Thick-film PTC thermistors and LTCC structures: The dependence of the electrical and microstructural characteristics on the firing temperature

The electrical and microstructural characteristics of 1 kΩ/sq thick-film thermistors with high positive temperature coefficients of resistivity, i.e., PTC 5093 (Du Pont) fired either on “green” LTCC (low-temperature co-fired ceramics) substrates or buried within LTCC structures, were evaluated. The thermistors were fired at different temperatures to study the influence of firing temperature on the electrical characteristics. The noise indices of the surface resistors fired at temperatures between 850 °C and 950 °C were very low, around −30 dB. The TCRs of the evaluated PTC thermistors were over 3000 × 10⁻⁶/K. The dependence of the resistivity on the temperature between −25 °C and 125 °C was linear, with the values of R² being better than 0.9999, regardless of the processing conditions. These results show that PTC thermistors co-fired on LTCC substrates can be used for temperature sensors in MCM-Cs as well as in MEMS structures. However, when the thermistors were buried in the LTCC substrates, the LTCC structures delaminated during firing and blisters formed, leading to high sheet resistivities and high noise indices. This delamination is attributed to the different sintering rates of the PTC and LTCC materials as well as to the expansion of the air bubbles captured in the viscous glass of the PTC material.     

Structural study of mullite based ceramics derived from a mica-rich kaolin waste

Mullite-based ceramics have been synthesized by reactive sintering of a mixture containing kaolin and a mica-rich kaolin waste. Samples fired in the temperature range from 1300 to 1500 °C were characterized by X-ray diffraction (XRD). The quantitative phase analysis and unit cell parameters of the mullite were determined by Rietveld refinement analysis of the XRD data. Mullite-based ceramics with 1.2 wt% quartz, 56.3 wt% glass (amorphous phase), 2.64 g/cm³ of apparent density, and 35±1.2 MPa of flexural strength were obtained after firing at 1500 °C. A liquid phase sintering mechanism activated by a total mica content of 13.3 wt% allowed to increase the mullite content to 47.6 wt% (2.3 wt% quartz and 50.1 wt% glass phase) and improve the flexural strength (70±3.9 MPa) after firing at 1400 °C.     

Microstructure and electrical conductivity of fast fired Sr- and Mg-doped lanthanum gallate

La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ solid electrolytes were consolidated by fast firing aiming to investigate the effects of the sintering method on densification, microstructure and ionic conductivity. Powder mixtures were prepared by solid state reaction at 1250 and 1350 °C for 12 h, and fast fired at 1450 and 1500 °C temperatures for 5 and 10 min. The content of impurity phases was found to be quite low with this sintering method. Relatively high density (>90% of the theoretical value) and low porosity (<1.5%) were readily obtained for powder mixtures calcined at 1250 °C. The activation energy for conduction was approximately 1 eV. Specimens fast fired at 1450 °C for 10 min with a mean grain size of 2.26 µm reached the highest value of total ionic conductivity, 22 mS cm⁻¹, at 600 °C.     

Combustion possibility of low rank Russian peat as a blended fuel of pulverized coal fired power plant

Combustion possibility of Russian peat as a blended fuel of commercial thermal power plant was investigated by thermogravimetric analysis (TGA), drop tube furnace (DTF) and ignition temperature (IT) tester. TGA results showed that the linear regression for the Arrhenius plot to the experimental data is very good, and activation energies for overall combustion of bituminous C & A (Coal & Allied, Australia coal name) and peat are 66.83 and 25.05 kJ/mol, respectively. It was derived that activation energies of 30%, and 50% blends produced through mixing of peat of 30%, and 50% to Design C & A (design criteria coal of 500 M coal fired power plant) are 27.76 and 24.22 kJ/mol in reciprocal proportion to blending ratio. The conversion behavior of the samples observed in DTF was similar to that reflected in TGA. DTF studies showed that the combustion of all blends was also completed at residence time of around 1 s, set temperature range of 1200 °C similar to commercial coal fired plant. Although the peat has the highest conversion than the blends, it was not appropriate as the single pulverized fuel of coal fired plant because its initial deformation (IDT) and ignition temperatures of about 1160 and 240 °C, respectively, were too low to cause the slagging in boiler, and the firing at pulverizer. The IDT and FT of the blends ashes of peat of less than 30% was about 1260 and 1410 °C, respectively, and was not expected to be associated with slagging and fouling in pulverized coal fired systems. The liability of spontaneous combustion of coal samples was increased with increasing the moisture and volatile contents whereas the same of peat was the lowest due to the high volatile content and specific heat (Cp). It was therefore proposed that the combustion of blends of peat with less than 30% was the most appropriate for the prevention of slagging and spontaneous combustion at the pulverized coal fired boilers, and has the excellent combustion efficiency.     

