Sintering and characterisation of ceramics containing paper sludge, glass cullet and different types of clayey materials

The present paper reports on the sintering behaviour of several ceramics prepared using a previously selected mixture of incinerated paper mill sludge and glass cullet in the ratio 60/40 which was blended with 10, 20, 30 and 40 wt.% of three different natural materials. The three natural products were: a red quartzitic clay, a yellow quartzitic clay and a kaolin. All mixtures were blended by attrition milling and dried; powders were sieved, pressed into specimens and fired for 1 h at temperatures ranging from 1040 to 1140 °C. The resulting materials were characterized by water absorption, shrinkage, crystallographic composition, microstructure and physico-mechanical properties. It was observed that materials containing kaolin display the best overall behaviour independently of the quantity of kaolin introduced. Conversely the optimal sintering temperature, and consequently the best properties of the materials prepared using red or yellow clay were measured on products fired at temperatures above 1080 °C; materials and temperatures are affected by the amount of clay added.     

Characterization of flat ceramic membrane supports prepared with kaolin-phosphoric acid-starch

The purpose of this work is the development of microporous ceramic materials based on kaolin for a filtration process. Flat ceramic membrane supports were prepared from the mixtures of kaolin, phosphoric acid and starch. Porosity, permeability and mechanical properties of those supports were studied as functions of the amount of phosphoric acid, the sintering temperature and the compaction pressure. The rupture strength and the permeability of the ceramic membrane, increase with the content of phosphoric acid until 5 mass%. The porosity decreases with both the sintering temperature rise and the addition of phosphoric acid. The addition of 5 to 10 mass% of phosphoric acid and 10 mass% of starch to the kaolin supports sintered at 1100 °C for 2 h leads to a satisfied permeability and mechanical proprieties in the filtration application. The elaborated support was characterized using two analytical methods: DRX, ²⁷Al and ³¹P MAS-NMR. The obtained analytical data indicate the presence of an AlPO₄ high temperature phase at 1100 °C.     

Effect of ZnO–WO3 additives on sintering behavior and microwave dielectric properties of 0.95MgTiO3–0.05CaTiO3 ceramics

The effect of WO₃ addition on the phase formation, the microstructures and the microwave dielectric properties of 1 wt% ZnO doped 0.95MgTiO₃–0.05CaTiO₃ ceramics system were investigated. Formation of second phase MgTi₂O₅ could be effectively restrained through the addition of WO₃, but should be in right amount. WO₃ as additives could not only effectively lower the sintering temperature of the ceramics to 1310 °C, but also promote the densification. A dielectric constant ε_r of 20.02, a Q×f value of 62,000 (at 7 GHz), and a τ_f value of −5.1 ppm/°C were obtained for 1 wt% ZnO doped 0.95MgTiO₃–0.05CaTiO₃ ceramics with 0.5 wt% WO₃ addition sintered at 1310 °C.     

Densification and characterization of SiO2B2O3CaOMgO glass/Al2O3 composites for LTCC application

A glass/ceramic composite using lead-free low melting glass (SiO₂B₂O₃CaOMgO glass) with Al₂O₃ fillers was investigated. X-ray diffraction analysis revealed that the anorthite and cordierite phase appeared in the sintered composites. The dilatometric analysis showed that the onset of shrinkage took place at ∼624 °C for all the samples and the onset temperature was independent on the content of glass. The low melting glass significantly promoted densification of the composites and lowered the sintering temperature to ∼875 °C. The addition of 50 wt% glass sintered at 875 °C showed ε_r of 7.3, tan δ of 1.15×10⁻³, TEC of 5.41 ppm/°C, thermal conductivity of 3.56 W/m °C, and flexural strength of 184 MPa. The results showed that the SiO₂B₂O₃CaOMgO glass/Al₂O₃ composites were strong potential candidates for low temperature cofired ceramic substrate applications.     

