Search

The calcium aluminosilicate-based glasses (CaO-Al2O3-SiO2, CAS) with different Fe2O3 content (0.10wt%, 0.50wt%, 0.90wt%, and 1.30wt%) were prepared by traditional melt-quenching method. The glass network structure, thermal and mechanical properties, and crystallization behavior changes were investigated by nuclear magnetic resonance spectrometer, Fourier-transform infrared spectro-photometer, X-ray diffractometer, differential scanning calorimetry and field emission scanning electron microscope measurements. The change of Qn in glass structures reveals the glass network connectivity decreases due to the increasing content of Fe2O3 addition, resulting in the increasing of non-bridging number in glass structure. The glass densities slightly rise from 2.644 to 2.681 g/cm3, while Vickers's hardness increases at first, from 6.469 to 6.901 GPa, then slightly drops to 6.745 GPa, with Fe2O3 content increase. There is almost no thermal expansion coefficient change from different Fe2O3 content. The glass transmittance in visible range gradually decreases with higher Fe2O3 content, resulting from the strong absorption of Fe2+ and Fe3+ ions. The calculated activation energy from thermal analysis results first decreases from 282.70 to 231.18 kJ/mol, and then increases to 244.02 kJ/mol, with the Fe2O3 content increasing from 0.10wt% to 1.30wt%. Meanwhile, the maximum Avrami constant of 2.33 means the CAS glasses exhibit two-dimensional crystallization. All of the CAS glass-ceramics samples contain main crystal phase of anorthite, the microstructure appears lamellar and columnar crystals. [ABSTRACT FROM AUTHOR]

Li2O–Al2O3–SiO2‐based glass–ceramics containing nanocrystals have attracted much attention due to their low expansion coefficient, good mechanical properties, for different applications, such as fire‐safe glass, dental materials, and electronic devices protectors. In current research, ultrahigh hardness Li2O–MgO–Al2O3–SiO2 glass–ceramics were prepared by conventional melt‐quenching and subsequent heat‐treatment method. MgO was introduced via Li2O substitution to change glass crystallization and mechanical properties. The differential scanning calorimetry analysis indicated the glass transition and crystallization temperatures increased with MgO/Li2O ratio increase, for better glass network connectivity from Raman spectra analysis. In addition, the main crystal phase changed from Li2SiO5 and LiAlSi4O10, to the combination of LiAlSi2O6 and MgAl2Si4O12, and finally to MgAl2Si4O12. The Vickers hardness of glass–ceramics was highly dependent on MgO/Li2O ratios in glass components and heat‐treatment temperatures, corresponding with the crystal phases in glass–ceramics. The highest hardness could reach 9.34 GPa, which was much higher than traditional silicate glass–ceramics. The scanning electron microscope images confirmed the crystal diameters that varied from 30 to 100 nm and were determined by MgO content. Transmission electron microscope images and energy‐dispersive spectroscopy mapping also confirmed the precipitation of multiphase nanocrystals in glass matrix. The changes of glass structure, and corresponding crystal combinations in glass–ceramics resulting from MgO introduction, were responsible for the ultrahigh hardness glass. [ABSTRACT FROM AUTHOR]

Transparent ZnO–MgO–Al 2 O 3 –SiO 2 (ZMAS) glass-ceramics comprising ZrO 2 and SnO 2 as nucleating agents were prepared via melting and two-step heat treatment. The effects of varying the SnO 2 concentration (1–3 mol%) on the crystallization behavior, microstructures, and mechanical properties of the glass-ceramics were investigated. Increasing the SnO 2 content promoted the precipitation of spinel and gahnite because SnO 2 improved the formation of ZrO 2 nanocrystals, which provided nucleation sites for subsequent crystallization. When the SnO 2 content was varied between 2 and 3 mol%, the crystallization behavior of the ZMAS glass-ceramics showed little difference, and SnO 2 crystals precipitated from the glass at a relatively low crystallization temperature. In addition, glass-ceramics containing 2 and 3 mol% SnO 2 exhibited average grain sizes of ∼26 and ∼29 nm, respectively, but their transmittance decreased to ∼52% and ∼50%, respectively, owing to grain aggregation. The best overall performance was achieved when the SnO 2 content of the glass-ceramic was 3 mol% and heat treatment was conducted at 780 °C for 3 h and 960 °C for 2 h, with the resulting glass-ceramic exhibiting a bending strength of ∼187 MPa and a Vickers hardness of ∼845 kgf/mm2. [ABSTRACT FROM AUTHOR]

