Characterization of high strength mortars with nano alumina at elevated temperatures

In this study, the effect of elevated temperatures on chemical composition, microstructure and mechanical properties of high strength mortars with nano alumina was investigated. Mortars with 1, 2 and 3% nano alumina as cement replacement were prepared and then exposed to 100 °C, 200 °C, 300 °C, 400 °C, 600 °C, 800 °C and 1000 °C. XRD, DSC and SEM tests were carried out to identify chemical composition and microstructure changes in the cement matrix after being exposed to elevated temperatures. Residual compressive strength, relative elastic modulus and gas permeability coefficient of samples were also obtained. A brittleness index was defined to monitor changes in brittleness of samples after being exposed to elevated temperatures. Nano alumina enhanced compressive strength of samples up to 16% and improved residual compressive strength. An increase in the relative elastic modulus, higher energy absorption and lower permeability were also observed when 1% nano alumina was added.     

Microstructure and electrical properties of ZnO–Bi2O3-based varistor ceramics by different sintering processes

The effect of sintering processes, such as open sintering, sintering inside a closed crucible, and sintering within a powder bed, on the microstructure and V–I characteristics of ZnO–Bi₂O₃-based varistor ceramics was investigated at sintering temperatures in the range 1000–1200 °C. The results from the experiments showed that the microstructure and electrical properties of the samples varied according to the sintering method and temperature. Optimal values for the electrical characteristics of the varistor ceramics by different sintering processes were obtained when the sintering was conducted at 1100 °C. At the same sintering temperature, the different processes affected the properties differently. At 1000 °C, the samples sintered within a powdered bed showed better electrical properties than those subjected to the other two processes, while at 1100 or 1200 °C, the samples sintered in an open crucible exhibited the best electrical properties.     

Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics

This study investigated the effect of sintering temperature on the microstructure and mechanical properties of dental zirconia-toughened alumina (ZTA) machinable ceramics. Six groups of gelcast ZTA ceramic samples sintered at temperatures between 1100 °C and 1450 °C were prepared. The microstructure was investigated by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The mechanical properties were characterized by flexural strength, fracture toughness, Vickers hardness, and machinability. Overall, with increasing temperature, the relative density, flexural strength, fracture toughness, and Vickers hardness values increased and more tetragonal ZrO₂ transformed into monoclinic ZrO₂; on the other hand, the porosity and pore size decreased. Significantly lower brittleness indexes were observed in groups sintered below 1300 °C, and the lowest values were observed at 1200 °C. The highest flexural strength and fracture toughness of ceramics reached 348.27 MPa and 5.23 MPa m^1/2 when sintered at 1450 °C, respectively. By considering the various properties of gelcast ZTA that varied with the sintering temperature, the optimal temperature for excellent machinability was determined to be approximately 1200–1250 °C, and in this range, a low brittleness index and moderate strength of 0.74–1.19 µm^−1/2 and 46.89–120.15 MPa, respectively, were realized.     

Fabrication of biomorphic SiC composites using wood preforms with different structures

Biomorphic SiC composites were fabricated from wood, including high-density compressed cedar, high-density fiberboard (HDF) and low-density paulownia followed by the fabrication of a preform and liquid silicon infiltration (LSI) process. The degree of molten silicon infiltration was strongly dependent on the cell wall thickness and pore size of the carbon preform. The mechanical properties of the biomorphic SiC composites were characterized by compressive tests at room temperature, 1000 °C and 1200 °C, and the relationship between the mechanical properties and the microstructural characteristics was analyzed. The compressive strength of the biomorphic composites was found to be strongly dependent on their bulk density and decreased as the test temperature increased to 1200 °C. Strength reduction in the biomorphic SiC composites occurred due to the deformation of the remaining Si at elevated temperatures under ambient atmospheric conditions.     

