–ceramics for LTCC substrates

In order to increase the flexural strength and to adjust the coefficient of thermal expansion (CTE), TiO2 was introduced into Li2O–Al2O3–SiO2 (LAS) glass–ceramics to satisfy LTCC applications. The effects of TiO2 addition on the microstructure, thermal, mechanical and electrical properties were studied. The results showed that the crystal phases were composed of Li2OAl2O37.5SiO2 (major), as well as TiO2 and CaMgSi2O6 (minor). The crystal phase TiO2 having higher CTE value could appropriately increase the CTE value of LAS to fit the silicon and remarkably improve the flexural strength of LAS glass–ceramic with low CTE. The dielectric and insulating properties also strongly depended on the amount of TiO2. LAS glass–ceramic sintered at 800 °C with high performances was achieved in the case of 1 wt% TiO2: α = 2.56 × 10−6/°C, σ = 151 MPa, e = 6.8, tanδ = 1.731 × 10−3, ρ = 8.29 × 1010 Ω cm.

Influences of ZrO2 on microstructures and properties of Li2O–Al2O3–SiO2 glass–ceramics for LTCC applications

In this work, Li2O–Al2O3–SiO2 (LAS) glass–ceramics for LTCC applications were prepared by the melt quench technique, and the influences of ZrO2 content on the microstructure, thermal, mechanical and electrical properties were investigated. XRD showed that the major crystalline phase was Li2OAl2O37.5SiO2 and the minor crystalline phases were ZrO2 and CaMgSi2O6. The formation of ZrO2 crystal phase with higher coefficient of thermal expansion (CTE) led to the increase of CTE value of LAS glass–ceramics, which was favorable for matching with silicon (3.1 × 10−6/°C). Moreover, it was found that the mechanical property enhanced markedly when the high thermal expansion phases generated in the glass–ceramic system with low CTE. The results indicated that the addition of ZrO2 was beneficial to promote the mechanical, thermal and dielectric properties. 3 wt% ZrO2-added LAS glass–ceramic sintered at 800 °C exhibited excellent properties: a high bulk density of 2.567 g/cm3, a high linear shrinkage of 20.08 %, a low CTE value (2.68 × 10−6/°C), a high flexural strength of 157 MPa, a low dielectric constant of 7.1, a low dielectric loss (tan δ) of 1.884 × 10−3 at 1 MHz, and a high resistivity (ρ > 1010 Ω cm).

Glass–ceramics of LAS (Li2O–Al2O3–SiO2) system enhanced by ion-exchange in KNO3 salt bath

The paper describes a low-temperature ion-exchange process on specially designed Li2O–Al2O3–SiO2 glass–ceramics (LAS). Due to thermal treatment, two main crystalline phases: lithium di-silicate and lithium alumina-silicate were obtained in glass–ceramic materials. As a result of proper time/temperature profile treatment of precursor glasses, flexural strength of final materials increased up to 300–450MPa.Crystallized samples were immersed in KNO3 molten salt, at temperatures of 400–420°C for 9 or 16h. The final materials achieved almost 100% greater mechanical resistance than glass–ceramics before ion-exchange.We claim that the process of ion-exchange improved flexural strength significantly. Furthermore, it resulted in some changes of the surface layer structure. We noticed that chemical tempering process leads to decrystallization of the multiphase LAS glass–ceramic surface layer consisting of lithium di-silicate and lithium alumina-silicate phases.

Structural evolution of Ni environment in lithium, magnesium and zinc aluminosilicate glasses and glass-ceramics

Three glasses in the Li2O–Al2O3–SiO2 (LAS), MgO–Al2O3–SiO2 (MAS) and ZnO–Al2O3–SiO2 (ZAS) systems were investigated to obtain spinel-based transparent glass-ceramics doped with TiO2 to induce internal nucleation and with NiO to provide optically active ion. The sequence of phase transformations of initial glasses during heat-treatments was studied by in situ X-ray absorption spectroscopy, X-ray diffraction and room temperature optical spectroscopy.For all glasses, there is a change of Ni2+ coordination with the increase of the heat-treatment temperature. The [5]Ni2+ sites in the parent glass disappear and new distorted [6]Ni2+ sites are formed within aluminotitanate and (Ni, or Ni,Mg, or Ni,Zn) aluminate amorphous regions. The local environment around Ni2+ then evolves from distorted octahedra to regular octahedra due to the nickel-doped spinel crystallization. In LAS glass-ceramics only, Ni2+ incorporates at elevated temperature octahedral and tetrahedral sites in crystals of β-quartz solid solution, whose content increases with time and temperature of heat treatment accompanied by a continuous decomposition or dissolution of spinel. In MAS and ZAS glass-ceramics, Ni-doped spinel exist in the whole temperature range of applied heat-treatments.