Electrical resistivity of porcelain bodies with natural zeolite addition

In this study, the effect of natural zeolite addition on the electrical properties of porcelain bodies was investigated. Clinoptilolite, which is a type of natural zeolite, was added partially or fully in replacement of quartz in selected electro-porcelain compositions. Samples were fired in an electric furnace with a heating rate of 10 °C/min at 1200 and 1250 °C with a period of 60 min. The electrical resistance measurements of samples were performed at 50, 200, 400 and 600 °C. It was shown that the resistivity of samples increased at 50 °C temperature after zeolite addition, while it was decreasing after zeolite addition at higher temperatures. At the same time, it was recognized that the resistivity of samples depends on sintering temperature. The activation energy of electrical resistivity of samples was found to be in the range of 0.79–0.87 eV.     

Pressureless sintering of silicon carbide ceramics containing zirconium diboride

SiC-5 wt.% ZrB₂ composite ceramics with 10 wt.% Al₂O₃ and Y₂O₃ as sintering aids were prepared by presureless liquid-phase sintering at temperature ranging from 1850 to 1950 °C. The effect of sintering temperature on phase composition, sintering behavior, microstructure and mechanical properties of SiC/ZrB₂ ceramic was investigated. Main phases of SiC/ZrB₂ composite ceramics are all 6H-SiC, 4H-SiC, ZrB₂ and YAG. The grain size, densification and mechanical properties of the composite ceramic all increase with the increase of sintering temperatures. The values of flexural strength, hardness and fracture toughness were 565.70 MPa, 19.94 GPa and 6.68 MPa m^1/2 at 1950 °C, respectively. The addition of ZrB₂ proves to enhance the properties of SiC ceramic by crack deflection and bridging.     

Effect of SiC content,additives and process parameters on densification and structure–property relations of pressureless sintered ZrB2–SiC composites

The effect of SiC content, additives, and process parameters on densification and structure–property relations of pressureless sintered ZrB₂–(10–40 vol%) SiC particulate composites have been studied. The ZrB₂–SiC composite powders mixed by ball-milling with 1.2 wt% C (added as phenolic resin) and 3 wt% B₄C have been uniaxially cold-compacted and sintered in argon environment at 1950–2050 °C for 2 h, or at 2000 °C for durations between 1/2 and 3 h. The amount of densification is found to increase with sintering duration, and by prior holding at 1250 and 1600 °C for reduction of oxide impurities (ZrO₂, B₂O₃ and SiO₂) on powder particle surfaces by the aforementioned additives. Presence of SiC with average size smaller than that of ZrB₂ appears to aid in densification by enhancing green density, increasing WC content by erosion of milling media, and inhibiting matrix grain growth. Both SiC and WC appear to aid in reduction of oxide impurities. Furthermore, the impurities enriched in W, Fe and Co obtained from milling media are found to be segregated at ZrB₂ grain boundaries, and appear to assist in densification by forming liquid phase, which completely wets the ZrB₂ grains. Hardness increases with SiC content or with sintering duration till 1 h, but decreases for periods ≥2 h due to grain growth. The experimentally measured elastic moduli approaches corresponding theoretically predicted values with increasing SiC content due to reduction in porosity.     

The phase structure and electric properties of low-temperature sintered (K,Na)NbO3-based piezoceramics modified by CuO

0.25 wt% CuO-doped (Li,K,Na)(Nb,Ta)O₃–AgSbO₃ lead-free piezoceramics with pure perovskite structure were successfully prepared at a sintering temperature below 1000 °C. The sintering temperature of KNN-based piezoceramics was effectively reduced by about 100 °C due to the enhanced densification process induced by the addition of CuO. Besides, the acceptable sintering temperature window was broadened by the addition of CuO. It is found that the CuO-doped samples show slightly higher tetragonal–orthorhombic phase transition point (T_T−O) but a lower Curie point (T_c), compared to undoped ones. The KNN-based piezoceramics became “hard” as CuO was added, supported by an increase of Q_m. Fairly good electrical properties of d₃₃^*=383 pm/V, ε_r=860, Q_m=188 and T_c=215 °C could be obtained in dense CuO-modified KNN-based piezoceramics sintered at 970 °C, demonstrating promising potential in practical applications.     

