Synthesis and characterization of borosilicate glass/β-spodumene/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites with low CTE value for LTCC applications

Glass?+?ceramic composites based on low-softening-point borosilicate (BS) glass, β-spodumene and Al₂O₃ were produced in this work. The influence of ceramic filler composition on the microstructure, sintering quality, mechanical properties, thermal properties and dielectric properties of composites were studied. XRD and DSC indicated that both kinds of ceramic filler as well as the BS glass maintained their characteristics after sintering. The addition of β-spodumene would decrease the coefficient of thermal expansion (CTE) value of composites to match with silicon well. The better wetting behavior between β-spodumene and BS glass would lead to better sintering quality, microstructure and dielectric properties for composites containing more β-spodumene. With appropriate Al₂O₃ content, the flexural strength of composites could be enhanced. Composite with 45 wt% BS glass, 30 wt% β-spodumene and 25 wt% Al₂O₃ sintered at 875 °C showed good properties which meet the requirements of low temperature co-fired ceramic applications: dense microstructure with high relative density of 96.27%, proper CTE value of 3.57 ppm/°C, high flexural strength of 156 MPa, low dielectric constant of 6.20 and low dielectric loss of 1.9?×?10^?3.     

Effect of sintering temperatures on properties of BaO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glass/silica composites for CBGA package

BaO–B₂O₃–SiO₂–Al₂O₃ (BBSA) glass/silica composites synthesized by solid-state reaction method were developed for CBGA packages, and the effects of sintering temperature (900–950 °C) on the phase transformation, microstructure, thermal, mechanical and electrical properties were investigated. XRD results show that the major phases quartz and cristobalite, and the minor phase BaSi₂O₅ are detected in BBSA composites. Furthermore, it was found that the quartz phase transforms to cristobalite phase at 930–940 °C. The formation of cristobalite phase with higher coefficient of thermal expansion (CTE) led to the increase of CTE value of BBSA composites. However, excessive cristobalite phase content would degrade the mechanical properties and the linearity of thermal expansion of the ceramics. BBSA composites sintered at 920 °C exhibited excellent properties: low dielectric constant and loss (ε_r = 6.2, tanδ = 10^?4 at 1 MHz), high bending strength (179 MPa), high CTE (12.19 ppm/°C) as well as superior linearity of the thermal expansion.     

The effect of microstructure on thermal expansion coefficients in powder-processed Al2Mo3O12

Orthorhombic Al₂Mo₃O₁₂ was investigated as a model anisotropic phase to understand the influence of powder preparation routes and bulk microstructure (mean grain size) on the bulk coefficient of thermal expansion (CTE) and to compare it to the intrinsic CTE of powder samples. A co-precipitation route was used for the synthesis of pure single-phase nanopowders, while a polyvinyl alcohol-assisted sol–gel method was utilized for the synthesis of micron-sized powders. Sintered samples prepared from both powders exhibited different microstructures in terms of mean crystal sizes and porosity. Bulk samples obtained from nanopowders were highly porous and contained crystals of approximately 100-nm diameter, while the bulk pieces produced from the micron-sized powders were denser, contained crystals larger than 5 μm, and showed occasional intergranular and transgranular microcracks. Such different microstructures hugely impact the bulk CTE: the nanometric sample possesses a bulk CTE (0.9 × 10^?6 °C^?1, from 200 to 700 °C) closer to the instrinsic CTE (2.4 × 10^?6 °C^?1) than for the micrometric sample, which showed a negative CTE (?2.2 × 10^?6 °C^?1) from 200 to 620 °C, and an even more negative CTE above 620 °C (?35 × 10^?6 °C^?1). A finite element analysis showed that the local maximum thermal tensile stresses could be as high as 220 MPa when simulating a temperature drop of 700 °C as an example of thermal treatment following sintering. This tensile stress is expected to exceed the tensile strength of Al₂Mo₃O₁₂, explaining the origin of microcracks in bulk samples prepared from the micron-sized powders. The thermal behavior of the microcracks leads to differences between the intrinsic and bulk thermal expansion; we show experimentally that such differences can be reduced by nanostructuring.     

