Structure and crystallization behaviour of some MgSiO3-based glasses

We report on the structure and crystallization behaviour of four enstatite based glasses. Two glasses with nominal compositions of Y_0.125Mg_0.875Si_0.875B_0.125O₃ and Y_0.125Mg_0.725Ba_0.15Si_0.875B_0.125O₃ were prepared as parent glasses while the other two glasses were derived by the addition of 8 wt.% Al₂O₃ to the parent glass compositions, respectively. Structural features of the glasses were accessed by Fourier transform infrared spectroscopy (FTIR). Non-isothermal crystallization kinetics and thermal stability of Al₂O₃-free glasses were studied using differential scanning calorimetry (DSC). It has been shown that these glasses exhibit higher activation energy of viscous flow and are prone to surface crystallization. Activation energy of crystallization decreases with the addition of BaO in the glasses. Crystallization behaviour of all the experimental glasses in the temperature range of 800–1000 °C was followed by X-ray diffraction (XRD) and FTIR. Clinoenstatite and orthoenstatite were the major crystalline phases in the BaO-free glass-ceramics while BaO-containing compositions featured the early formation and stabilization of protoenstatite.     

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.     

Effects of alumina on the crystallization behavior, densification and dielectric properties of BaO–ZnO–SrO–CaO–Nd2O3–TiO2–B2O3–SiO2 glass–ceramics

The alumina addition effects on the crystallization, sintering behaviors and dielectric properties of BaO–ZnO–SrO–CaO–Nd₂O₃–TiO₂–B₂O₃–SiO₂ (Ba–Zn–Sr–Ca–Nd–Ti–B–Si) glass powder were investigated using the differential thermal analyzer (DTA), thermo-mechanical analyzer (TMA), X-ray diffractometer (XRD). The results showed that the addition of alumina powder into Ba–Zn–Sr–Ca–Nd–Ti–B–Si glass changed the crystallization sequence from Nd₂Ti₄O₁₁–Nd_0.66TiO₃ to Nd₂Ti₃O_8.7–Nd₂Ti₂O₇–Nd₂Ti₄O₁₁and increased the densification activation energy due to the dissolution of Al³⁺ ions into the glass structure. Fully densified 30 vol.% alumina-added Ba–Zn–Sr–Ca–Nd–Ti–B–Si glass can be obtained via glass viscous flow before the second and third crystalline phases, Nd₂Ti₂O₇ and Nd₂Ti₄O₁₁crystallization. The 30 vol.% alumina-added Ba–Zn–Sr–Ca–Nd–Ti–B–Si glass–ceramics sintered at 900 °C exhibited a high dielectric constant of 17 and a quality factor of about 820, which provided a promising candidate for LTCC applications.     

Effect of sintering temperature and Ho2O3 on the properties of TiO2-based varistors

In this study, a series of TiO₂-based ceramics doped with different contents of Ho₂O₃ in the range of 0–0.6?mol% are prepared by means of a conventional solid-state reaction method. Phase composition, microstructure and element distribution are studied by use of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) separately. The influence of sintering temperature and Ho₂O₃ on the properties of samples is explored. The results show that the breakdown voltage decreases continuously while both the nonlinear coefficient and the relative dielectric constant ascend firstly and then descend with the sintering temperature increasing. Meanwhile, the relative dielectric constant and nonlinear coefficient of samples firstly ascend and then descend with the increasing of Ho₂O₃. Although the minimum breakdown voltage (3.3?V/mm) is obtained when sample is sintered at 1450?°C, the sample doped with 0.45?mol% Ho₂O₃ sintered at 1400?°C exhibits high comprehensive electrical properties, with breakdown voltage of 6?V/mm, the nonlinear coefficient of 5.5 and the relative dielectric constant of 1.88?×?10⁵.     