Effect of Bi2O3 additives on sintering and microwave dielectric behavior of La(Mg0.5Ti0.5)O3 ceramics

Bi₂O₃ was selected as liquid phase sintering aid to lower the sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics. The sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics is generally high, about 1600 °C. However, the sintering temperature was significantly lowered about 275 °C from 1600 °C to 1325 °C by incorporating in 15 mol% Bi₂O₃ and revealed the optimum microwave dielectric properties of dielectric constant (?_r) value of 40.1, a quality factor (Q × f) value of 60,231 GHz, and the temperature coefficient (τ_f) value of 70.1 ppm/°C. During all addition ranges, the relative dielectric constants (?_r) were different and ranged from 32.0 to 41.9, the quality factors (Q × f) were distributed in the range of 928–60,231 GHz, and the temperature coefficient (τ_f) varies from 0.3 ppm/°C to 70.3 ppm/°C. Noticeably, a nearly zero τ_f can be found for doping 5 mol% Bi₂O₃ sintering at 1325 °C. It implies that nearly zero τ_f can be achieved by appropriately adjusting the amount of Bi₂O₃ additions and sintering temperature for La(Mg_0.5Ti_0.5)O₃ ceramics.     

Effect of sintering temperature electrical properties of ZNR doped with Pr–Co–Cr–La

The microstructure, electrical properties, and dielectric characteristics of the ZNR (zinc oxide-based nonlinear resistors), which are composed of zinc oxide-based ceramics doped with Pr–Co–Cr–La, were investigated at different sintering temperatures (1240, 1245, 1250, 1255, 1260, and 1300 °C). The increase of sintering temperature led to more densified ceramics, whereas it decreased the nonlinear properties and breakdown voltage. The highest nonlinearity was obtained from 1240 °C, with 79.3 in nonlinear coefficient and 0.3 μA in leakage current. As the sintering temperature increased, the donor density increased from 0.90 × 10¹⁸ to 2.59 × 10¹⁸/cm³, and the barrier height decreased from 1.90 to 0.67 eV, and the dielectric dissipation factor increased from 0.0874 to 0.2839.     

High piezoelectric properties of BaTiO3-xLiF ceramics sintered at low temperatures

BaTiO₃-xLiF ceramics were prepared by a conventional sintering method using BaTiO₃ powder about 100 nm in diameter. The effects of LiF content (x) and sintering temperature on density, crystalline structure and electrical properties were investigated. A phase transition from tetragonal to orthorhombic symmetry appeared as sintering temperatures were raised from 1100 °C to 1200 °C or as LiF was added from 0 mol% to 3 mol%. BaTiO₃-6 mol% LiF ceramic sintered at 1000 °C exhibited a high relative density of 95.5%, which was comparable to that for pure BaTiO₃ sintered at 1250 °C. BaTiO₃-4 mol% LiF ceramic sintered at 1100 °C exhibited excellent properties with a piezoelectric constant d₃₃ = 270 pC/N and a planar electromechanical coupling coefficient k_p = 45%, because it is close to the phase transition point in addition to high density.     

The effects of rare-earth oxide additives on the densification of pressureless sintering B4C ceramics

Boron carbide ceramics used as neutron absorbing materials in fast breeder reactor were fabricated with boron carbide powders and different rare-earth oxide additives by pressureless sintering. The effects of rare-earth oxide as well as phenolic resin on densities and mechanical properties of the composites were studied. The addition of Dy₂O₃, Eu₂O₃, and Sm₂O₃ was found to be beneficial in the densification of B₄C ceramics. B₄C with 4 wt% rare-earth oxide and 18 wt% phenolic resin, exhibiting bulk density of 90–96% T.D., flexural strength of 276–358 MPa, could be prepared by pressureless sintering at 1960–2080 °C, which are capable of meeting the requirement of fast breeder reactor.     