Effect of SiC nanowires on the thermal shock resistance of joint between carbon/carbon composites and Li2O–Al2O3–SiO2 glass ceramics

Abstract In order to improve the bonding property of joint between SiC modified carbon/carbon (C/C) composites and Li2O–Al2O3–SiO2 (LAS) glass ceramics, SiC nanowires were attempted as the reinforcement materials in the interface region of SiC transition layer and Li2O–MgO–Al2O3–SiO2 (LMAS) gradient joining interlayer. The C/C–LAS joint with SiC nanowire-reinforced interface layer was prepared by a three-step technique of pack cementation, in situ reaction and hot-pressing. The microstructure and thermal shock resistance of the as-prepared joints were examined. The average shear strength of the joined samples with SiC nanowires increased from 24.9 MPa to 31.6 MPa after 40 thermal cycles between 1000 °C and room temperature, while that of the joined samples without SiC nanowires dropped from 21.4 MPa to 8.3 MPa. The increase of thermal shock resistance of the C/C–LAS joints was mainly attributed to the toughening mechanism of SiC nanowires by pullout, bridging and crack deflection.

Thermal and spectroscopic characterization of glasses and glass–ceramics of Li2O–Al2O3–SiO2 (LAS) system

Research were focused on obtaining Li2O–Al2O3–SiO2 glass–ceramics where crystallization of both Li-di-silicate and Li-alumina-silicates occurs. DTA techniques were used to determine the basic glass temperature/time crystallization profile of the set chemical composition (wt%): SiO2 66.3%; Al2O3 10.2%; Li2O 12.1%; CaO 1.46%; K2O 0.53%; Na2O 2.59%; P2O5 4.28%; ZrO2 0.56%; CeO2 1.5%; V2O5 0.04%. In order to define in material the crystallization degree and phase composition after particular thermal treatments, XRD method was used. On this base the proportion of main, Li-di-silicate and Li-Al-silicates, crystalline phases was defined. As a complementary, the spectroscopic FTIR examinations has been performed. After temperature/time thermal treatment of basic glass, FTIR spectra were changing: bands correlated with crystalline phases became sharper and split. This indicated increase the crystallization in material after particular heat treatments. For the investigated glasses and glass–ceramics 27Al MAS NMR and 29Si MAS NMR spectra was also measured. These spectra indicated what was the local structure of the alumina Al and silica Si in examined glasses and glass–ceramics. On this base various Qn units of aluminum Al3+ and silicon Si4+ were identified.

Effects of MgO on properties of Li2O–Al2O3–SiO2 glass–ceramics for LTCC applications

Li2O–Al2O3–SiO2 (LAS) glass–ceramics for low temperature co-fired ceramics (LTCC) application were prepared by melting method, and the effects of MgO on the sinterability, microstructure, dielectric property, thermal expansion coefficient (CTE) and mechanical character of this glass–ceramics have been studied. The X-ray diffraction images represent that the main phase is β-spodumene solid solutions. And some ZrO2 and CaMgSi2O6 phases in LAS glass–ceramics are detected. The LAS glass–ceramics without additive (MgO) sintered at 800° had the dielectric properties: dielectric constant (er) of 5.3, dielectric loss (tanδ) of 2.97 × 10−3 at 1 MHz, CTE value of 1.06 × 10−6 K−1, bulk density of 2.17 g/cm3, and flexural strength of 73 MPa. 5.5 wt% MgO-added LAS glass–ceramic achieves densification at 800° exhibited excellent properties: low dielectric constant and loss (er = 7.1, tanδ = 2.02 × 10−3 at 1 MHz), low CTE (2.89 × 10−6 K−1), bulk density = 2.65 g/cm3 as well as high flexural strength (145 MPa). The results indicate that the addition of MgO is helpful to improve the dielectric and mechanical properties. The formation of CaMgSi2O6 crystal phase with higher CTE leads to the increase of CTE value of LAS glass–ceramics due to the increasing MgO content, and the increase of CTE is favourable for matching with silicon (3.1 × 10−6 K−1). The prepared LAS glass–ceramics have the potential for LTCC application.

