Influence of Al2O3/SiO2 ratio on the microstructure and properties of low temperature co-fired CaO–Al2O3–SiO2 based ceramics

In this work, in order to obtain the materials for low temperature co-fired ceramics applications, CaO–Al₂O₃–SiO₂ (CAS) based ceramics were synthesized at a low sintering temperature of 900 °C. The influences of Al₂O₃/SiO₂ ratio on the microstructure, mechanical, electrical and thermal properties were studied. According to the X-ray diffractomer and scanning electron microscopy results, the addition of the Al₂O₃ is advantageous for the formation of the desired materials. Anorthite(CaAl₂Si₂O₈) is the major crystal phase of the ceramics, and the SiO₂ phase is identified as the secondary crystal phase. No new crystal phase appears in the ceramics with the increasing Al₂O₃ content. More or less Al₂O₃ addition would all worsen the sintering, mechanical and dielectric properties of CAS based ceramics. The ceramic specimen (Al₂O₃/SiO₂ = 20/18.5) sintered at 900 °C shows good properties: high bending strength = 145 MPa, low dielectric constant = 5.8, low dielectric loss = 1.3 × 10^?3 and low coefficient of thermal expansion value = 5.3 × 10^?6 K^?1. The results indicate that the prepared CAS based ceramic is one of the candidates for low temperature co-fired ceramic applications.     

Effect of MoO3, Nd2O3, and RuO2 on the crystallization of soda–lime aluminoborosilicate glasses

The effect of adding Nd₂O₃, MoO₃, and RuO₂ separately or simultaneously on the crystallization of a soda–lime aluminoborosilicate glass during cooling from the melt or glass heating was studied by DTA, XRD (at room and high temperature), SEM, Raman, and optical absorption. Nd₂O₃ addition strongly reduces liquid–liquid phase separation and crystallization of calcium and sodium molybdates (CaMoO₄ (powellite) and Na₂MoO₄) in Mo-rich compositions as long as Nd³⁺ ions remain solubilized in the glassy network. This suggests that (MoO₄)^2? entities and Nd³⁺ ions are close to each other in the glass structure (Nd³⁺ ions would prevent the clustering of molybdate entities). The effect of MoO₃ addition in Nd-rich compositions is more complex since an increase of the solubility of Nd₂O₃ is observed, whereas the nucleation rate of an Nd-rich silicate apatite (Ca₂Nd₈(SiO₄)₆O₂) in the bulk of the glass increases as soon as molybdates crystallized. The addition of RuO₂ has a nucleating effect on apatite crystallization in the bulk but not on molybdates crystallization.     

Microchemistry and microstructure of a multiphase aluminosilicate ceramic

The microstructure and microchemistry of a sintered (≈ 1700° C) aluminosilicate ceramic (60 wt % Al₂O₃-40 wt% SiO₂) was investigated by optical, scanning (SEM and EDAX), and analytical electron microscopy (TEM and STEM). The microstructural features of the fired ceramic consisted of unreacted Al₂O₃, glass, porosity, and equilibrium and metastable mullite phases. Residual Al₂O₃ agglomerates (≈ 15 to 30 μm in size) were surrounded by a ≈ 6 μm layer of equilibrium mullite (≈ 71.3 to 73.5 wt% Al₂O₃). The unreacted Al₂O₃-equilibrium mullite assembly formed islands embedded in a silica rich glass (≈ 4.5 to 14wt% Al₂O₃) which also contained 2 to 3 μm thick metastable mullite needles (≈ 70 to 77 wt% Al₂O₃). Phase separation and alumina rich glass compositions (≈ 57 to 59 wt% Al₂O₃) were also observed in some areas of the microstructure.     