Low-temperature sintering and microwave dielectric properties of 3Z2B glass–Al2O3 composites

A potential low temperature co-fired ceramics system based on zinc borate 3ZnO–2B₂O₃ (3Z2B) glass matrix and Al₂O₃ filler was investigated with regard to phase development and microwave dielectric properties as functions of the glass content and sintering temperature. The densification mechanism for 3Z2B–Al₂O₃ composites was reported. The linear shrinkage of 3Z2B glass–Al₂O₃ composites exhibited a typical one-stage densification behavior. XRD patterns showed that a new crystalline phase, ZnAl₂O₄ spinel, formed during densification, indicating that certain chemical reaction took place between the 3Z2B glass matrix and the alumina filler. Meanwhile, several zinc borate phases, including 4ZnO·3B₂O₃, crystallized from the glass matrix. Both of the reaction product phase and crystallization phases played an important role in improving the microwave dielectric properties of composites. The optimal composition sintered at 850–950 °C showed excellent microwave dielectric properties: ?_r = ∼5.0, Q·f₀ = ∼8000 GHz, and τ_f = ∼−32 ppm/°C at ∼7.0 GHz.     

Low-temperature pressureless sintering of Al2O3-SiC-Ni nanocermets in air environment

This paper introduces a simplified method for low-temperature pressureless sintering of Al₂O₃-Ni-SiC nanocermets in air environment. In this method, a thin and continuous Ni shell was coated on the surface of Al₂O₃ particles using electroless deposition method. The composite powders were subsequently compressed to prepare bulk specimens. By preventing the ceramic particles from direct contact during the densification of green specimens, sintering temperature of cermet materials was reduced from that of Al₂O₃ (>?1400?°C) to the range of Ni solid-phase sintering temperature. Furthermore, dissolution of a low amount of phosphorus in the composition of Ni coatings caused the further decrease of the sintering temperature to 800?°C. At such low temperatures, pressureless sintering of the cermets in the air environment was successfully performed instead of the common hot pressing process in a reducing atmosphere. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and X-ray diffraction (XRD) characterizations indicated that the microstructure of such sintered samples consists of a continuous Ni network surrounding Al₂O₃ grains, without any structural defects or Ni oxidation. Furthermore, mechanical properties of the cermet materials were improved through reinforcement of the continuous Ni network by different amounts of SiC nanoparticles. The results showed that Al₂O₃-Ni-5?wt% SiC nanocermets sintered at 800?°C obtain the highest compressive strength of 242.5?MPa, hardness of 56.8 RA, and the lowest wear weight loss of 0.04?mg/m.     

Preparation of Al2O3 and MgAl2O4-based samples with tailored macroporous structures

In this work we successfully obtained freeze-cast alumina (Al₂O₃) and magnesium aluminate spinel (MgAl₂O₄) samples. Camphene was used as the freezing vehicle in this study. The specimens prepared herein were examined by Archimedes tests, scanning electron microscopy, and X-ray powder diffraction. Cold crushing tests were also carried out at room temperature. It was observed that the pore structure of Al₂O₃ samples can be tailored by changing the solid loading and freezing rate; the higher the solid loading and freezing rate, the finer the pore structure of the freeze-cast sample. MgAl₂O₄-based specimens were fabricated by keeping the solid loading in the starting slurry at 30 vol% and using liquid nitrogen as the cooling agent. The material obtained from a 60 Al₂O₃?40 MgO slurry showed a spinel amount of about 90%, an expressive total porosity (63 ± 3%), and a significant cold crushing strength (58 ± 6 MPa). In addition, this material exhibited the finest pore structure among the composition studied herein, showing a mean pore size of about 4 µm.     