Evolution of microstructure,mineralogy and properties during firing of clay-based ceramics with borates

The effect of sieve boron waste (SBW) and borate-containing Evansite^® on the thermal behaviour, microstructure and properties of a clay-based body was investigated. SBW and Evansite^® were introduced in quantities that correspond to 0.6 wt.% B₂O₃ addition in the dry body for both cases. Cylindrical samples were extruded and fired at three different peak temperatures 900, 950 and 1000 °C. The reference body, R, and the body with SBW, RB, demonstrate a comparable dilatometric behaviour whereas the densification for the body with Evansite^®, RV, initiated 50 °C approximately lower and resulted in higher firing shrinkage. After firing at 900 °C, the physico-mechanical properties as well as the microstructure are comparable. Nonetheless, åkermanite is formed in RB, whereas hercynite and mullite, the latter at 1000 °C, are formed in RV. For firing at 1000 °C, the role of borates is intensified. Water absorption is reduced by 16.1% and 18.0%, whereas bending strength increased by 27.6% and 40.8%, for RB and RV respectively, compared to the reference formulation. This is attributed predominantly to the enhanced vitrification that took place in the boron-containing bodies.     

The effect of boron waste in phase and microstructural development of a terracotta body during firing

In the present study, a dewatering sieve waste (TSW) of Etibor Kırka Borax company (Turkey) was employed in different amounts in order to develop an experimental terracotta floor tile body composition in combination with a feldspathic waste provided from a local sanitaryware plant and a ball clay. Several formulations were prepared and shaped by dry pressing under laboratory conditions. The obtained samples were fired at selected peak temperatures (1050, 1100 and 1150 °C) to establish their optimum firing temperatures. Some technological properties of the resultant products, namely linear firing shrinkage, water absorption and breaking strength were determined as a function of increasing TSW content in place of the sanitaryware waste at these temperatures. The phase content of the starting raw materials and that of the fired compositions was determined by XRD. The relevant polished surfaces of selected fired samples were also examined using SEM. According to the results, increased presence of TSW compared to the standard mixture of clay and the sanitaryware waste, as a co-fluxing material, in the experimental terracotta body considerably accelerated the vitrification process. The overall results indicated a prospect for using the TSW as a raw material in mixtures with both clay and sanitaryware waste for the production of a terracotta floor tile body.     

Different shades of red: The complexity of mineralogical and physico-chemical factors influencing the colour of ceramics

Different techniques (X-ray diffraction, field emission scanning electron microscope, colorimetry, visible-near infrared reflectance spectroscopy) were carried out to investigate the cause of colour changes of traditional ceramic materials. Two clayey materials of different composition, collected in the Bay of Naples, were fired in oxidising atmosphere at different temperatures resulting in different shades of red colour.Hematite is responsible of the reddish hue of ceramics and its nucleation is strictly related to firing temperature and chemical composition of the raw materials. A low CaO concentration allowed hematite to form in higher amounts providing a more intense reddish hue at high firing temperatures (over 950 °C). At the highest temperature (1100 °C) all samples showed darker colour due to increased size of iron oxide particles. Black core developed in Ca-rich ceramics fired at low temperatures as the short time of firing is insufficient to complete iron oxidation within the matrix, except in those containing high temper amounts. Indeed, microstructural modification occurs due to the presence of discontinuities among temper grains and matrix, which improves the circulation of oxygen in the core of ceramics.     