Magnetic,optical, dielectric,and sintering properties of nano-crystalline BaFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> synthesized by a polymerization method

A one-pot polymerization method using citric acid and glucose for the synthesis of nano-crystalline BaFe_0.5Nb_0.5O₃ is described. Phase evolution and the development of the crystallite size during decomposition of the (Ba,Fe,Nb)-gel were examined up to 1100 °C. Calcination at 850 °C of the gel leads to a phase-pure nano-crystalline BaFe_0.5Nb_0.5O₃ powder with a crystallite size of 28 nm. The shrinkage of compacted powders starts at 900 °C. Dense ceramic bodies (relative density ≥ 90%) can be obtained either after conventional sintering above 1250 °C for 1 h or after two-step sintering at 1200 °C. Depending on the sintering regime, the ceramics have average grain sizes between 0.3 and 52 µm. The optical band gap of the nano-sized powder is 2.75(4) eV and decreases to 2.59(2) eV after sintering. Magnetic measurements of ceramics reveal a Néel temperature of about 23 K. A weak spontaneous magnetization might be due to the presence of a secondary phase not detectable by XRD. Dielectric measurements show that the permittivity values increase with decreasing frequency and rising temperature. The highest permittivity values of 10.6 × 10⁴ (RT, 1 kHz) were reached after sintering at 1350 °C for 1 h. Tan δ values of all samples show a maximum at 1–2 MHz at RT. The frequency dependence of the impedance can be well described using a single RC-circuit.     

Synthesis and some properties of Ti3SiC2 by hot pressing of Ti,Si and C powders Part 2 – Mechanical and other properties of Ti3SiC2

Abstract

Some properties of the remarkable Ti₃SiC₂ based ceramic synthesised by hot pressing of elemental Ti, Si, and C powders have been investigated. Its flexural strength by using three point bending tests and fracture toughness by using single edge notched beam tests were measured at room temperature to be in the range 310–427 MPa and about 7·MPa m^1/2, respectively. This material is a relative 'soft' ceramic with a low hardness of 4 GPa. Ti₃SiC₂ is similar to the soft metals and is a damage tolerant material that is able to contain the extent of microdamage. An oxidation test has been performed in the temperature range 1000–1400°C in air for 20 h. The oxidation resistance below 1100°C was good. Two oxidized layers were formed, the outer layer consisting of pure rutile-type TiO₂, and the inner layer a mixture of SiO₂ and TiO₂. The average coefficient of thermal expansion (CTE) of Ti₃SiC₂ was measured to be 9·29 × 10^?6 K^?1 in the temperature range 25–1400°C. The thermal shock resistance of Ti₃SiC₂ was evaluated by quenching the samples from 800°C, 1200°C, and 1400°C, respectively. The retained flexural strength drops dramatically at quenching temperature, but shows a slight increase after quenching from 1400°C compared with quenching from 800°C and 1200°C.     

Effect of Sintering on Magnetic Properties of Ni0.55Zn0.45Fe2O4

Bulk Ni_0.55Zn_0.45Fe₂O₄ samples were obtained by sintering their nanopowder at 1100 ^°C, 1200 ^°C, and 1300 ^°C. Improvement in crystallinity on sintering was identified from increase in intensity of the XRD peaks and grain development in SEM micrographs. Saturation magnetization increased from 81.7 emu/g to 85.3 emu/g as the sintering temperature increased from 1100 ^°C to 1300 ^°C. Initial permeability increases whereas the relative loss factor, resonance frequency, and DC resistivity decreases with increasing the sintering temperature. Curie temperatures obtained from low field AC normalized susceptibility and permeability measurements are in good agreement. The DC resistivity of the samples in the present case is two orders higher than the reported values of samples prepared using conventional ceramic method.     

