Clays from the Bay of Naples (Italy): New insight on ancient and traditional ceramics

The features of two clayey raw materials from the Bay of Naples and their fired products were investigated via minero-petrographic and physical techniques.Clay preparation and firing dynamics were performed following a process similar to that performed by ancient and traditional potters. A high-CaO marine clay from Ischia was mixed with different amounts of volcanic temper in order to replicate most common ware. These mixtures show a fair mechanical resistance starting from relatively low firing temperatures (>850 °C). The addition of temper resulted in different technological characteristics. A low-CaO weathered pyroclastics from the Sorrento Peninsula was prepared to simulate heat resistant and refractory ceramics. Fired products are characterised by a less resistant ceramic body up to 1000 °C compared to Ischia ceramics. Despite worse strength these ceramics show a porous structure, yielding better refractory performances.     

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.     

Gravimetric and spectroscopic studies of the chemical combination of moisture by as-fired and reheated terracotta

The application of microbalance measurements to investigate the chemical recombination of moisture with fired clay ceramics is demonstrated. The kinetics of mass gain at constant temperature and relative humidity (RH) are studied for terracotta fired at temperatures between 800 °C and 1200 °C. The experimental results show that mass gain proceeds in two stages. The second stage mass gain is taken as a measure of the capacity for long-term chemical combination with moisture based on the (time)^1/4 law. A maximum rate of mass gain is obtained for terracotta fired at 1000 °C. It is shown that reheating a sample of fired terracotta at any temperature between 500 °C and the original firing temperature will return the material to its as-fired state. This is supported by Raman spectroscopy. Exceeding the original firing temperature during reheating alters the subsequent rate of reaction to moisture, suggesting a method for determining the original firing temperature of ceramic artefacts.     

Low-valued raw materials challenge the common eligibility criteria for triaxial ceramics

In this work, two low-valued natural raw materials were used, namely a low plasticity and high iron content clay and a powdered rock waste generated during crushing of igneous rocks to produce construction aggregates. After characterization (chemical, mineralogical, thermal and granulometric), mixtures containing up to 75 wt% of rock powder were prepared and extruded closely following industrial practice, and fired for 60 min from 900 to 1100 °C. Property development was evaluated on as-extruded, dried and fired pieces. The results obtained were interpreted based on chemical compositions, the estimates from the Al₂O₃–SiO₂–CaO phase diagram and presumed reaction kinetics. Such interpretation showed that physical characteristics dominate not only during shaping and drying, but also during low temperature firing (slow reaction kinetics), when rock additions act as inert material; at high firing temperatures, however, the rock promotes liquid phase development after first melting (fluxing effect) and thermodynamics prevail. The dominant fluxing effect results in improved fired properties, which were shown to depend almost linearly on the liquid phase content, predicted by the phase diagram and determined by the chemical composition. These results enabled the identification of behavioural composition ranges to best exploit the materials’ industrial use potential and demonstrate that current paradigms in raw material evaluation and processing in traditional ceramics industries need a re-assessment.     

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.     

Aluminum titanate based ceramics from aluminum sludge waste

This work aims at obtaining aluminum titanate-based ceramics (Al₂TiO₅: AT) composites from industrial wastes. Al-sludge waste and rutile ore were used as rich sources of alumina and titania instead of pure materials. Sludge-(0–40 wt%) rutile mixtures were mixed, formed and fired at 1350 °C for various times. Phase composition, microstructure, densification, mechanical and thermal behaviors of the obtained AT composites have been investigated. Complete conversion of the starting materials to AT with bulk density of 3.199 g/cm³, compressive strength and modulus of rupture of 326.425 MPa and 30.84 MPa, respectively and very low CTE (−0.927*10⁻⁶ K⁻¹) were achieved by firing the sludge-(30 wt%) rutile at 1350 °C for 4 h. These results suggest that the obtained AT-ceramics from Al-sludge waste-rutile ore are a promising and an ecofriendly route.     

The effect of heat treatment and cooling rate on the properties of lightweight aggregates

Lightweight aggregates (LWA) were produced from clay in the laboratory. After firing different heat treatments and cooling rates were applied and the resulting material was investigated with respect to strength and microstructure. Fast cooling led to the formation of micro cracks and weakened the material whereas slow cooling enhanced the strength of LWA. The residence time at temperatures between 700 °C and 900 °C led to differences in average oxidation state of iron in the matrix phase leading to substantial changes in thermal behaviour of the matrix phase. The combination of a highly oxidized shell and a reduced core proved to enhance the strength of LWA. A two hour heat treatment at 800 °C in air combined with a subsequent slow cooling rate (0.7 °C/min) applied to LWA produced in an industrial rotary kiln led to a strength increase of 114% compared to material of the normal production without changing any other property.     

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.     

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.     

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.     

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.     

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.     

Preparation and properties of porous alumina ceramics with oriented cylindrical pores produced by an extrusion method

Porous alumina ceramics with unidirectionally-oriented pores were prepared by extrusion. Carbon fibers of 14 μm diameter and 600 μm length to be used as the pore-forming agent were kneaded with alumina, binder and dispersing agent. The resulting paste was extruded, dried at 110 °C, degreased at 1000 °C and fired at 1600 °C for 2 h. SEM showed a microstructure of dispersed highly oriented pores in a dense alumina matrix. The pore area in the cross section was 25.3% with about 1700 pores/mm². The pore size distribution of the fired body measured by Hg porosimetry showed a sharp peak corresponding to the diameter of the burnt-out carbon fibers. The resulting porous alumina ceramics with 38% total porosity showed a fracture strength of 171 MPa and a Young's modulus of 132 GPa. This strength is significantly higher than the reported value for other porous alumina ceramics even though the present pore size is much larger.     