Utilization of recycled paper processing residues and clay of different sources for the production of porous anorthite ceramics

Production of porous anorthite ceramics from mixtures of paper processing residues and three different clays are investigated. Suitability of three different clays such as enriched clay, commercial clay and fireclay for manufacturing of anorthite based lightweight refractory bricks was studied. Porous character to the ceramic was provided by addition of paper processing residues (PPR). Samples with 30–40 wt% PPR fired at 1200–1400 °C contained anorthite (CaO·Al₂O₃·2SiO₂) as major phase and some minor secondary phases such as mullite (3Al₂O₃·2SiO₂) or gehlenite (2CaO·Al₂O₃·SiO₂), depending on the calcite to clay ratio. Anorthite formation for all clay types was quite successful in samples with 30–40 wt% of paper residues fired at 1300 °C. A higher firing temperature of 1400 °C was needed for the fireclay added samples to produce a well sintered product with large pores. Gehlenite phase occurred mostly at lower temperatures and in samples containing higher amount of calcium (50 wt% PPR). Compressive strength of compacted and fired pellets consisting of mainly anorthite ranged from 8 to 43 MPa.     

Sintering of zirconia ceramics by intense high-energy electron beam

A comparative analysis of the efficiency of zirconia ceramics sintering by thermal method and high-energy electron beam sintering was performed for compacts prepared from commercial TZ-3Y-E grade powder. The electron energy was 1.4 MeV. The samples were sintered in the temperature range of 1200–1400 °C. Sintering of zirconia ceramics by high-energy accelerated electron beam is shown to reduce the firing temperature by about 200 °C compared to that in conventional heating technique. Ceramics sintered by accelerated electron beam at 1200 °C is of high density, microhardness and smaller grain size compared to that produced by thermal firing at 1400 °C. Electron beam sintering at higher temperature causes deterioration of ceramics properties due to radiation-induced acceleration of high-temperature recrystallization at higher temperatures.     

Low-temperature sintering behavior of the nano-sized AlN powder achieved by super-fine grinding mill with Y2O3 and CaO additives

For low-temperature sintering, mixtures of AlN powder doped with 3.53 mass% Y₂O₃ and 0–2.0 mass% CaO as sintering additives were pulverized and dispersed in a vertical super-fine grinding mill with very small ZrO₂ beads. The particle sizes achieved ranged between 50 and 100 nm after grinding for 90 min. The mixtures were then fired at 1000–1500 °C for 0–6 h under nitrogen gas pressure of 0.1 MPa. All nano-sized powders showed pronounced densification from 1300 °C as revealed by shrinkage measurement. The larger amounts of sintering additives enhanced AlN sintering at lower temperatures. Densified AlN ceramics with very fine and uniform grains of 0.3–0.4 μm were obtained at a firing temperature of 1500 °C for 6 h.     

The influence of nano-silica and zircon additions on the sintering and mechanical properties of in situ formed forsterite

The influence of nano-silica and zircon additions on the sintering and mechanical properties of in situ formed forsterite fired at 1550 °C for 2 h was investigated. The results indicated that, nano-silica improved in situ formed forsterite at the firing temperature, while zircon additions enhanced the sintering of the investigated samples. XRD analysis and SEM examination observed a good crystallinity of in situ formed forsterite with nano-silica and/or zircon additions. Densification parameter (BD ∼3.22 g/cm³ and AP ∼5.82%), cold crushing strength (CCS ∼285 MPa) and micro-hardness (Hv ∼660) were enhanced with zircon additions.     

Low temperature preparation and machinability of porous ceramics from talc and foamed glass particles

Porous ceramics were prepared by firing mixtures of talc (Mg₃Si₄O₁₀(OH)₂) and foamed glass particles (ceramic balloons, CB) with and without LiCl as a sintering acid. The mixing ratios of the starting materials were talc:CB = 7:3, 8:2, 9:1 and 10:0, with additions of LiCl of 0, 2 and 5 mass%. The mixtures were formed into pellets and fired at 600–1000 °C. The pellets without LiCl showed very poor strength even when fired at 1000 °C but those containing LiCl were much stronger, even when fired at only 600 °C. The crystalline phases in these samples changed to enstatite (MgSiO₃) at ≥ 700 °C by decomposition of the talc under the fluxing action of the LiCl. The resulting samples were machinable and easily cut and drilled. The cutting rate decreased with increasing bending strength, for example, from 105 mm²/s and 6.3 MPa to 50 mm²/s and 16.3 MPa, respectively. The drilling rate of the present sample was found to be only slightly less than Teflon (polytetrafluoroethylene, PTFE) but much faster than graphite, glass ceramics, etc.     