The comparative influences of structural ordering, grain size, Li-content, and bulk density on the Li+-conductivity of Li0.29La0.57TiO3

The lattice and total Li⁺-ionic conductivity of Li_0.29La_0.57TiO₃ ceramic (LLTO) sintered at 1200 °C were determined as functions of powder calcination temperature and sintering duration, and these results were correlated with the relative degrees of Li⁺-ordering, Li-content, grain size, and bulk density to assess the relative impact of these parameters on material performance. Under all conditions, LLTO formed with a high degree of tetragonal superstructure to its perovskite related framework, and the lattice conductivity closely followed the relative amounts of the superstructure, as evaluated via determination of the sample ordering parameter from X-ray diffraction data. LLTO powders that were calcined at 900 °C for 1 h and sintered at 1200 °C for 6 h gave lattice conductivity values (~1.14 × 10⁻³ S cm⁻¹) comparable within the highest ranges reported in the literature. This coincided with the lowest degree of tetragonal superstructure formation, and it was also found to be largely independent of the values of Li-content measured on sintered ceramic despite significant Li₂O volatilization at longer sintering times (up to 23 % after 12 h at 1200 °C). Samples of LLTO powder that were calcined at 1100 °C and sintered at 1200 °C for 12 h resulted in the highest total Li-ion conductivity value ~6.30 × 10⁻⁵ S cm⁻¹. The total conductivity of LLTO varied inversely with grain size when the grains were <20 μm but was insensitive to that parameter above that size threshold. The strongest influence on total conductivity was primarily the bulk ceramic density. It was estimated from measured values that as the bulk ceramic density approached the full theoretical value for LLTO the total conductivity could near the lattice conductivity of ~1.2 × 10⁻³ S cm⁻¹.     

Preparation and characterization of NiMn2O4 negative temperature coefficient ceramics by solid-state coordination reaction

The influence of synthesis parameters, such as calcination temperature and sintering temperature, on the microstructure, phase composition, and electrical properties of NiMn₂O₄ negative temperature coefficient (NTC) ceramics was systematically investigated. The NiMn₂O₄ NTC ceramics were synthesized via solid-state coordination reaction. With increasing sintering temperatures, the relative density increased, whereas the porosity decreased. Single-phase, cubic spinel ceramic was obtained following sintering at 900 and 1,050 °C, whereas a secondary phase, i.e., NiO, was detected when the sintering temperature was higher than 1,100 °C. High-density ceramics were obtained when the sintering temperature was higher than 1,100 °C, and featured the lowest room temperature resistivity of 2,924 Ω cm and thermal constant B of 3,429 K. The latter parameter reflects the temperature sensitivity of the NTC ceramics. Variations of the electrical property were because of increases in density and onset of decomposition.     

Spark plasma reaction sintering of ZrO2–mullite composites from plasma spheroidized zircon/alumina powders

Mullite–zirconia ceramic composites are prepared by reaction sintering of plasma spheroidized (PS) zircon–alumina powders in a spark plasma sintering (SPS) system at 1000, 1100, 1200 and 1300 °C with duration of 10 and 30 min. At SPS temperature of 1000 °C, evidence of zircon decomposition is detected, while at 1200 °C, mullite formation dominates the process, resulting in significant increases in microhardness, Young's modulus and fracture toughness values. At SPS temperature of 1300 °C, due to re-crystallization, rapid grain growth, and intergranular micro cracking, there is a slight decrease of microhardness and Young's modulus values. Yet, fracture toughness as high as 11.2±1.1 MPa m^1/2 is obtained by the indentation technique. The results indicate that with optimized sintering parameters, a combination of PS and SPS is effective in preparing high performance mullite/ZrO₂ composites from zircon/alumina mixtures at a relatively low reaction sintering temperature.     