Influence of CaO-ZrO2-Al2O3-SiO2 glass-ceramic frits on the technological properties of porcelain stoneware bodies

In the present work, the effect of the addition to a porcelain stoneware body of glass-ceramic frits belonging to the CaO-ZrO₂-Al₂O₃-SiO₂ (CZAS, containing 1, 3, 5, and 10 mol% of Al₂O₃) system as replacement of the “state of the art” frit belonging to CaO-ZrO₂-SiO₂ system (CZS) was evaluated. The firing process was performed in a furnace able to complete the thermal cycle in 50 min, in order to simulate the industrial process.Technological properties such as water absorption, firing shrinkage, flexural strength, and thermal expansion behaviour were measured. Aesthetical properties were also evaluated. X-ray diffraction and scanning electron microscopy studies were also carried out to analyse the microstructure and the phase compositions of the studied samples.It was found that the sample containing the CZAS frit with 5 mol% of Al₂O₃ shows, in general, the best combination of properties in term of mechanical, physical and aesthetical properties.     

Sintering kinetics of a 18.8Li2O 8.3ZrO2 64.2SiO2 8.7Al2O3 glass ceramic

The LZSA (Li₂O-ZrO₂-SiO₂-Al₂O₃) glass ceramic system has shown high potential as low temperature co-fired ceramics, the so called LTCC, for several applications, such as screen-printed electronic components, due to its low sintering temperature (< 1000 °C). However, a good microstructural control must be achieved in order to obtain a low porosity material. The 18.8Li₂O 8.3ZrO₂ 64.2SiO₂ 8.7Al₂O₃ LZSA glass ceramic composition was prepared by melting, quenching in water, and grinding in order to obtain a very fine powder (3.78 μm mean particle size). Subsequently, compacted bodies were obtained by uniaxial pressing (40 MPa) and drying. Sintering kinetics was investigated by optical dilatometry measurements and scanning electron microscopy (SEM) observations. The activation energy for sintering was found to be 314 kJ.mol⁻¹, whilst a maximum linear shrinkage of 22% and porosity of 3.5% were obtained with 10 min swelling time at 800 °C.     

Low-temperature reaction-sintering of mullite ceramics with an Y2O3 addition

Mullite ceramics were fabricated at relatively low temperatures from powder mixtures of -Al₂O₃ and quartz, with an Y₂O₃ addition. The mullitization process was analyzed by X-ray diffraction. The densification behavior was investigated as a function of the Y₂O₃ content, sintering temperature and holding time as well as mullite seeds. It has been shown that mullitization occurs via a nucleation and growth mechanism within an yttrious aluminosilicate glass, but lattice and grain-boundary diffusion becomes important during the densification process. Moreover, the incorporation of mullite seeds was observed to enhance both mullitization and densification. At 1400°C for 5 h or 1450°C for 2 h, 15 mol% Y₂O₃-doped and 5 mol% mullite-seeded specimens can be sintered to almost full density.     

Some studies on nucleation,crystallization, microstructure and mechanical properties of mica glass-ceramics in the system 0.2BaO·0.8K2O·4MgO·Al2O3·6SiO2·2MgF2

The crystallization, microstructure, microhardness and theoretical machinability have been investigated by DTA, XRD, SEM and Microhardness Indenter of resulting glass-ceramics. Two distinct crystallization exotherms in the DTA curve are observed and resolved. The first peak corresponds to the initial formation of potassium fluorophlogopite and the second is due to the formation of barium fluorophlogopite. The activation energy for precipitation of each crystalline phase has been evaluated, and the crystallization mechanism has been studied. DTA analyses were conducted at different heating rates and the activation energy was determined graphically from Kissinger and Ozawa equation. The average activation energy is calculated as 276 KJ/mol for the first and 366 KJ/mol for the second crystallization peak. The Avrami exponent for first and second crystallization peak temperature determined by Augis and Bennett method is found to be 3 and 3.9, respectively. The results indicate that the growth of mica is a two and three dimensional process, controlled by the crystal-glass interface reaction. The Vicker's hardness decreased steadily at intermediate heat treatment temperature with the formation of barium and potassium fluorophlogopite phase, but the decrease in hardness is more rapid at higher temperature with the development of an interconnected ‘house of cards’ microstructure.     