Effect of La2O3 addition on crystallization and properties of Li2O–Al2O3–SiO2 glass-ceramics

La-doped Li 2 O–Al 2 O 3 –SiO 2 (LAS) glass-ceramics were prepared by solid phase reaction. Their crystallization, morphologies and properties were investigated. The crystallization temperature of LAS glass-ceramics decreased due to the La 2 O 3 addition. For the La-doped LAS glass, the β -quartz solid solution (s.s.) was initially formed, and then the β -quartz s.s. transformed to β -spodumene at higher temperature. The La 2 O 3 addition played a role of network modifier in the LAS glass altering their network structure. With increasing the La 2 O 3 content, the flexural strength decreased, and the thermal expansion coefficient increased.

Preparation and Thermal Properties of P<sub>2</sub>O<sub>5</sub> Doped Li<sub>2</sub>O-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> Glass-Ceramics

Using TiO2 and ZrO2 as compound nucleation agent, serials Li2O–Al2O3–SiO2 (LAS) glasses containing different content of P2O5 were prepared by conventional melt quenching technique and subsequent converting to glass–ceramics. The influence of P2O5 content on viscosity of LAS glasses was investigated in the temperature range from the softening point to melting temperature. DSC was undertaken to analyze the change of characteristic temperature Tg, Tc and Tp. Thermal expansion coefficient (TEC) of matrix glass and glass-ceramic containing different P2O5 were measured. The results show introducing P2O5 effectively decreased the melting temperature and viscosity of LAS glasses. TECs of the matrix glass and glass-ceramic range from 4.1×10-6 °C -1 to 5.3×10-6 °C -1 and from -0.2×10-6 °C -1 to 1.2×10-6 °C -1, respectively. XRD analysis indicates that β-spodumene is the main crystalline phase in the P2O5 doped LAS glass-ceramics, resulting in the low thermal expansion coefficient of the glass-ceramics.

Crystallization mechanism and microstructure evolution of Li2O–Al2O3–SiO2 glass-ceramics with Ta2O5 as nucleating agent

Li2O–Al2O3–SiO2 glass-ceramics were prepared with Ta2O5 as nucleating agent, the crystallization mechanism and microstructure evolution were investigated by DTA, XRD, and SEM technologies. With increasing amount of Ta2O5 from 2 to 6 mol%, the crystallization activation energy decreased from 297.73 to 218.66 kJ mol−1, while the crystallization index increased from 1.76 to 3.39. In addition, the cluster of dendritic crystals and lamellar structure obtained in T-2 glass-ceramics indicated a typical two-dimensional crystallization mechanism, and the formation of spherical β-quartz solid solution in T-4 specimens, with average size of 50–70 nm, was mainly due to bulk crystallization mechanism. It was considered that Ta2O5 promoted the nucleation and crystallization of LAS glass by precipitating the crystalline precursor phase of Ta2O5, which acted as nuclei for the subsequent crystal growth. Eventually, the diffusion and crystallization process, microstructure morphology, as well as the secondary grain growth were also investigated.