Thermal conductivity improvement of copper–carbon fiber composite by addition of an insulator: calcium hydroxide

The effects of adding calcium hydroxide (Ca(OH)₂) to a copper–CF (30 %) composite (Cu–CF(30 %)) were studied. After sintering at 700 °C, precipitates of calcium oxide (CaO) were included in the copper matrix. When less than 10 % of Ca(OH)₂ was added, the thermal conductivity was similar to or higher than the reference composite Cu–CF(30 %). A thermal conductivity of 322 W m^?1 K^?1 was measured for the Cu–Ca(OH)₂(3 %)–CF(30 %) composite. The effects of heat treatment (400, 600, and 1000 °C during 24 h) on the composite Cu–Ca(OH)₂(3 %)–CF(30 %) were studied. At the lower annealing temperature, CaO inside the matrix migrated to the interface of the copper matrix and the CF. At 1000 °C, the formation of the interphase calcium carbide (CaC₂) at the interface of the copper and CFs was highlighted by TEM observations. Carbide formation at the interface led to a decrease in both thermal conductivity (around 270 W m^?1 K^?1) and the coefficient of thermal expansion (CTE (10.1 × 10^?6 K^?1)).     

Synthesis and thermal behaviour of potassium sialate geopolymers

The geopolymers potassium polysialate (K-PS) and potassium sialate disiloxo (K-PSDS) have been found to possess extremely good thermal stability. K-PS shows little sign of melting up to 1400 °C, its amorphous structure being replaced by the crystalline feldspars leucite and kalsilite at 1000 °C. ²⁷Al and ²⁹Si MAS NMR confirm the ease of this thermal reaction which involves only slight changes to the tetrahedral Al environment. Silica-rich K-PSDS becomes friable and porous at >1200 °C, with less complete crystallisation of K-feldspar, the formation of some cristobalite and the retention of a degree of amorphous geopolymer. ³⁹K NMR suggests that the charge-balancing alkali ions in the K-PS and K-PSDS geopolymer networks behave similarly to those of Na geopolymers, dehydrating on heating and moving into the feldspar lattice >1000 °C.     

Preparation of forsterite-based glass ceramics for LTCC from potassium feldspar

A forsterite-based glass ceramic material has been developed from potassium feldspar for low temperature co-fired ceramics (LTCC). The crystalline phases and microstructure of forsterite-based glass ceramics were investigated using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The results show only forsterite was formed in temperature range 900–1,050 °C, and sapphirine was formed in temperature range 1,080–1,100 °C. The glass compact could be well densified at 950 °C, and full densification samples were obtained in temperature range 1,000–1,050 °C. The physical properties including dielectric properties, bending strength and thermal expansion of the specimens were also evaluated. The dielectric constants are in the range 7.00–8.25 and dielectric loss is below 0.01 in the frequency range 1–10 MHz. The specimens obtained in temperature range 950–1,100 °C are of high bending strength (69–106 MPa). The linear coefficient of thermal expansion of the specimen sintered at 1,080 °C is 9.76 × 10^?6 K^?1. All of these qualify the forsterite-based glass ceramic for further investigation as a candidate suitable for applications in LTCC field.     

Effect of sintering temperature on the dielectric and varistor properties of SnO<Subscript>2</Subscript>–Zn<Subscript>2</Subscript>SnO<Subscript>4</Subscript> composite ceramics

The effect of sintering temperature from 1350 to 1450 °C on the dielectric and varistor properties of SnO₂–Zn₂SnO₄ composite ceramics has been systematically investigated. With the increasing of sintering temperature, the average grain size increased from about 1 to 5 μm and the breakdown electric field decreased from 117 to 3 V/mm. The relative dielectric constant increased with sintering temperature and it achieved the maximum of 1.2 × 10⁴ (40 Hz, 0 °C) at 1425 °C. With excessive increasing of sintering temperature, the relative dielectric constant decreased and the microstructure of the ceramic bulk became porous. In the spectra of imaginary part of the complex modulus, a peak was exhibited and the peak’s position shifted to high frequency with increasing testing or sintering temperature. The activation energy related to the peak was about 0.4 eV and this value was thought to be associated with the oxygen vacancies. Based on the sintering effect, the mechanism of oxygen vacancies in SnO₂–Zn₂SnO₄ composite ceramics was proposed and accordingly, the varistor and giant permittivity properties are well understood based on the grain boundary barrier model.     