Effect of cure temperature on the formation of metakaolinite-based geopolymer

A series of metakaolinite-based geopolymer was prepared at several curing temperatures and its relationship with porosity, infrared spectrometry (FT-IR) and mechanical properties was investigated. The samples were cured at the following temperatures: 55, 65 and 80 °C for 1 h. After a post cure of 28 days, the samples were investigated by using the following techniques: compressive strength, mercury intrusion porosimetry (MIP), helium pycnometry, X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared spectrometry (FT-IR). All samples were amorphous by XRD. The sample thermally treated at 65 °C (C65) presented the highest values of compressive strength and relative integrated area of peak at 792 cm⁻¹. This peak was attributed to a higher concentration of tetra-coordinated aluminum, indicating a higher efficiency of the geopolymeric reaction. The C65 also presented the lowest volume of closed pores. The values of the skeletal and the true densities for C65 were very similar and consistent with the volume of the closed pores. On the other hand, this sample showed the highest bulk density obtained by MIP and the greatest difference between the open and closed porosity measured by MIP and helium pycnometry, respectively. All these results are coherent and clearly indicate that the amount of open pores is directly related to a better mechanical performance of the geopolymeric sample.     

Constrained sintering of YSZ/Al2O3 composite coatings on metal substrates produced from eletrophoretic deposition

Yttria stabilized zirconia/alumina (YSZ/Al₂O₃) composite coatings were prepared from electrophoretic deposition (EPD), followed by sintering. The constrained sintering of the coatings on metal substrates was characterized with microstructure examination using electron microscopy, mechanical properties examination using nanoindentation, and residual stress measurement using Cr³⁺ fluorescence spectroscopy. The microstructure close to the coating/substrate interface is more porous than that near the surface of the EPD coatings due to the deposition process and the constrained sintering of the coatings. The sintering of the YSZ/Al₂O₃ composite coating took up to 200 h at 1250 °C to achieve the highest density due to the constraint of the substrate. When the coating was sintered at 1000 °C after sintering at 1250 °C for less than 100 h, the compressive stress was generated due to thermal mismatch between the coating and metal substrate, leading to further densification at 1000 °C because of the ‘hot pressing’ effect. The relative densities estimated based on the residual stress measurements are close to the densities measured by the Archimedes method, which excludes an open porosity effect. The densities estimated from the hardness and the modulus measurements are lower than those from the residual stress measurement and the Archimedes method, because it takes account of the open porosity.     

Mechanical properties of NiO/Ni–YSZ composites depending on temperature,porosity and redox cycling

The Impulse Excitation Technique (IET) was used to determine the elastic modulus and specific damping of different Ni/NiO–YSZ composites suitable for use in solid oxide fuel cells (SOFC). The porosity of the as-sintered samples varied from 9 to 38% and that of the reduced ones from 31 to 52%. For all samples a linear relation between Young's modulus and porosity was found. The temperature dependency of the mechanical properties of both as-sintered and reduced composites was investigated by IET up to 1200 °C. In the as-sintered state, first an increase and peak of stiffness coinciding with the Néel temperature, 250 °C, of NiO was observed. Above this temperature, a linear decrease occurred. Specific damping showed a peak at 170–180 °C and increased above ca. 1000 °C in NiO–YSZ. In the reduced state the elastic modulus decreased linearly with temperature; specific damping increased above ca. 600 °C and was found to be very dependent on microstructure. Damage caused by redox cycling degraded the elastic properties of the composites. Degradation started linearly from 0.5 to 0.6% redox strain leading to macroscopic sample failures at about 2.5% dL/L_o. A simple continuum elastic damage model was fitted to the degradation data.     

Quantitative microstructural analysis and piezoelectricity of highly dense,submicron-structured NaNbO3 ceramics from mechanically activated precursors

We study in this work the processing of NaNbO₃ ceramics prepared in a single thermal treatment of highly reactive precursors obtained by mechanical activation of different reagents, aiming to determine optimum conditions for piezoelectric ceramics production. Pressure-less sintering at 1200 °C leads to dense ceramics (<5% porosity) with poor mechanical stability, unsuitable for practical uses. Dense hot-pressed ceramics were also obtained at lower temperatures (900–1100 °C), all of them in the submicron range of average grain sizes (<400 nm). Their microstructure was quantitatively characterized and their elastic and electromechanical properties determined by an automatic iterative method from impedance measurements at resonance. A noticeable ensemble of piezoelectric and elastic properties (d₃₃ = 38 pC N^?1 and N_p = 3252 kHz mm) was measured for hot-pressed ceramics, from precursors obtained by a combined route of wet-chemistry and mechanical activation, with a microstructure characterized by 0.4% residual porosity and a bimodal lognormal distribution of grain size.     