Evaluation of sintering stresses of an Al2O3 powder with a self-loading apparatus

The sintering stress of an Al₂O₃ powder, σ_s, is evaluated from an equation σ_s = F_s/ρS_a, where F_s is the uniaxial tensile force necessary to just stop the sintering contraction, ρ is the relative density, and S_a is the cross-sectional area. During densification, σ_s increases to the maximum at a relative density of 85%, and then abruptly decreases. The variation of σ_s in the intermediate sintering stage suggests a simple cubic packing of particles. Densification in the final sintering stage is explained by shrinkage of the pores at the corners where four tetrakaidecahedra meet. The present surface tensions of the Al₂O₃ powder evaluated from the σ_s roughly equal a reported value.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

Improving sintering behavior of MWCNT/BaTiO3 ceramic nanocomposite with Bi2O3-B2O3 addition

The present research systematically investigated the novel low-temperature fabrication of a multi-walled carbon nanotube (MWCNT)/barium titanate nanocomposite using a two-step mixing technique. The synthesis was conducted using different amounts of MWCNT (0.25%, 0.5%, 1%, 2%, 4%, and 8% wt) with different compositions of (Bi₂O₃ + B₂O₃) as a sintering aid. Scanning and transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, three-point bending strength, Vickers hardness indentation and Archimedean technique were used to characterize the as-synthesized specimens. It was found that the appropriate content of sintering aid (Bi₂O₃+B₂O₃) strongly decreased the sintering temperature from 1200 °C to 950 °C. The results also revealed that any sintering aid with the optimum composition that included 30% (mol) Bi₂O₃ was optimal for a sintering aid content of 6% (wt). Consequently, the highest values of the flexural strength and fracture toughness were achieved by applying the optimal amounts of MWCNT (1% wt) and sintering aid (6% wt).     

Early-stage sintering in a powder compact of polyhedral particles II. Experimental analysis with a highly sinterable Al2O3

A model of early-stage sintering for polyhedral particles is examined with a highly sinterable Al₂O₃ powder. Combining of the initial and intermediate stages based on the model concerned describes quantitatively its sintering data such as power dependence of time and reduction in a specific surface area. Valid estimations of a model and/or mechanisms of sintering necessitate the accurate measurement of both shrinkage versus sintering time and specific surface area versus shrinkage.     

Influence of Al2O3 addition on microstructure,defects level and magnetic properties of LiTiZn ferrite ceramics

In the present work, the results of the influence of diamagnetic additives on the defects level of ferrite ceramics, its microstructure and magnetic properties are presented. A method based on a mathematical analysis of the experimental temperature dependences of the initial permeability was used for estimation of the defects level in the samples. Model samples containing a controlled amount of the diamagnetic additive Al₂O₃ served to test the possibility of monitoring this method of nonmagnetic phases of ferrite ceramics. It was shown that with an increase in the concentration of the Al₂O₃ additive in the range of (0–0.5) wt%, a significant increase in the defects level was observed almost 6-fold. The data from SEM micrographs showed that the addition of Al₂O₃ affects the type of grains of ferrite ceramics, but does not affect their grain size. Grains are highly agglomerated and show large grain size dispersion and also pore. Obtained data were compared to hysteresis loop parameters. It is shown that with an increase in the concentration of the Al₂O₃ addition, there is a regular decrease in the residual induction and an increase in the coercive force. However, such changes in hysteresis loop parameters are small in comparison to defects level. Investigations of the true physical broadening of the diffraction reflections were performed for the same model samples in order to compare the change in the defects level to the direct X-ray diffraction measurements of micro deformations. The defects level as a characteristic of the elastic stress of a ferrite ceramics is proposed. This assumption follows from a linear relationship between the defects level and the width of the diffraction reflections. The consistency of the obtained results made it possible to evaluate the high efficiency and sensitivity of the method for defects level estimating.     