Effect of nanosilica addition on the physicomechanical properties,pore morphology,and phase transformation of freeze cast hydroxyapatite scaffolds

Freeze casting technique is a simple and effective method for the fabrication of porous ceramic structures. The objective of this work is to study the production and characterization of hydroxyapatite/nanosilica (HA/nSiO₂) scaffolds fabricated through this method. In the experimental procedure, the solidified samples were prepared by slurries containing different concentration of HA and nSiO₂ followed by sintering procedure at 1200 and 1350 °C. The phase composition, microstructure, and compressive strength of the scaffolds were characterized by X-ray diffraction, scanning electron microscopy, and mechanical strength test. It was found that the porosity of the scaffolds was in the range of 30–86.5 % and the value of compressive strengths lied between 0.16 and 71.96 MPa which were influenced by nSiO₂ content, cooling rate, and sintering temperature. With respect to porosity, pore size, and compressive strength, the scaffolds with 5 % nSiO₂, the cooling rate of 1 °C/min and the sintering temperature of 1350 °C showed preferable results for bone tissue engineering applications.     

Influence of cryogenic thermal cycling treatment on the thermophysical properties of carbon/carbon composites between room temperature and 1900 °C

Influence of cryogenic thermal cycling treatment (from ?120 °C to 120 °C at 1.3 × 10^?3 Pa) on the thermophysical properties including thermal conductivity (TC), thermal diffusivity (TD), specific heat (SH) and coefficient of thermal expansion (CTE) ranging from room temperature to 1900 °C of carbon/carbon (C/C) composites in x-y and z directions were studied. Test results showed that fiber/matrix interfacial debonding, fiber pull-out and microcracks occurred after the cryogenic thermal treatment and they increased significantly with the cycle number increasing, while cycled more than 30 times, the space of microdefects reduced obviously due to the accumulation of cyclic thermal stress. TC, TD, SH and CTE of the cryogenic thermal cycling treated C/C composites were first decreased and then increased in both directions (x-y and z directions) with the increase of thermal cycles. A model regarding the heat conduction in cryogenic thermal cycling treated C/C composites was proposed.     

The effect of ZrO2 and TiO2 on solubility and strength of apatite–mullite glass–ceramics for dental applications

The effect of ZrO₂ and TiO₂ on the chemical and mechanical properties of apatite–mullite glass–ceramics was investigated after sample preparation according to the ISO (2768:2008) recommendations for dental ceramics. All materials were characterized using differential thermal analysis, X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. X-ray fluorescence spectroscopy was used to determine the concentrations of elements present in all materials produced. The chemical solubility test and the biaxial flexural strength (BFS) test were then carried out on all the samples. The best solubility value of 242 ± 61 μg/cm² was obtained when HG1T was heat-treated for 1 h at the glass transition temperature plus 20 °C (T_g + 20 °C) followed by 5 h at 1200 °C. The highest BFS value of 174 ± 38 MPa was achieved when HG1Z and HG1Z+T were heat-treated for 1 h at the T_g + 20 °C followed by 7 h at 1200 °C. The present study has demonstrated that the addition of TiO₂ to the reference composition showed promise in both the glass and heat-treated samples. However, ZrO₂ is an effective agent for developing the solubility or the mechanical properties of an apatite–mullite glass–ceramic separately but does not improve the solubility and the BFS simultaneously.     

Influence of calcination temperature of kaolin on the structure and properties of final geopolymer

In this paper, the structure of two types of metakaolins from kaolin calcined at 800 and 900 °C, respectively, and the obtained geopolymer were systematically characterized. It was found that calcination temperature had little effect on the environment of silicon atoms but had great effect on that of aluminum ones. ²⁷Al NMR analysis showed that tetrahedral aluminums in the metakaolin from kaolin calcined at 800 and 900 °C were in different environment, of the type AlQ₃(3Si) and AlQ₄(4Si), respectively, leading to different environment of aluminum atoms in the resulted geopolymer. Aluminum atoms in the geopolymer based on metakaolin from kaolin calcined at 800 °C were in the types of tetrahedral and octahedral, and silicon atoms were in the types of tetrahedral Q₄(3Al) together with a small amount of Q₄(0Al). However, geopolymer based on metakaolin from kaolin calcined at 900 °C consisted of Q₄(4Si) unit aluminum and Q₄(3Al) unit silicon. The results revealed that the calcination temperature had a great effect on environment of the aluminum atoms of the metakaolin, thus led to the different structure and properties including mechanical strength and thermal conductivity of the post obtained geopolymer.     