Low temperature sintering and characteristics of K2O–B2O3–SiO2–Al2O3 glass/ceramic composites for LTCC applications

The K₂O–B₂O₃–SiO₂, K₂O–B₂O₃–SiO₂–2 %Al₂O₃, K₂O–B₂O₃–SiO₂–4 %Al₂O₃ glasses with different Al₂O₃ content were prepared. Different proportions (50, 55, 60, 65, 70 %) of the three glasses were respectively mixed with alumina ceramic-filler, then the mechanical and dielectric properties were investigated. The results showed K₂O–B₂O₃–SiO₂–2 %Al₂O₃ glass/alumina filler (glass:alumina = 60:40) had the excellent comprehensive properties, so further study was continued with part of alumina ceramic-filler replaced by the silica ceramic-filler on this composite. Then the X-ray diffraction analysis revealed that the alumina and silica fillers existed as the crystal phase, and the densification was seriously damaged when the silica content reached to three quarters of the fillers. With the increase of the silica-filler, the composites’ density and dielectric constant exhibited uniform decrease, but thermal expansion coefficient (TEC) uniformly increased. When the glass:alumina:silica was equal to 60:30:10, a best composite property was presented as a bulk density of 2.582 (g cm^?1), a dielectric constant of 6.1 and a dielectric loss of 2 × 10^?3 at 1 MHz, a flexural strength of 168 MPa, and a TEC of 8.62 × 10^?6 °C^?1.     

Effect of starting melt composition on growth of La3Ta0.5Ga5.5O14 crystal

The phase diagram of the La₂O₃-Ta₂ O₅-Ga₂ O₃ system around La₃ Ta_0.5 Ga_5.5 O₁₄ (LTG) was investigated by a differential thermal analysis and the quenching method. La₃ Ta_0.5 Ga_5.5 O₁₄ single crystals were subsequently grown from different starting melt compositions using the Czochralski technique. The effect of the starting melt composition on crystal quality was examined by measuring the variation in the chemical composition and lattice parameters along the growth axis. On the basis of these experimental results, we have grown LTG single crystals over 2 inches in diameter.     

Structural and microstructural transitions during phase separation and crystallization of Al2O3-SiO2-P2O5-Y2O3 glass

A highly refractory glass in the system Al₂O₃-SiO₂-P₂O₅-Y₂O₃ has been designed and produced such that, upon heating, an essentially fully crystalline glass—ceramic evolves containing mullite (nominally 3Al₂O₃·2SiO₂) and xenotime (YPO₄) as the final principal phases. Phase separation in this glass occurred during cooling from the melt and continued during annealing. XPS of the Al 2p, P 2p and the Y 3d electrons revealed that the average chemical environment of each of these elements is measurably different in the annealed glass and in the completely crystallized material. This indicates that the compositions of the separated glass phases are very different from those of the crystal phases which form from them. Additional rearrangement of the glass structure was observed at 1173 K. Extensive formation of mullite was initially detected at 1223 K and was followed by the crystallization of xenotime and the transient compounds of Y₄Al₂O₉, Y₂Si₂O₇, AlPO₄ and YP₅O₁₄. The optimum crystal nucleation and crystallization temperatures of 1173 and 1473 K, respectively, were determined from DTA, XRD, SEM and TEM studies.     

Control of properties of PZT ceramics during liquid-phase sintering

A PZT ceramic modified by 0.4Pb-(1–x)(B₂O₃)–x(GeO₂) glass, and glass-ceramics modified by V₂O₅, have been obtained. The influence of the chemical composition of the glasses on the mechanical strength and dielectric parameters of these ceramics is investigated. The mechanical and electrical parameters of PZT ceramics are considerably improved by small additions of glass ( 1 mass %) of suitable composition. During heat treatment, the mobility of the domain walls may be changed as a result of the interaction of smelt glass with the surface layer of the crystallites (resulting from a change of V _Pb ^t" vacancy concentration in grains and decreased mechanical tension on the grain boundaries). By introducing the glass, we can decrease the porosity of the ceramics and thus decrease the attenuation of surface acoustic waves in piezoelectric filters. The origin of the liquid phase during sintering of the PZT glass-ceramics prevents PbO evaporation. This determines the preservation of the solution's stoichiometry. The glass addition leads to a decrease in the sintering temperature of PZT, which simplifies the technological process. The glass ceramics are obtained by common ceramic technology which is used for commercial piezoceramic production. The main technological problem is choosing the chemical composition and concentration of the glass and thermal process parameters.     