Transparent Na 2 O–MgO–Al 2 O 3 –SiO 2 glass-ceramics containing spinel crystals were prepared by melting and heat treatment method, and the effects of substitution of 2–8 mol% MgO for Na 2 O on the crystallization behavior, microstructure, and properties were investigated. As Na 2 O is replaced by MgO, there is higher concentration of 5-coordinated Al units in the glass structure, which played an important role in the crystallization behavior of the glass. The increasing substitution of MgO for Na 2 O promotes the formation of spinel but inhibits the nepheline and carnegieite. When containing 2 mol% MgO, the large-sized nepheline and carnegieite are the main crystalline phases, making the glass ceramics opaque. When containing 6 mol% MgO, there is no NaAlSiO 4 crystal formed, but only spinel crystals with small size and uniform distribution are detected in glass-ceramics. The glass-ceramics with spinel as the main crystalline phase show high transparency above 82% in the visible wavelength range. The Vicker's hardness of transparent glass-ceramics can be further improved by ion-exchanged with KNO 3 molten salt, which shows significant potential in the applications for the protective covers of mobile displays. [ABSTRACT FROM AUTHOR]

Transparent glass-ceramics containing eucryptite and nepheline crystalline phases were prepared from alkali (Li, Na) aluminosilicate glasses with various mole substitutions of Al 2 O 3 for SiO 2. The relationships between glass network structure and crystallization behavior of Li 2 O–Na 2 O–Al 2 O 3 –SiO 2 (LNAS) glasses were investigated. It was found that the crystallization of the eucryptite and nepheline in LNAS glasses significantly depended on the concentration of Al 2 O 3. LNAS glasses with the addition of Al 2 O 3 from 16 to 18 mol% exhibited increasing Q4 (mAl) structural units confirmed by NMR and Raman spectroscopy, which promoted the formation of eucryptite and nepheline crystalline phases. With the Al 2 O 3 content increasing to 19–20 mol%, the formation of highly disordered (Li, Na) 3 PO 4 phase which can serve as nucleation sites was inhibited and the crystallization mechanism of glass became surface crystallization. Glass-ceramics containing 18 mol% Al 2 O 3 showed high transparency ∼84% at 550 nm. Moreover, the microhardness, elastic modulus and fracture toughness are 8.56 GPa, 95.7 GPa and 0.78 MPa m1/2 respectively. The transparent glass-ceramics with good mechanical properties show high potential in the applications of protective cover of displays. [ABSTRACT FROM AUTHOR]

Cordierite glass-ceramics usually begin to crystallize from the surface. As an efficient nucleating agent, TiO 2 can promote the rapid transformation of glass to bulk crystallization, but it is easy to cause the increase of dielectric constant and light absorption. High crystallinity cordierite glass-ceramics were prepared by optimizing the heat treatment process without or with different nucleating agents in stoichiometric cordierite glasses. The results show that the crystallization mechanisms of glasses without and with ZrO 2 +P 2 O 5 as nucleation agents are controlled by surface crystallization. While, the glass with TiO 2 +ZrO 2 +P 2 O 5 as nucleation agents have the tendency to be bulk crystallization. The studied glasses are crystallized from surface and have different crystallization orientations with the inner glass. The thickness of crystalline layer increased with the increasing of heating temperatures, but the "surface-center" crystallization process cannot complete by further increasing heating temperatures because of softening deformation of glasses. At 1020 ℃, the glasses complete the "surface-center" crystallization for long durations. The glasses without nucleation agents and with ZrO 2 +P 2 O 5 require 10 h, but the glass with TiO 2 +ZrO 2 +P 2 O 5 complete for 5 h. Although all the three glasses complete the "surface-center" crystallization, the glasses with nucleation agents show the higher crystallinity upon the same heat treatments. Finally, glass-ceramics with excellent performance were obtained, for example, the Z1# glass-ceramic have the high microhardness ∼7.4 GPa, low thermal expansion coefficient ∼1.4☓10−6 ℃−1 at 20–300 ℃, and relatively high thermal conductivity ∼2.4 W/mK. It also exhibits low dielectric constant and loss, which was ∼4.5 and ∼1.2☓10−3 at 1 MHz, ∼ 4.9 and 2.3☓10−3 at 10.5 GHz.. [ABSTRACT FROM AUTHOR]