Low temperature sintering of CCTO using P2O5 as a sintering aid

Recent work on CCTO is directed towards decreasing its dissipation factor and further raising its dielectric constant by using different dopants. Also attempts have been made to lower its sintering temperature by adding different sintering aids so as to save energy and use low-cost electrodes (Ag–Pd or base metal) for making multilayer capacitors. Normally, CCTO needs a processing temperature of 1100 °C and above for densification. We report the formation of dense CCTO ceramics at a temperature as low as 1000 °C by adding P₂O₅ as a sintering aid. The samples showed dielectric constant value as high as 40,000, though the dissipation factor values remained high like those reported for pure CCTO.     

Composite wasteform based on SiO2–PbO–CaO–ZrO2–TiO2–(B2O3–K2O) parent glass with zircon as the second component

Many different types of glass and ceramic wasteforms have been investigated for nuclear waste immobilization. This study deals with synthesizing composite wasteforms based on a parent glass belonging to the SiO₂–PbO–CaO–ZrO₂–TiO₂–(B₂O₃–K₂O) system with the use of zircon as a second component. The fabrication involves powder mixing, pressing and pressureless sintering. The processing conditions were investigated so as to achieve the highest density and the best sintering temperature for different amounts of zircon, i.e. 5, 10 and 15 wt%. The sintered products were studied by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM); as well as ICP-MAS for leaching experiments. The most promising composite containing zirkelite and titanite crystals in a lead-rich glassy matrix was obtained at 700 °C for 10 wt% zircon.     

Optimisation of sintered glass–ceramics from an industrial waste glass

Industrial plasma melting of municipal solid waste (MSW) incinerator fly ashes leads to a glass that may be easily crystallised to gehlenite glass–ceramics, by the sintering of fine glass powders. However, since the glass composition is not optimised for glass–ceramic manufacturing, the viscous flow is much hindered by a very significant surface crystallisation and dense glass–ceramics are feasible only by sintering above 1000 °C. This paper reports a new strategy for obtaining dense and strong glass–ceramics at 950 °C, with a holding time of only 30 min, consisting of the mixing of waste glass with a secondary recycled glass, such as soda-lime–silica glass or borosilicate glass. For an optimum balance between the two types of glass also the addition of kaolin clay, in order to favour the shaping, was found to be feasible. The approach had a positive effect, besides on the mechanical properties (e.g. bending strength exceeding 100 MPa), on the chemical stability.     

Effects of CaTiO3 addition on the densification and microwave dielectric properties of BiSbO4 ceramics

In this study, the effects of CaTiO₃ addition on the sintering characteristics and microwave dielectric properties of BiSbO₄ were investigated. Pure BiSbO₄ achieved a sintered density of 8.46 g/cm³ at 1100 °C. The value of sintered density decreased with increasing CaTiO₃, and sintering at a temperature higher than 1100 °C led to a large weight loss (>2 wt%) caused by the volatile nature of the compound. Samples either sintered above 1100 °C or with a CaTiO₃ content exceeding 3 wt% showed poor densification. SEM micrographs revealed microstructures with bimodal grain size distribution. The size of the smaller grains ranged from 0.5 to 1.2 μm and that of the larger grains between 3 and 7 μm. The microwave dielectric properties of the (1−x) BiSbO_4−x CaTiO₃ ceramics are dependent both on the x value and on the sintering temperature. The 99.0 wt% BiSbO₄–1.0 wt% CaTiO₃ ceramic sintered at 1100 °C reported overall microwave dielectric properties that can be summarized as ε_r≈21.8, Q×f≈61,150 GHz, and τ_f≈−40.1 ppm/°C, all superior to those of the BiSbO₄ ceramics sintered with other additives.     