Thermo-optic properties of B2O3 doped Li2O–Al2O3–SiO2 glass–ceramics

Reduction in the temperature coefficient of the optical path length, dS/dT of Li2O–Al2O3–SiO2 glass–ceramics with near-zero thermal expansion coefficient was attempted using control of the temperature coefficient of electronic polarizability, ϕ, and the thermal expansion coefficient, α. The dS/dT value of 2.6mol% B2O3-doped glass–ceramic was 12.5 ×10−6/°C, which was 0.9× 10−6/°C smaller than that of B2O3-free glass–ceramic. On the other hand, reduction in dS/dT through B2O3 doping was not confirmed in precursor glasses. Results showed that reduction in dS/dT of the glass–ceramic through B2O3 doping is caused by the reduction in ϕ. The reduction in ϕ from B2O3 doping was probably attributable to numerical reduction in non-bridging oxide ions with larger ϕ value by the concentration of boron ions in the residual glass phase. In addition, application of hydrostatic pressure during crystallization was effective to inhibit precipitation of β-spodumene solid solution, which thereby decreases dS/dT. The dS/dT value of B2O3-doped glass–ceramic crystallized under 196MPa was 11.7× 10−6/°C. That value was slightly larger than that of silica glass. The α value of this glass–ceramic was smaller than that of silica glass.

Structural characterization of Er-doped Li2O–Al2O3–SiO2 glass ceramics

Particularly favourable properties of glass ceramics are developed on the basis of two key advantages of these materials: the variation of chemical composition and of microstructure. Therefore, detailed structural and chemical information are necessary to get insight in novel glass ceramic materials. We present here two examples of Er-doped Li2O–Al2O3–SiO2, with different quantities of ZrO2, both obtained with sol–gel synthesis. Different transmission electron microscopy techniques: conventional TEM, HRTEM, and EELS are used and the results are compared with those previously obtained with XRD and Rietveld analysis. We also demonstrate the 3D reconstruction, obtained from HAADF-STEM imaging, to determine the morphology of nanosize precipitates in these composites.

Sintering and Phase Transformation in B2O3/P2O5-Doped Li2O.Al2O3.4SiO2Glass-Ceramics

The effects of the additives B2O3and P2O5on the sintering and phase transformation in Li2O.Al2O3.4SiO2glass-ceramics were investigated. The addition of B2O3 enhanced the crystallization of the parent glasses, whereas the addition of P2O5 delayed it; however, both additives improved the sinterability. Glass-ceramics that contained appropriate amounts of B2O3 and P2O5 showed good sinterability at temperatures as low as 750°C. The phase transformation of metastable high-quartz solid solution (h-Q ss) to stable β-spodumene (β-S) occurred when the crystallization of parent glass to h-Q ss was almost complete. Both B2O3 and P2O5 affected the relative amount of β-S that transformed to h-Q ss. A glass that contained a larger amount of B2O3 revealed an increase in the relative ratio of β-S to h-Q ss with increasing temperature, which was consistent with the generally known phase development in Li2O–Al2O3–SiO2 (LAS) glass-ceramics. A glass that contained a larger amount of P2O5 showed an anomalous temperature dependence of the transformation of metastable h-Q ss to stable β-S; this dependence had not been observed in other LAS glass-ceramics. A possible explanation for this anomaly is that h-Q ss (and β-S) that has been formed at different temperatures may have different phase compositions.

Crystallization behaviour and microstructural evolution of a Li2O–Al2O3–SiO2 glass derived from spodumene mineral

A naturally occurring mineral deposit of α-spodumene has been successfully used to fabricate glasses and glass ceramics in the Li2O–Al2O3–SiO2 (LAS) system. TiO2 is an effective nucleating agent in promoting the crystallization of Li2O–Al2O3–4SiO2 glass to produce LAS glass-ceramics in which β-quartzss and β-spodumeness are the major crystalline phases evolved. The crystallization process and microstructural evolution were monitored using X-ray diffraction and transmission electron microscopy.

Preparation and crystallization of ultrafine Li2O-Al2O3-SiO2 powders

Ultrafine powders of Li2O-Al2O3-SiO2 (LAS) glass-ceramic were prepared by the sol-gel process using tetraethoxysilane, titanium butoxide, lithium, magnesium, aluminium (and zinc) inorganic salts as starting materials. The effect of pH on the sol-gel transition and particle sizes of the Li2O-Al2O3-SiO2 system was studied. The nucleation and crystallization process of LAS powders were also investigated by differential thermal analysis and X-ray diffraction. The results show that a surface nucleation process occurs for ultrafine LAS powders. The LAS glass-ceramics fabricated from ultrafine LAS powders have a low thermal expansion coefficient, α<10×10−7 °C.