Magnetic and Electrotransport Properties of the Anion-Deficient Manganites with Perovskite Structure

The chemical phase content and microstructure as well as magnetic and magnetotransport properties for the doped anion-deficient La³⁺_1-xSr²⁺_xMn³⁺O^2–_3-x/2 orthomanganites have been experimentally studied. The high-quality anion-deficient ceramic with controlled chemical phase content and microstructure has been obtained from the parent stoichiometric one at the 800 C in vacuum. The average grain size is ~5 m. It is established that the samples in the region of 0 x 0.125 are O-orthorhombic perovskites whereas in the 0.175 x 0.30—rhombohedric. As doping level increases along with the oxygen vacancies number the samples in ground state undergo a transition from the antiferromagnet A-type (x=0) through the inhomogeneous magnetic state (0 < x 0.175 to the cluster spin glass one (0.175 < x 0.30. The temperature of magnetic moment freezing is ~ 45 K. All the samples are semiconductors and show considerable magnetoresistance over a wide temperature range with peak for x = 0.175 only. Fitting of the electrical resistivity to T^–1 and T^–1/4 realized. The concentration dependences of the activation energy, spontaneous magnetization, coercivity as well as magnetic transition temperatures for the anion-deficient La³⁺_1-xSr²⁺_xMn³⁺O^2–_3-x/2 ortomanganites have been established. Obtained experimental results are interpreted in terms of the oxygen vacancies, isotropic superexchange Mn³⁺-O–Mn³⁺ interactions and phase separation models.     

Raman spectroscopic study of structure of WO3La2O3B2O3 glasses with no color and crystallization of LaBWO6

Glasses with the nominal compositions of xWO₃25La₂O₃(75 − x)B₂O₃ (mol%) with x = 15, 25, and 50 were prepared using a conventional melt quenching method, and their structure and crystallization behavior were examined from Raman scattering spectra and X-ray diffraction analyses. The glasses are colorless in the visible light region and give the optical band gap energy of 3.49-3.61 eV. The glass transition and crystallization temperatures and the thermal stability against crystallization decrease with increasing WO₃ content. The strong Raman bands at 840 and 940-960 cm⁻¹ suggest that the main coordination state of W⁶⁺ ions in the glasses is isolated (WO₄)²⁻ tetrahedral units. The formation of WO₆ octahedral units is also suggested in the glasses with high WO₃ contents. The main crystallization mechanism in the glasses is the surface crystallization, and the glass of 50WO₃25La₂O₃25B₂O₃ shows the crystallization of LaBWO₆ single phase. The present study proposes that WO₃La₂O₃B₂O₃ glasses and crystallized glasses are very interesting as optical functional materials.     

Effects of NiNb<Subscript>2</Subscript>O<Subscript>6</Subscript> doping on dielectric property,microstructure and energy storage behavior of Sr<Subscript>0.97</Subscript>La<Subscript>0.02</Subscript>TiO<Subscript>3</Subscript> ceramics

Sr_0.97La_0.02TiO₃ ceramics with samll amounts of NiNb₂O₆ additives were prepared by the traditional solid state sintering method, and the phase purity, microstructure, dielectric properties and energy storage behavior of the NiNb₂O₆-added Sr_0.97La_0.02TiO₃ ceramics were investigated. The results show that the grain size of the ceramics firstly decreases and then increases with increasing NiNb₂O₆ concentration. The average grain size reaches 0.55 um for the sample with 4.5 wt% NiNb₂O₆. Moreover, impedance spectroscopy (IS) analysis was employed to study the electrical conductive behavior of NiNb₂O₆-doped Sr_0.97La_0.02TiO₃ ceramics. IS results reveale that the NiNb₂O₆-doped Sr_0.97La_0.02TiO₃ ceramic has large R _gb /(R _gb  + R _g ) ratios due to the decreased grain sizes. The breakdown strength is notably improved, and the highest breakdown strength of 324 kV/cm can be achieved for the sample with 4.5 wt% NiNb₂O₆ additive. The Sr_0.97La_0.02TiO₃ sample with 4.5 wt% NiNb₂O₆ possesses the maximum theoretical energy density of 1.36 J/cm³, which is about 2 times higher than that of pure SrTiO₃ in the literature. And its energy storage efficiency reaches 91.4 % under applied electric field of 80 kV/cm. This study provides the NiNb₂O₆ added ceramic as an attractive candidate for making high-energy density capacitors.     