The influence of different CaO source in the production of anorthite ceramics

The effects of starting raw materials and firing conditions on anorthite (CaAl₂Si₂O₈) phase formation are investigated by differential thermal analysis (DTA)–thermogravimetry (TG) and X-ray powder diffraction (XRD). Four different sources of CaO were used for anorthite production such as Ca(OH)₂, CaCO₃, marble powder and gypsum mould waste. The mixture of raw materials was prepared in stoichiometric ratio of anorthite. Sintering of samples was carried out at various temperatures (1000–1300 °C). In all samples before the formation of anorthite phase, formation of layered alimunosilicate phase (LAS) and of gehlenite phase were observed at low temperatures (<1200 °C). All the samples showed similar crystallization behaviour at 1200 °C. The densification characteristic and the flexural strength of samples were affected by the nature of starting raw materials. The maximum density (～80%) was reached in sample ACH which was prepared from Ca(OH)₂.     

Synthesis of fly ash based geopolymer mortar considering different concentrations and combinations of alkaline activator solution

Geopolymerisation is a process that can transform alumina and silica rich waste materials into valuable binding materials, having excellent mechanical properties. The present experimental study shed a light on the variation in compressive strength of fly ash based geopolymer mortar by varying the molarity of sodium hydroxide as 12 M, 14 M, 16 M and accompanying by sodium silicate (Na₂SIO₃) in 2:1 (Na₂SIO₃/ NaOH) with same molarities. All the geopolymer mixes were oven cured at 80 °C for 24 h and after that kept at room temperature up to the time of testing. The compressive strength was checked subsequently at the ages of 3, 7, 14 and 28 days. The experimental results reveal that the addition of sodium silicate enhances the strength development in geopolymer mortar. The ultimate compressive strength of 40.42 MPa was obtained by incorporating sodium silicate along with 16 M concentrated sodium hydroxide. Furthermore, increasing trend of the compressive strength was found with increasing molar concentration of sodium hydroxide and curing period.     

Improving compressive strength of fly ash-based geopolymer composites by basalt fibers addition

In this study, ASTM Class C fly ash used as an alumino-silicate source was activated by metal alkali and cured at low temperature. Basalt fibers which have excellent physical and mechanical properties were added to fly ash-based geopolymers for 10–30% solid content to act as a reinforced material, and its influence on the compressive strength of geopolymer composites has been investigated. XRD study of synthesized geopolymers showed an amorphous phase of geopolymeric gel in the 2θ region of 23°–38° including calcium-silicate-hydrate (C-S-H) phase, some crystalline phases of magnesioferrite, and un-reacted quartz. The microstructure investigation illustrated fly ash particles and basalt fibers were embedded in a dense alumino-silicate matrix, though there was some un-reacted phase occurred. The compressive strength of fly ash-based geopolymer matrix without basalt fibers added samples aged 28 days was 35 MPa which significantly increased 37% when the 10 wt%. basalt fibers were added. However, the addition of basalt fibers from 15 to 30 wt% has not shown a major improvement in compressive strength. In addition, it was found that the compressive strength was strong relevant to the Ca/Si ratio and the C-S-H phase in the geopolymer matrix as high compressive strength was found in the samples with high Ca/Si ratio. It is suggested that basalt fibers are one of the potential candidates as reinforcements for geopolymer composites development.     