Study of the Drying Kinetics of Green Bell Pepper and Chemical Characterization

The present work aimed, on one hand, the study of the drying of green peppers, in terms of drying kinetics evaluated at 30°C, 40°C, 50°C, 60°C and 70°C, having the experimental data been fitted to different empirical kinetic models from literature. This kinetic study was then complemented with the modelling in terms of Fick's diffusion equation.On the other hand, the chemical characterization in fresh and after drying at the lowest and highest temperatures was analysed, for evaluation of the effect of drying and drying temperature on the chemical composition of the product. In this way, the analyses made were: moisture content, sugar content, proteins, ash, fat, fibre and acidity.From the results obtained, it was concluded that the empirical models that best describe the dehydration kinetics were the Page and Newton models. From the experimental data was possible to estimate the diffusivities, which range between 9.0 × 10⁻¹⁰ at 30°C and 8.0 × 10⁻⁹ m² s⁻¹ at 70°C.Moreover, it was verified that drying influences the chemical composition of the peppers, but, conversely, the influence of the drying temperature was not very significant.     

Development of fireclay aluminosilicate refractory from lithomargic clay deposits

Lithomargic clay until now has not been utilised to produce refractory bodies due to its low plasticity. In this work, the development and evaluation of fireclay refractory material produced from lithomargic clay deposit has been studied by addition of clay binder. Three formulations were prepared by mixing, semi-dry moulding, drying and firing at temperatures ranging from 1200 to 1400 °C. The fired samples were investigated to determine their physical properties such as bulk density, apparent porosity, linear firing shrinkage, and cold crushing strength. The chemical and mineralogical compositions were also determined. The results show that the linear firing shrinkage values were within limits acceptable for refractory clays. The cold crushing strength increases as temperature increased to 1400 °C. Cold crushing strength increased with increasing binder content. The increase of the highly refractory phases (cristobalite and mullite) and the densification of the bricks due to the presence of fluxing agents were responsible for the high cold crushing strength values. The investigated properties indicate that lithomargic clay underlying bauxite deposits could be used to produce fire clay aluminosilicate refractories.     

Nitridation of silicon powder studied by XRD, 29Si MAS NMR and surface analysis techniques

The nitridation of elemental silicon powder at 900–1475 °C was studied by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), XRD, thermal analysis and ²⁹Si MAS NMR. An initial mass gain of about 12% at 1250–1300 °C corresponds to the formation of a product layer about 0·2 μm thick (assuming spherical particles). XPS and XAES show that in this temperature range, the surface atomic ratio of N/Si increases and the ratio O/Si decreases as the surface layer is converted to Si₂N₂O. XRD shows that above 1300 °C the Si is rapidly converted to a mixture of α- and β-Si₃N₄, the latter predominating >1400 °C. In this temperature range there are only slight changes in the composition of the surface material, which at the higher temperatures regains a small amount of an oxidised surface layer. By contrast, in the interval 1400–1475 °C, the ²⁹Si MAS NMR chemical shift of the elemental Si changes progressively from about −80 ppm to −70 ppm, in tandem with the growth of the Si₃N₄ resonance at about −48 ppm. Possible reasons for this previously unreported change in the Si chemical shift are discussed. ©     

Accurate mass determination of melamine–formaldehyde synthetic polymers after separation on preparative HPLC

Separation of 57 species from solutions of commercial melamines and analysis by high resolution mass spectrometry using electrospray ionisation and atmospheric pressure chemical ionisation is reported. Structures for melamine-based crosslinked polymers were also confirmed with two orthogonal techniques for characterisation of polymers: Fourier Transform Infrared Spectrometry (FTIR) and Nuclear Magnetic Resonance spectroscopy (NMR), respectively.Accurate mass measurements of melamine–formaldehyde resin structures were achieved in the ppm range, using an internal calibration standard reserpine. Results were within 5 ppm of calculated formulae. Relative isotopic abundances of [M+H]⁺ ions were evaluated for different fractions. Results were independent of the mobile phase used. NMR studies also confirmed structures of fractions and their thermal stability up to 70 °C. Differences in NMR spectra were explained by change of conformation and it was confirmed that this is a reversible process by reheating samples. Stability at 70 °C was confirmed by MS, since when the temperature of the ion transfer capillary in MS was set above this temperature decomposition occurred.     