Investigation of the crystal structure and electrical properties of La3+-doped SrBi2Ta2O9 ceramics

The layered-perovskite ferroelectric ceramics of La³⁺-doped SrBi₂Ta₂O₂ (SBT), with the chemical formula of SrBi_{(2 - x)}La _x Ta₂O₉ (SBLT), have been prepared by the conventional mixed-oxide method. The effect of substitution of La³⁺ for Bi³⁺ in the crystal structure and electrical properties of SBT ceramics was explored with the aid of X-ray diffraction, (T) curve and ferroelectric hysteresis loop measurements. The electrical properties such as dielectric constant () and remanent polarization (P _r) showed a dependence on the crystal structure, and both reached maxima of 243 and 25 C cm^–2, respectively, with 6 at % La³⁺ substitution, accompanying the greatest structure change. Theoretical considerations were presented to suggest that the atomic displacements and the crystal deformation implied by the crystal structure change are responsible for the improvement of electrical properties. On the other hand, degradation of fatigue resistance was observed in SBLT ceramics, which is believed to be caused by the chemical environment change of the perovskite layers arising from La³⁺ substitution on Bi³⁺ sites.     

Preparation and Magnetic Properties of La-Substituted Strontium Hexaferrite by Microwave-Assisted Sol-Gel Method

Sr_1?x La _x Fe₁₂O₁₉ (x = 0, 0.15, 0.25, 0.5) hexaferrites were prepared by microwave-assisted sol-gel method. The thermal decomposition process, structural, and magnetic properties of the products were studied by thermal differential scanning calorimeter (DSC), thermogravimetry (TG), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM). The phase of α-Fe₂O₃ appeared at x = 0.25 and x = 0.5. The coercivity of La³⁺-substituted strontium hexaferrites is improved to 5960.2 Oe at x = 0.25.     

Formation of plate-like lanthanum-β-Aluminate crystal in Ce-TZP matrix

The reaction route and morphology of lanthanum--aluminate (LBA) crystals formed in Ce-TZP matrix were studied by examining the crystal phase changes and the microstructures in relation to the heat-treatment time, heat-treatment temperatures and the particle size of raw Al₂O₃ powders. In the Ce-TZP matrix, the LBA crystal was formed by the reaction between La₂O₃ and Al₂O₃ through the LaAlO₃ phase as the intermediate. La₂Zr₂O₇ forms at 800 °C and remains in the temperature range 800–1500 °C, and LaAlO₃ forms between 1200 and 1400 °C. The LaAlO₃ reacts with Al₂O₃ to form LBA above 1500 °C. The diffusion of La³⁺ through the La₂Zr₂O₇ phase was faster than that of Al³⁺. The morphology of LBA crystals was dependent on the particle size of the starting raw Al₂O₃ particle. When submicrometre size Al₂O₃ (0.4m) particles were used as the starting particles, anisotropic, plate-like LBA crystals, about 10m long, were formed during heat treatments. On the other hand, Al₂O₃ of larger grain sizes (3.6, 10.3m) yield conglomerates of LBA crystals. The size of the conglomerates is similar to that of the raw Al₂O₃ particle. The dependence of the morphology of LBA on the particle size of Al₂O₃ can be attributable to the sintering process of the Ce-TZP matrix, leading to the control of the mechanical properties of Ce-TZP ceramics with LBA crystals.     