Characterization of submicrometer-sized NiZn ferrite prepared by spark plasma sintering

High-density submicrometer-sized Ni_0.5Zn_0.5Fe₂O₄ ferrite ceramics were prepared by spark plasma sintering in conjunction with sufficient high energy ball milling. They were evaluated by different characterization techniques such as X-ray diffraction, scanning electron microscopy, and dielectric and magnetic measurements. All samples prepared at sintering temperatures ranging from 850 to 925 °C exhibit a single spinel phase and their relative densities and grain sizes range from 90% to 99% and ~100 nm to ~300 nm, respectively. The dielectric constant increases with decreasing grain size until ~250 nm, and then decreases dramatically with further decreasing grain size. The saturation magnetization increases continuously with increasing grain size/density but the magnetic coercivity decreases. The highest dielectric constant and saturation magnetization at room temperature are approximately 1.0×10⁵ and 84.4 emu/g, respectively, while the lowest magnetic coercivity is only around 15 Oe. These outstanding properties may be associated with high density and uniform microstructure created by spark plasma sintering. Therefore, the spark plasma sintering is a promising technique for fabricating high-quality NiZn ferrites with high saturation magnetization and low coercivity.     

Effect of YF3 on the phase stability and sinterability of hydroxyapatite—Partially stabilized zirconia composites

Pure hydroxyapatite (HA), HA and partially stabilized zirconia composites (PSZ) with YF₃ and HA–PSZ composite containing 5 wt% PSZ without YF₃ were sintered in air at 900 °C, 1100 °C and 1300 °C for 1 h. The reactions and transformation of the phases in the composites were determined by X-ray diffraction. All the composites with or without YF₃ showed desirable thermal stability below 1300 °C and besides various amounts of CaZrO₃, any amount of tri-calcium phosphate (TCP) was not observed. Above 1100 °C, composites with YF₃ showed higher thermal stability than the composites without YF₃. On the other hand, pure HA started to decompose and TCP was observed at 1300 °C. Composites with YF₃ showed improved thermal stability than the composite containing 5 wt% PSZ without YF₃ and pure HA at lower sintering temperatures such as 900 °C and 1100 °C. However, it was observed that the increasing amount of YF₃ addition caused negative effect on the thermal stability of the composites. 5ZHA composites with YF₃ showed the highest relative density among all of the composites with or without YF₃.     

Enhanced piezoelectric properties of (K0.40Na0.60)0.94Li0.06Nb0.94SbO3 lead-free ceramics by optimizing the sintering temperature

Effects of sintering temperature on the microstructure and electrical properties of (K_0.40Na_0.60)_0.94Li_0.06Nb_0.94SbO₃ (KNLNS) lead-free ceramics are investigated. The grain size gradually becomes larger with increasing sintering temperature from 1055 °C to 1105 °C, and the piezoelectric property could be enhanced by optimizing their sintering temperature. The ceramic sintered at 1075 °C has optimum electrical properties, i.e., d₃₃~272 pC/N, k_p~43.5%, ε_r~1152, tan δ~0.026, and T_C~346 °C. These results show that the sintering temperature can optimize electrical properties of KNLNS ceramics.     

Fabrication and electrical properties of Pb(Co1/3Nb2/3)O3 ceramics

Pb(Co_1/3Nb_2/3)O₃ (PCN) ceramics have been produced by sintering PCN powders synthesized from lead oxide (PbO) and cobalt niobate (CoNb₂O₆) with an effective method developed for minimizing the level of PbO loss during sintering. Attention has been focused on relationships between sintering conditions, phase formation, density, microstructural development, dielectric and ferroelectric properties of the sintered ceramics. From X-ray diffraction analysis, the optimum sintering temperature for the high purity PCN phase was found at approximately 1050 and 1100 °C. The densities of sintered PCN ceramics increased with increasing sintering temperature. However, it is also observed that at very high temperature the density began to decrease. PCN ceramic sintered at 1050 °C has small grain size with variation in grain shape. There is insignificant change of dielectric properties with sintering temperature. The P–E hysteresis loops observed at −70 °C are of slim-loop type with small remanent polarization values, which confirmed relaxor ferroelectric behavior of PCN ceramics.