Mullite development on firing in porcelain stoneware bodies

Microstructural evolution on heating was investigated in a reference industrial composition (50% kaolinitic clay, 40% feldspar and 10% quartz) of porcelain stoneware, fast fired at different temperatures (500–1400 °C). The evolution of mullite crystals, regarding shape and size progress, was examined by scanning electron microscopy (SEM). The proportion of Type I mullite crystals decreases with firing temperature and simultaneously, the size of crystals increases, reaching the maximum value of aspect ratio (3:1) at 1400 °C. Type II and Type III secondary mullite needles increase with temperature in both number and length, which leads to an increase in the aspect ratio from 5:1 to ～20:1 in Type II crystals and from ～33:1 to 50:1 in Type III mullite needles. Finally, clusters of Type III mullite fibres are observed in porcelain stoneware samples fast fired in the 1250–1280 °C interval.     

Convective drying of onion: Kinetics and nutritional evaluation

The present work aimed, on one hand, to study of the drying of onions in terms of drying kinetics, which was evaluated at 30 °C, 50 °C and 60 °C. The experimental data was fitted to different empirical kinetic models from the literature, and this kinetic study was then complemented with the modelling if terms of Fick's diffusion equation, for estimation of the diffusion coefficients. On the other hand, the chemical characterization in fresh and dried onions at different temperatures (varying from 30 °C to 70 °C) was analysed, to evaluate the effect of drying and drying temperature on the chemical composition of the product. In this way, the analyses of moisture content, sugar content, crude protein, ash, fat, crude fibre, acidity and vitamin C were made and reported in this paper.From the results obtained it was verified that some chemical components of the onions are not affected by drying (ash, fat, protein and fibre) whereas some others are considerably influenced by drying (sugars, acidity and vitamin C). The present work allowed concluding that the three empirical models tested (Newton, Modified Page and Logarithmic) all describe relatively well the dehydration kinetics at the three temperatures analysed. Moreover, from the experimental data it was possible to estimate the diffusivities, which range between 3.33 × 10^?09 m²/s at 30 °C and 8.55 × 10^?09 m²/s at 60 °C.     

Investigation of the electrical conductivity of sintered monoclinic zirconia (ZrO2)

High-density monoclinic ZrO₂ was manufactured through sintering at ~1200 °C by using nanosized powders. Then, the electrical conductivity was measured at a range of high temperatures (700–900 °C) by electrical impedance spectroscopy (EIS). For the as-sintered monoclinic ZrO₂, the measured electrical conductivity was 3.2×10⁻⁵ s/cm (for 80% TD) and 4.4×10⁻⁵ s/cm (for 89% TD) at 900 °C. After aging at 900 °C for 100 h, the electrical conductivity of the monoclinic ZrO₂ of 80%-TD decreased by more than 50%. However, after reheating at 1200 °C for 1 h, approximately 80% of the conductivity was recovered compared to the value of the as-sintered monoclinic ZrO₂. The pure monoclinic crystal structure was retained despite the aging and reheating treatment. Based on microstructural observations of the aged and reheated monoclinic ZrO₂, the changes in electrical conductivity after aging and reheating were explained by the formation and recovery of micro-cracks, respectively.     

Recycled granulate obtained from waste alumina-rich refractory powder by the cold bonding process

Within the scope of the present study the cold-bonding process was used for the recycling of waste filter powder which was mixed with two different binders in different concentrations; alumino-silicate cement and potassium water glass, and combinations of these two materials, and hardened at room temperature. Selected samples were also fired at 1200 °C. Tests to determine tensile and compressive strength, density and porosity, as well as dilatometry and SEM analyses, were performed. As expected, compressive strength increased as a function of the concentration of the potassium water glass. When combinations of both binders were used, compressive strengths were higher, but a significant increase in strength was also achieved by firing the samples. The compressive strengths of the non-fired samples were in the range from 0.8 to 2.4 MPa, whereas after firing strengths of up to 36 MPa were obtained. During the firing density increased, and porosity was reduced, while the average pore size increased. The results of dilatometric analysis showed that the granulate produced with cement shrink upon firing up to 300 °C, but then start slowly to expand, whereas the granulate produced by water glass first expanded on firing up to 800 °C, and then began to shrink swiftly. In the case of combinations of the two binders, shrinkage as well as expansion on firing was less pronounced. Selected granulate prepared using potassium water glass were also tested in a refractory concrete matrix in order to verify their usability.