Columnar and DVC-structured thermal barrier coatings deposited by suspension plasma spray: high-temperature stability and their corrosion resistance to the molten salt

High-temperature stability of SPS YSZ coatings with the columnar and deep vertically cracked (DVC) structures and their corrosion resistance to 56 wt% V₂O₅+44 wt% Na₂SO₄ molten salt mixture were investigated. Both the columnar and DVC-structured YSZ coatings were sintered at 1000 °C, but a significant increase in porosity in combination with significant reductions in Vickers’ hardness and Young's modulus were observed at the temperatures from 1200 °C to 1400 °C. The DVC-structured YSZ coating exhibited superior corrosion resistance against the molten salt mixture attack to the columnar-structured one due to its higher density behaving as a sealing protective top layer at 950 °C.     

Effect of pre-oxidation on the microstructure,mechanical and dielectric properties of highly porous silicon nitride ceramics

Highly porous Si₃N₄ ceramics have been fabricated via freeze casting and sintering. The as-sintered samples were pre-oxidized at 1200–1400 °C for 15 min. The effect of pre-oxidation temperature on the microstructure, flexural strength, and dielectric properties of porous Si₃N₄ ceramics were investigated. As the pre-oxidation temperature increased from 1200 °C to 1400 °C, firstly, the flexural strength of the pre-oxidized specimens remained almost constant at 1200 °C, and then decreased to 14.2 MPa at 1300 °C, but finally increased to 25.6 MPa at 1400 °C, while the dielectric constant decreased gradually over the frequencies ranging from 8.2 GHz to 12.4 GHz. This simple process allows porous Si₃N₄ ceramics to have ultra-low dielectric constant and moderate strength, which will be feasible in broadband radome applications at high temperatures.     

Improvement in sinterability and phase stability of hydroxyapatite and partially stabilized zirconia composites

Composites of hydroxyapatite with partially stabilized zirconia with MgO or MgF₂ were pressureless sintered between 1000 °C and 1300 °C. The reactions and transformations of phases were verified by X-ray diffraction. For the hydroxyapatite and zirconia composites with MgO, calcium from the hydroxyapatite diffused into the zirconia phase, and the hydroxyapatite decomposed to tri-calcium phosphate at sintering temperatures higher than 1000 °C. Above about 1200 °C, CaZrO₃ was formed. Composites containing the MgF₂ decomposed slower than the composites with MgO, which was verified by the changes in the lattice volume of the hydroxyapatite left in these composites. Fluorine ions in MgF₂ diffused into hydroxyapatite, which resulted in thermal stability at high sintering temperatures. Composites with MgF₂ had higher hardness than those with MgO. The lowest porosity was found in a composite initially containing 10 wt% partially stabilized zirconia and 5 wt% MgF₂.     

In situ processing of MWCNTs/leucite composites through geopolymer precursor

A serial of multi-walled carbon nanotubes (MWCNTs) reinforced geopolymer composites were prepared, and then heated at elevated temperature to fabricate MWCNTs/leucite composites by in situ transformation. Effects of high-temperature treatment on the microstructure evolution and mechanical performance of the composites were investigated. The results indicated that the introduction of MWCNTs could improve the mechanical properties of geopolymer, and the optimum content was 3 wt%. The mechanical performance declined instead with the further increase in MWCNTs content up to 5 wt%, which could be attributed to the agglomeration of MWCNTs. Significant improvements in mechanical properties were achieved after the composites were treated in a temperature range from 950 °C to 1200 °C relative to their original state before heat treatment. The significant improvements could be described to the matrix densification, and leucite formation as well as the proper interface bonding state between carbon nanotube and leucite matrix.     