Ion exchange has the potential to improve the mechanical properties of glass ceramics. In this work, ZrO 2 nanocrystals embedded transparent glass ceramics were prepared and effect of the crystallization on ion-exchange properties was investigated. The crystallization of ZrO 2 did not affect the transmittance and Vickers hardness due to the small nanocrystal size and the low crystallinity, but significantly enhanced the ion exchange depth of layer (DOL). X-ray diffraction, high resolution transmission electron microscope, Raman spectra and nuclear magnetic resonance analysis demonstrated that with the crystallization of ZrO 2 , the charge compensator (Na+) was released, which promoted the transformation of highly coordinated Al into [AlO 4 ]− tetrahedral units and the formation of Na+ balanced non-bridging oxygens. These changes in structure of glass made the Na+ more mobile and increased the DOL upon the crystallization. Results reported here may be useful for the development of glass-ceramic materials suitable for chemical strengthening. [ABSTRACT FROM AUTHOR]

The effects of ZrO 2 addition on the glass network structure, crystallization behavior and microwave dielectric properties of ZnO-B 2 O 3 glass-ceramics were investigated. The introduction of ZrO 2 decreases the connectivity of the superstructure in the borate glass network and thus affects the crystallization behavior of glass, leading to the transformation of the main crystalline phase from Zn 4 B 6 O 13 to ZnB 4 O 7 and Zn 3 B 2 O 6. With the increase of ZrO 2 content, the microstructure changed from the branch-like structure into the net-like structure. For the sintered glass-ceramics, the relative permittivity increased with the ZrO 2 content, which relates to the decrease in the vibrational energy of the B-O bond. In addition, the glass-ceramic with 0.25 mol% ZrO 2 exhibits the maximum Q×f value due to the low disorder of B3+ cations. The ZnO-B 2 O 3 glass-ceramic with 0.25 mol% ZrO 2 sintered at 660 °C for 5 h exhibits the optimum microwave dielectric properties with ε r = 6.18, Q×f = 23,670 GHz, τ f = −65 ppm/°C. The high Vickers hardness of 662 kgf/mm2 and excellent chemical compatibility with Ag electrode demonstrated that the glass-ceramic is a good candidate for ULTCC application. • Raman spectroscopy was used to characterize the change of borate superstructures units in the glass at the intermediate-range order scale, which is an important reason for the change of the main crystalline phase when the introduction of ZrO 2 into the ZnO-B 2 O 3 glass system • The Zr4+ ions in the glass-ceramics have a strong accumulation effect on the O2- ions, which leads to the reduction of the vibration energy of the B-O bond and affect the polarization behavior, finally the relative dielectric constant increased. • In the glass-ceramic with 0.25mol% ZrO 2 content, the B3+ions show the highest degree of cationic order, which indicates that the crystal structure in glass-ceramic is closer to the standard lattice structure, so the highest Q×f value is obtained. [ABSTRACT FROM AUTHOR]