Sintering behavior and microwave dielectric properties of Bi2O3–ZnO–Nb2O5-based ceramics sintered under air and N2 atmosphere

The sintering behavior and dielectric properties of the monoclinic zirconolite-like structure compound Bi₂(Zn_1/3Nb_2/3)₂O₇ (BZN) and Bi₂(Zn_1/3Nb_2/3−xV_x)₂O₇ (BZNV, x = 0.001) sintered under air and N₂ atmosphere were investigated. The pure phase were obtained between 810 and 990 °C both for BZN and BZNV ceramics. The substitution of V₂O₅ and N₂ atmosphere accelerated the densification of ceramics slightly. The influences on microwave dielectric properties from different atmosphere were discussed in this work. The best microwave properties of BZN ceramics were obtained at 900 °C under N₂ atmosphere with _r = 76.1, Q = 850 and Q_f = 3260 GHz while the best properties of BZNV ceramics were got at 930 °C under air atmosphere with _r = 76.7, Q = 890 and Q_f = 3580 GHz. The temperature coefficient of resonant frequency τ_f was not obviously influenced by the different atmospheres. For BZN ceramics the τ_f was −79.8 ppm/°C while τ_f is −87.5 ppm/°C for BZNV ceramics.     

Gas pressure sintering of Al2O3/TiCN composite

Al₂O₃/TiCN composites have been fabricated by gas pressure sintering, which overcomes the limitations of hot pressing. The densification behavior and mechanical properties of the Al₂O₃ gas pressure sintered with 30 wt.% TiCN at different temperatures have been investigated. The gas pressure sintered Al₂O₃–30 wt.%TiCN composite achieved a relative density of 99.5%, a bending strength of 772 MPa, a hardness of 19.6 GPa, and a fracture toughness of 5.82 MPa·M^1/2.     

Fabrication of Yb-doped Lu2O3-Y2O3-La2O3 solid solutions transparent ceramics by self-propagating high-temperature synthesis and vacuum sintering

A comparative analysis of sintering and grain growth processes of lutetium oxide and lutetium-yttrium-lanthanum oxides solid solutions, as well as optical properties, luminescence and laser generation of (Lu_xY_0.9-xLa_0.05Yb_0.05)₂O₃ transparent ceramics are reported. Fabrication of highly dispersed initial powders of these compounds was performed via nitrate-glycine self-propagating high-temperature synthesis (SHS) method. The powders were compacted at 300?MPa and vacuum sintered at temperatures up to 1750?°С. Optical ceramics of (Lu_0.65Y_0.25La_0.05Yb_0.05)₂O₃ elemental composition were shown to have the highest in-line transmittance, which achieved 78% at the wavelength of 800?nm. Generation of laser radiation at a wavelength of 1032?nm with the differential efficiency of 20% was demonstrated in the (Lu_0.65Y_0.25La_0.05Yb_0.05)₂O₃ ceramics.     

Effect of Y2O3 additive on conventional and microwave sintering of mullite

Mullite has become a strong candidate material for advanced structural and functional ceramics. Much interest has recently focused on sintering aids for mullite. The aim of this study was to evaluate the effect of Y₂O₃ as a sintering aid in the conventional and microwave sintering of mullite. To accomplish this study, a highly pure industrial mullite was used. Mullite with and without Y₂O₃ was pressed under a cold isostatic pressure of 200 MPa. Samples were sintered conventionally at 1400, 1450, 1500, 1550 and 1600 °C for 2 h and microwave-sintered for up to 40 min using a large range of power. The microstructure and physical properties of the microwave-sintered samples were compared to those of the conventionally sintered samples. The results showed that Y₂O₃ improved the densification of mullite bodies in the conventional and microwave sintering processes, but high densifications were achieved in just a few minutes when Y₂O₃ was used with microwave processing.