Degradation of plasma-sprayed yttria-stabilized zirconia coatings via ingress of vanadium oxide

V₂O₅ reaction and melt infiltration in plasma-sprayed 7 wt% Y₂O₃–ZrO₂ (YSZ) coatings were investigated at temperatures ranging from 750 °C to 1200 °C using SEM and TEM combined with EDS. The interlamellar pores and intralamellar cracks, common in plasma-sprayed materials, provide pathway for the molten species. The microstructure of the contaminated coatings is therefore the result of the interplay between the dissolution/reaction rates of the V₂O₅ with YSZ coating and the infiltration rates of the molten species. Near the coating surface, the reaction front proceeds in a planar fashion, via dissolution of the lamella and precipitation of fine-grained reaction products composed of ZrV₂O₇ (for reactions at 750 °C and below), m-ZrO₂ and YVO₄. The thickness of this planar reaction zone or PRZ was found to increase as reaction time and temperature increased. The melted V₂O₅ was observed to infiltrate along the characteristic microstructure of plasma-sprayed coatings, i.e. the interconnected pores and cracks, and react with the YSZ. The thickness of this melt infiltrated reaction zone or MIRZ ranged from 5 μm for reactions at 750 °C for 30 min to 130 μm for reactions at 1000 °C for 90 min. At 1200 °C, only a PRZ was observed (i.e. the thickness of the MIRZ was nominally zero), suggesting that the dissolution reaction within the pores/cracks and subsequent formation of reaction products may limit infiltration. Fifty-hour heat-treatments at 1000 °C and 1200 °C prior to reaction with the V₂O₅ at 800 °C for 90 min were used to change the microstructural features of the coating, such as crack connectivity and pore size. The heat-treatment at 1000 °C was found most deleterious to the coating due to large cracks created via a desintering process that afforded deep penetration of the molten V₂O₅.     

Formation of hollow MgO-rich spinel whiskers in low carbon MgO–C refractories with Al additives

MgO–C refractories with different carbon contents have been developed to meet the requirement of steel-making technologies. Actually, the carbon content in the refractories will affect their microstructure. In the present work, the phase compositions and microstructure of low carbon MgO–C refractories (1 wt% graphite) were investigated in comparison with those of 10 wt% and 20 wt% graphite, respectively. The results showed that Al₄C₃ whiskers and MgAl₂O₄ particles formed for all the specimens fired at 1000 °C. With the temperature up to 1400 °C, more MgAl₂O₄ particles were detected in the matrix and AlN whiskers occurred locally for high carbon MgO–C specimens (10 wt% and 20 wt% graphite). However, the hollow MgO-rich spinel whiskers began to form locally at 1200 °C and grew dramatically at 1400 °C in low carbon MgO–C refractories, whose growth mechanism was dominated by the capillary transportation from liquid Al at these temperatures.     

In-situ thermo-mechanical testing of fly ash geopolymer concretes made with quartz and expanded clay aggregates

The mechanical and microstructural properties of geopolymer concretes were assessed before, during and after high temperature exposure in order to better understand the engineering properties of the material. Fly ash based geopolymer concretes with either quartz aggregate or expanded clay aggregate were exposed to various temperatures up to 750 °C using a thermo-mechanical testing apparatus. Microstructural investigations were also undertaken to better understand the measured changes in the mechanical properties. It was found that dehydration of capillary water caused cracking and strength losses at temperatures ≤ 300 °C, an effect that was more severe in the quartz aggregate geopolymer due to its lower permeability. At higher temperatures (T ≥ 500 °C) sintering promoted strength increases which enabled both concrete types to yield significant strength advantages over conventional materials. Stress–mechanical strain curves, which form the basis of the fire design of concrete structures, are reported.     

High temperature deformation of non-directionally solidified Al2O3/YAG/ZrO2 eutectic bulk ceramic

Excellent high temperature mechanical properties of melt-grown Al₂O₃-based eutectics have previously been demonstrated in samples prepared by directional solidification methods. In this study, the deformation behaviour of melt-grown Al₂O₃/YAG/ZrO₂ eutectic bulk prepared by a non-directional solidification method was investigated by means of compressive tests in a temperature range of 1200–1700 °C. The non-directionally solidified eutectic bulk ceramic has a colony structure and is polycrystalline. It begins to show ductility and has a compressive strength of 320 MPa at 1500 °C, which is much higher than that of the sintered ceramic with the same composition. However, its plastic deformability is insufficient, even at 1700 °C (just below the melting point of 1715 °C), and cracking occurs during compressive deformation.     

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.