Preparation and characterization of sintered ceramics made with spent foundry olivine sand and clay

Two different types of clay (a yellow and a red clay) were used to prepare two sets of materials containing spent foundry olivine sand. They were blended by attrition milling in varying proportions to obtain powders of different composition.All mixtures were dried, sieved, uniaxially pressed into specimens and air sintered for 1 h at temperatures ranging from 900 to 1140 °C. The resulting materials were characterized by density, water absorption, shrinkage, crystallographic composition, microstructure and physico-mechanical properties. Mechanical and crystallographic properties were determined on samples fired at 1040 °C in order to compare materials with similar characteristics. It was observed that, after sintering, all compositions show the presence of the glassy phase which surrounds the crystalline grains and the set of materials prepared using the red clay displayed best overall behavior. XRD analysis performed on the free surface of the fired samples did not show the presence of compounds containing heavy metals present in the starting materials.     

The influence of mineralogical composition on the colour of clay body

By using methods of Mössbauer spectroscopy, X-ray diffraction and colorimetrics analysis, the dependence of the colour of a ceramic body on its mineralogical composition was determined. In order to achieve this objective, the mineralogical composition and colour parameters of the Girininkai deposit (Lithuania) easily fusible clay and its mixture with the glauconite rock during firing at 600, 800, 1000, and 1025 °C were investigated in detail. By changing firing temperature from 600 to 1025 °C and duration of isothermal exposure from 0.5 to 4 h the amount of iron in hematite increases 5 times, whereas the amount of Fe³⁺ incorporated into the crystal lattice of other compounds diminishes 3–4 times if compared with natural clay. For this reason, the value of red colour coordinate increases from 18.44 to 23.65 NBS units and the ceramic body darkens and becomes redish brown. The colour can be intensified even more by adding to the forming mass finely ground glauconite.     

Microstructural and optical features of a Eu-monazite

The obtention of europium-phosphate nanoparticles by the precipitation method and its thermal evolution to become ceramics materials is presented. The monazite structure was obtained from the rabdophane phase after firing at 1000 °C during several hours. The powder characteristics made easy the pressing and sintering processes and it was possible to obtain high density bodies (relative density > 97%) at only 1200 °C. The fluorescence spectra and the lifetimes were investigated as a function of the heating temperature, as well, the microstructure and the residual glassy phase. It was found that higher sintering temperatures (>1500 °C) resulted in lower fluorescence emission than lower temperatures (maximum at 1200 °C) as consequence of the microstructure detrimental. The gamma irradiation up to the dose of 18 kGys did not produce any appreciable effect in the optical properties; however, the sintering temperature modified the optical absorption in the UV range.     

Effect of firing temperature and atmosphere on ceramics made of NW Peloponnese clay sediments. Part I: Reaction paths,crystalline phases,microstructure and colour

Archaeometric investigation on ancient ceramic collected from excavations in NW Peloponnese demonstrated that the ancient potters used the local Plio-Pleistocene clay sedimentary deposits for a large historical period. Three representative raw materials of these local sediments were chosen for experimental work aiming to evaluate their firing behaviour in a propane-fired kiln, with a different atmosphere and temperature. The determination of mineralogy and microstructure was carried out by XRD and SEM-EDS analysis. For ceramics fired at 850 and 950 °C, no significant mineralogical and microstructural differences were observed between the oxidising and reducing atmosphere. The main pyrometamorphic phases are fassaite, gehlenite, anorthite and wollastonite. On the contrary, at 1050 °C in reducing atmosphere, gehlenite and wollastonite are diminished whereas the content of anorthite, fassaite and amorphous phase is higher. The higher vitrification is attributed to Fe²⁺ that participates either in the formation of eutectic phases or in low melting crystalline phases.     

Sintering behaviour and translucency of dense Eu2O3 ceramics

Eu₂O₃ ceramics have been obtained at sintering temperatures of between 1000 °C and 1550 °C. X-ray diffraction and scanning electron microscopy, in combination with dilatometry experiments, allowed understanding the sintering behaviour. Moderate grain growth followed an efficient densification process between 1400 °C and 1550 °C, which yielded high-density ceramics with an average grain size of 4 μm. The ceramics had Young modulus of 125 GPa, in agreement with the previously published data. The dense Eu₂O₃ ceramics were translucent (35.1% transmittance at 800 nm of 0.8 mm thick discs), showing in addition a slightly pink colour. We propose that the combination of high density and an average grain size of 4 μm is responsible for this novel functionality.     

Grain size effect in conductive phosphate / carbon nanotube ceramics

Composite materials based on aluminium phosphate matrix with different grain sizes and small inclusions of carbon nanotubes were studied by means of broadband dielectric spectroscopy in a wide frequency (20 Hz to 36 GHz) and temperature (25–600 K) ranges. The highest electrical percolation threshold was observed for ceramics with the grain size of 0.8 µm, which is higher than the carbon nanotubes cluster size. The electrical transport in ceramics occurs due to the thermal activation at higher temperatures (above room temperature) and the tunneling at lower temperatures. The potential barrier for electron hopping is the lowest in nanosized ceramics. The distance for electron tunneling is also lowest in nanosized ceramics. The electrical properties of ceramics are stable up to 560 K.     

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.