The thermal expansion coefficient as a key design parameter for thermoelectric materials and its relationship to processing-dependent bloating

The coefficient of thermal expansion (CTE) is a key design parameter for thermoelectric (TE) materials, especially in energy harvesting applications since stresses generated by CTE mismatch, thermal gradients, and thermal transients scale with the CTE of the TE material. For the PbTe–PbS-based TE material (Pb_0.95Sn_0.05Te)_0.92(PbS)_0.08—0.055 % PbI₂ over the temperature ranges of 293–543 and 293–773 K, a CTE, α_avg, of 21.4 ± 0.3 × 10^?6 K^?1 was measured using (1) dilatometry and (2) high-temperature X-ray diffraction (HT-XRD) for powder and bulk specimens. The CTE values measured via dilatometry and HT-XRD are similar to the literature values for other Pb-based chalcogenides. However, the processing technique was found to impact the thermal expansion such that bloating (which leads to a hysteresis in thermal expansion) occurred for hot pressed billets heated to temperatures >603 K while specimens fabricated by pulsed electric current sintering and as-cast specimens did not show a bloating-modified thermal expansion even for temperatures up to 663 K. The relationship of bloating to the processing techniques is discussed, along with a possible mechanism for inhibiting bloating in powder processed specimens.     

Effect of spark plasma sintering temperature on the phase equilibria and dielectric properties of BaTi2O5 ceramics

The effect of sintering temperature (ranging from 1055 to 1200 °C) on the phase ingredient and dielectric property of the nominal BaTi₂O₅ ceramics (starting with the Ba/Ti of 1:2) fabricated by a spark plasma sintering method were systematically studied. At the first stage, BaTi₂O₅ component was enhanced in the sintering temperature range of 1055–1120 °C; it turned out to be the dominant phase. For these BaTi₂O₅ phase dominated ceramics, the Curie temperature T _c rised on increasing the sintering temperature and saturated around 440 °C with the maximum dielectric constant of 500. Further increasing the sintering temperature, the decomposition of the obtained BaTi₂O₅ into BaTiO₃ extensively happened; the ceramics turned to be the BaTi₂O₅ and BaTiO₃ coexisting state. These ceramics can be characterized by two dielectric anomalies. One at ~420 °C stood for the phase transition of BaTi₂O₅ while the other at ~150 °C stood for the transition of BaTiO₃, which is exceptionally high as the normal BaTiO₃ ceramics. Further increasing the sintering temperature (until 1200 °C) would dramatically enhance the BaTiO₃ phase; the ceramics showed T _c at 130 °C with the maximum dielectric constant of 1800.     

The effect of phase transformation on the thermal expansion property in Al/ZrW2O8 composites

This article studied the effect of phase transformation on the thermal expansion property in Al/ZrW₂O₈ composites. The Al/ZrW₂O₈ composites of low-thermal expansion were fabricated by a squeeze casting method. The coefficient of thermal expansion (CTE) of as-made composites was discovered sharply increased at around 130 °C. The X-ray diffraction (XRD) spectra showed the existence of high-pressure γ-phase in the as-made composites. This high-pressure γ-phase was considered to be induced by the compressive residual stress originated from the thermal mismatch between Al matrix and ZrW₂O₈ particles. The in situ high-temperature XRD and the differential scanning calorimetry technique were used to study this thermally expanded abruption phenomenon. It was found that the phase transformation from high-pressure γ-phase to the low-pressure phases (α/β phase) in the composites should be responsible for fluctuation in the CTE of composites. Furthermore, using a proper heat treatment to eliminate the high-pressure phase in the composite, the Al/ZrW₂O₈ composites of low and uniform CTE (from 20 to 200 °C) could be achieved. And when temperature increased again, the thermal mismatch stresses between the metal matrix and ceramic particles in the composite were not large enough to re-induce the α-γ transformation.