Alkali resistant non-silicate porous glass-like material

Substitution of the end-member oxides in the ternary sodium borosilicate system has been studied. Replacing SiO₂ with a combination of alkali resistant oxides, Th, Zr, Ce with or without Y₂O₃, was found to produce glasses which, after heat treatment, decomposed into two immiscible microphases, one of which was water soluble. The structure of the leached material or material sintered after leaching (ThO₂, ZrO₂, CeO₂, Y₂O₃ or ThO₂, ZrO₂, CeO₂) was predominantly glass-like. Some Na₂O and B₂O₃ may be expected to remain unleached in the pores as has been observed in silica-based material. However, no evidence of this in crystalline form was found during X-ray diffraction analysis. The specific surface areas of the materials so formed ranged between 58 and 315 m² g^?1, having calculated pore radii of between 0.8 and 13.6 nm. Alkali resistance of up to 1.96×10^?2 mg dm^?2 and water resistance between 5 and 16.18 mg Na₂Og^?1 were measured.     

Role of Calcium in the Evolution of Superconductivity in a (La2?xRx)Ba2(CayCu4+y)Oz (R = Y, Er, Gd) System

It has been reported that, by adding equal amounts of CaO and CuO to non superconducting La₃Ba₃Cu₆O _z (La-336), a series of superconductors with nominal compositions La₃Ca _y Ba₃Cu_6+y O _z were prepared with maximum T^on _c 80K. Similar studies on addition of CaO and CuO in nonsuperconducting LaBaCu₂O _z (La-112) resulted into superconducting LaCaBaCu₃O _z (La-1113). To date no attempt has been made to synthesize La₂CaBa₂Cu₅O _z (La-2125) superconducting phase by addition of CaO and CuO to non superconducting La₂Ba₂Cu₄O _z (La-224) system. Also no reports are published to study the effect of replacing larger La³⁺-ions (1.01Å) by smaller rare earth ions viz Y³⁺(0.89Å), Er³⁺(0.91Å), Gd³⁺(0.91Å) on the structural and superconducting properties of (La_2–x R _x )Ba₂(Ca _y Cu_4+y )O _z (LRBCaC); 0.0 x 0.5; y=2x system. In this paper, we report the method of synthesis, structural and superconducting property characterization using X-ray diffraction, oxygen content measurements using iodometry, resistivity measurements using d.c. four probe technique and a.c. susceptibility measurements in the temperature range RT to 15K. Also a comparative study, on the evolution of superconducting phase with Ca-concentration for different rare earth substitutions for LRBCaC system in the context of hole doping mechanism, is carried out.     

Magnetic and mössbauer study of rare-earth-substituted M-, W- and X-type hexagonal ferrites

The effects of substitution of Ba²⁺ ions for some trivalent rare-earth ions on the magnetic properties and phase formation of M-, W- and X-type hexaferrites are studied. The compounds considered were: Ba_1-RE_x/2Na_x/2Fe₁₂O₁₉ with RE = gadolinium, lanthanum, lutetium, samarium and 0.0 x 0.3; Ba_0.9La_0.05Na_0.05Zn₂Fe₁₆O₂₇ and Ba_1.9La_0.05Na_0.05Zn₂Fe₂₈O₄₆. The samples were prepared by standard ceramic procedures and have been investigated by thermomagnetic analysis, X-ray diffraction, Mössbauer spectroscopy, scanning electron microscope and magnetic measurements. Monophasic BaRE _x/2-M compounds were found for 0.0x<0.2 giving an indication of the solubility range of the rare-earth (RE) atoms in this phase. The intrinsic magnetic properties and the coercive field are lowered with the exception of the lanthanum-substituted hexaferrite. No significant differences in the phase formation and magnetic properties were observed for the W- and X-type hexaferrites.     