Transparent glass-ceramics were prepared in MgO–Al 2 O 3 –SiO 2 (MAS) glass by two-step heat-treatments, and the effect of CeO 2 and La 2 O 3 on crystallization mechanism of MAS glass-ceramics was investigated. The micro-structure of the transparent glass-ceramic samples was studied by FE-SEM. Crystallization activation energy, E , and Avrami constants, n , were determined by DSC through Kissinger and Augis–Bennett methods. The crystallization mechanism of specimens without rare-earth oxides represents two-dimensional crystallization, and with the addition of CeO 2 and La 2 O 3 , the mechanism tends to three-dimensional crystallization. After nucleation at 800 °C for 4 h and crystallized at 950 °C for 1.5 h, spinel crystals with the mean diameter ~ 20 nm precipitated from matrix glass without rare-earth oxides, and the sample maintained the transparency over ~ 80% in visible light. The glass without rare-earth oxides showed the highest value of E (362.74 kJ/mol) and the lowest value of n (2.09), which is corresponding to the precipitation of spinel. With the addition of CeO 2 and La 2 O 3 , though the spinel is still the dominate crystal phase, the sizes increased and distributions turned to be broad. The over-growth of crystals leads the transmittance to decrease to ~ 30%. [ABSTRACT FROM AUTHOR]

• Rare-earth doped NAS transparent glass ceramic have been prepared. • Effect of rare-earth ions on crystallization behavior of NAS glass ceramics • The light transmittance of transparent glass-ceramics can reach over 70%. • Nd2O3 is conductive to the preparation of transparent nepheline glass-ceramics. Transparent nepheline glass-ceramics were prepared from Na 2 O-Al 2 O 3 -SiO 2 (NAS) glasses with TiO 2 , ZrO 2 and P 2 O 5 as multiple nucleation agents. Effects of Eu 2 O 3 and Nd 2 O 3 on the crystallization behavior on the NAS glasses were discussed. As the calculation results of crystallization kinetics, the NAS glass without rare-earth oxides is surface crystallization mechanism with the Avrami parameter n ~ 1.4. With the addition of Eu 2 O 3 and Nd 2 O 3 , the crystallization activation energy decreased and the Avrami parameter n increased to 2.40 and 2.44, respectively. The mechanism of crystallization tend to be bulk crystallization and two dimensional crystal growth. In all the studies NAS glasses without and with RE oxides, ZrO 2 , low- carnegieite and nepheline crystals successively precipitated with the increasing of heating temperatures from 650 to 780 °C for 5 h. The glass with 0.4 mol% Nd 2 O 3 containing nepheline as main crystalline phase kept the visible transparency ~65% upon the heat treatment 750 °C for 5 h. While, the glass without RE oxides is opaque, and the transmittance of glass with 0.4 mol% Eu 2 O 3 significantly decrease to 21%. It is suggested that Nd 2 O 3 is more effective to promote crystallization and form ultra-fine microstructure of NAS glass-ceramics than Eu 2 O 3. [ABSTRACT FROM AUTHOR]

ZnAl 2 O 4 nanocrystals (NCs) embedded transparent glass-ceramics containing relatively large amount of alkali oxide were prepared and effects of the crystallization on ion-exchange properties of glass-ceramics were investigated. Precipitation of ZnAl 2 O 4 NCs not only improves the Vickers hardness of the glass-ceramics, but also reduces the content of zinc and aluminum with concomitant increase in the content of sodium in the residual glassy phase. These changes in the residual glassy phase decrease the activation energy of K–Na ion-exchange and enhance the diffusion coefficient of potassium, resulting in the large ion-exchange depth and significantly improved Vickers hardness of the ion-exchanged glass-ceramics. These results show that glass-ceramics containing non-alkali nanocrystals have promising applications in fields of covers for optoelectronic devices. • ZnAl 2 O 4 NCs were precipitated in transparent aluminosilicate GCs containing large amount of alkali oxides. • Crystallization of ZnAl 2 O 4 NCs can improve ion exchange properties. • Compositional changes in residual glassy phase in GCs was responsible for the enhanced ion-exchange. [ABSTRACT FROM AUTHOR]