Magnetic properties of LaInO<Subscript>3</Subscript>-based perovskite-structure photoluminescent materials doped with Nd<Superscript>3+</Superscript>, Cr<Superscript>3+</Superscript>, and Mn<Superscript>3+</Superscript> ions

Single-phase ceramic samples of La_1–xNd_xInO₃ (0.007 ≤ x ≤ 0.05), LaIn_0.99M_0.01O₃, and La_0.95Nd_0.05In_0.995M_0.005O₃ (M = Cr³⁺ and Mn³⁺) solid solutions have been prepared by solid-state reactions, and their crystal structure, magnetic field dependences of their specific magnetization at 5 and 300 K, and temperature dependences of their molar magnetic susceptibility have been studied. It has been shown that the 300-K specific magnetization of the La_1–xNd_xInO₃ (x = 0.02, 0.05), La_0.95Nd_0.05In_0.995M_0.005O₃ (M = Cr³⁺ and Mn³⁺), and LaIn_0.99Mn_0.01O₃ solid solutions increases linearly with increasing magnetic field strength up to 14 T and that the magnitude of the 300-K specific magnetization of the La_0.993Nd_0.007InO₃ and LaIn_0.99Cr_0.01O₃ solid solutions increases linearly, but they have diamagnetic magnetization. At a temperature of 5 K, the magnetization of all the indates studied here increases nonlinearly with increasing magnetic field strength, gradually approaching magnetic saturation, without, however, reaching it in a magnetic field of 14 T. In the temperature range where the Curie–Weiss law is obeyed (5–30 K), the effective magnetic moments obtained for the Nd³⁺ ion (\({\mu _{effN{d^{3 + }}}}\)) in the La_1–xNd_xInO₃ solid solutions with x = 0.007, 0.02, and 0.05 are 2.95μ_B, 3.09μ_B, and 2.75μ_B, respectively, which is well below the theoretical value \({\mu _{effN{d^{3 + }}}}\)= 3.62μ_B. The effective magnetic moments of the Cr³⁺ and Mn³⁺ ions in the LaIn_0.99Cr_0.01O₃ and LaIn_0.99Mn_0.01O₃ solid solutions are 3.87μ^B and 5.11μ^B, respectively, and differ only slightly from the theoretical values \({\mu _{effC{r^{3 + }}}}\)= 3.87μ^B and \({\mu _{effM{n^{3 + }}}}\)= 4.9μ^B.     

Phase equilibria in the La<Subscript>2</Subscript>O<Subscript>3</Subscript>-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>-CoO system at 1100°C

The phase equilibria in the La₂O₃-Ga₂O₃-CoO system have been studied at 1100°C in air, using samples prepared by a standard ceramic processing technique from oxides and by a glycine-nitrate combustion process. The composition ranges and structures of solid solutions in this system have been determined by x-ray powder diffraction: LaGa_1?x Co _x O₃ (0 < x ≤ 0.05), LaCo_1?x Ga _x O₃ (0 < x ≤ 0.10), La₄Ga_2?x Co _x O₉ (0 < x ≤ 0.20), Co_1?x Ga_2+x O₄ (?0.20 ≤ x ≤ 0.05). The unit-cell parameters of the solid solutions vary little with composition, in accordance with the small difference in ionic radius between gallium and cobalt. The 1100°C section through the phase diagram of the La₂O₃-Ga₂O₃-CoO system in air is presented.     

Low-temperature synthesis,pyrolysis and crystallization of tantalum oxide gels

Tantalum oxide gels in the form of transparent monoliths and powders have been prepared from hydrolysis of tantalum pentaethoxide under controlled conditions using different mole ratios of Ta(OC₂H₅)₅C₂H₅OHH₂OHCl. Alcohol acts as the mutual solvent and HCl as the deflocculating agent. For a fixed alkoxide water HCl ratio, the time of gel formation increased with the alcohol to alkoxide molar ratio. Thermal evolution of the physical and structural changes in the gel has been monitored by differential thermal analysis, thermogravimetric analysis, X-ray diffraction, and infrared spectroscopy. On heating to 400 °C, the amorphous gel crystallized into the low-temperature orthorhombic phase -Ta₂O₅, which transformed into the high-temperature tetragonal phase -Ta₂O₅ when further heated to 1450 °C. The volume fraction of the crystalline phase increased with the firing temperature. The -Ta₂O₅ converted back into the low-temperature phase, -Ta₂O₅, on slow cooling through the transformation temperature of 1360 °C, indicating a slow but reversible transformation.