Fabrication and technological properties of nanoporous spinel/forsterite/zirconia ceramic composites

In this work, nanoporous spinel/forsterite/zirconia ceramic composites were fabricated at 1600 °C for 2 h. The influence of zirconia content (up to 10 mass%) on the technological properties, nanopores formation, phase compositions, microstructure and thermal diffusivity of nanoporous ceramic composites was investigated. Nanospinel and nanoforsterite powders were synthesized via a modified co-precipitation and sol–gel techniques, respectively. Results indicated that apparent porosity of the fired nanoporous ceramic composites is mostly in the range 14.26–56.14% with the average pores diameter 35.8 nm. Using of nanopowders (spinel and forsterite) as the staring materials were achieved high mechanical (cold crushing strength ∼ 235–164 MPa) and elastic (Young’s modulus ∼ 123.6–4.5 GPa) properties of the prepared nanoporous ceramic composites. Microstructure analysis exhibited all of the crystalline phases and pores of the nanoporous ceramic composites are in the nanosize (35–40 nm). These nanoporous ceramic composites are promising porous ceramic materials for using in advanced applications due to their excellent combination properties.     

In situ formation of sintered cordierite–mullite nano–micro composites by utilizing of waste silica fume

This study aims at in situ formation of sintered cordierite–mullite nano–macro composites having high technological properties using waste silica fume, calcined ball clay, calcined alumina, and magnesia as starting materials. The starting materials were mixed in different ratios to obtain different cordierite–mullite composite batches in which the cordierite contents ranged from 50 to 100 wt.%. The batches were uni-axially pressed at 100 MPa and sintered at 1350, 1400 and 1450 °C to select the optimum temperature required for cordierite–mullite nano–macro composites formation. The formed phases were identified by X-ray diffraction (XRD) pattern. The sintering parameters in terms of bulk density (BD) and apparent porosity (AP) were determined. The microstructure of composites has been investigated by scanning electron microscope (SEM). Cold crushing strength (CCS) of the sintered batches was evaluated. The result revealed that the cordierite–mullite nano–macro composites were in-situ formed at 1400 °C. The batch containing 70 wt.% cordierite showed good physical and mechanical properties.     

Temperature stable microwave dielectric ceramic 0.3Li2TiO3–0.7Li(Zn0.5Ti1.5)O4 with ultra-low dielectric loss

In the present work, the 0.3Li₂TiO₃–0.7Li(Zn_0.5Ti_1.5)O₄ ceramic was prepared via the conventional solid state reaction route, and the phase composition, microstructure, and sintering behavior were investigated. The ceramic sample sintered at 1100 °C for 2 h demonstrated high microwave dielectric performance with a relative permittivity of 23.5, a high quality factor (Qf) ~ 88,360 GHz (at 7.4 GHz), and near zero temperature coefficient of resonant frequency about ? 0.8 ppm/°C. These results indicate that the 0.3Li₂TiO₃–0.7Li(Zn_0.5Ti_1.5)O₄ ceramic might be a good candidate for dielectric resonators, filters and other microwave electronic device applications.     

Mechanical properties of in-situ toughened Al2O3/Fe3Al

Mechanical properties of in-situ toughened Al₂O₃/Fe₃Al nano-/micro-composites were measured. Effects of Fe₃Al content, sintering temperature and holding time on properties and microstructure of the composites were investigated. The addition of Fe₃Al nano-particles decreased the aspect ratio and grain size of Al₂O₃, and changed the fracture mode of composites. The maximum bending strength and fracture toughness were 832 MPa and 7.96 MPa m^1/2, which were obtained in Al₂O₃/5 wt.% Fe₃Al sintered at 1530 °C and Al₂O₃/10 wt.% Fe₃Al sintered at 1600 °C, respectively. Compared to monolithic alumina, the strength increased by 132% and the toughness increased by 73%. The improvement in the mechanical properties of the composites was attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the “in-situ reinforced effect” arising from the platelet grains of Al₂O₃ matrix, refined microstructure by dispersoids, as well as crack deflection and bridging of intergranular and intragranular Fe₃Al.     

Influence of the microstructure evolution of ZrO2 fiber on the fracture toughness of ZrB2–SiC nanocomposite ceramics

ZrB₂–SiC nanocomposite ceramics toughened by ZrO₂ fiber were fabricated by spark plasma sintering (SPS) at 1700 °C. The content of ZrO₂ fiber incorporated into the ZrB₂–SiC nanocomposites ranged from 5 mass% to 20 mass%. The content, microstructure, and phase transformation of ZrO₂ fiber exhibited remarkable effects on the fracture toughness of the ZrO_2(f)/ZrB₂–SiC composites. Fracture toughness of the composites greatly improved to a maximum value of 6.56 MPa m^1/2 ± 0.3 MPa m^1/2 by the addition of 15 mass% of ZrO₂ fiber. The microstructure of the ZrO₂ fiber exhibited certain alterations after the SPS process, which enhanced crack deflection and crack bridging and affected fracture toughness. Some microcracks were induced by the phase transformation from t-ZrO₂ to m-ZrO₂, which was also an important reason behind the improvement in toughness.     

Preparation and characterization of the bismuth sodium titanate (Na0.5Bi0.5TiO3) ceramic doped with ZnO

This study investigates effects of the zinc oxide (ZnO) addition and the sintering temperature on the microstructure and the electrical properties (such as dielectric constant and loss tangent) of the lead-free piezoelectric ceramic of bismuth sodium titanate (Na_0.5Bi_0.5TiO₃), NBT, which was prepared using the mixed oxide method. Three kinds of starting powders (such as Bi₂O₃, Na₂CO₃ and TiO₂) were mixed and calcined. This calcined NBT powder and a certain weight percentage of ZnO were mixed and compressed into a green compact of NBT–ZnO. Then, this green compact of NBT–ZnO was sintered to be a disk doped with ZnO, and its characteristics were measured. In this study, the calcining temperature was 800 °C, the sintering temperatures ranged from 1000 to 1150 °C, and the weight percentages of ZnO doping included 0.0, 0.5, 1.0, and 2.0 wt%. At a fixed wt% ZnO, the grain size increases with increase in the sintering temperature. The largest relative density of the NBT disk obtained in this study is 98.3% at the calcining temperature of 800 °C, the sintering temperature of 1050 °C, and 0.5 wt% ZnO addition. Its corresponding dielectric constant and loss tangent are 216.55 and 0.133, respectively.     

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

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

Mechanical alloying and sintering of nanostructured TiO2 reinforced copper composite and its characterization

Mechanical alloying is a suitable method for producing copper based composites. Cu–TiO₂ composite was fabricated using high energy ball milling and conventional consolidation. Ball milling was performed at different milling durations (0–24 h) to investigate the effects of the milling time on the formation and properties of produced nanostructured Cu–TiO₂ composites. The amount of the TiO₂ in the final composition of the composite assumed to be 0, 1, 3, 5 and 7 wt%. The milled composite powders were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy to investigate the effects of the milling time on the formation of the composite and its properties. Also hardness, density and electrical conductivity of the sintered specimen were measured. High energy ball milling causes a high density of defects in the powders. Thus the Cu crystallite size decreases, generally to less than 50 nm. The maximum hardness value (105 HV) of the sintered compacts belongs to Cu–5 wt%TiO₂ which has been milled for 12 h.     

Effect of 10Ce-TZP/Al2O3 nanocomposite particle amount and sintering temperature on the microstructure and mechanical properties of Al/(10Ce-TZP/Al2O3) nanocomposites

A zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al₂O₃ nanocomposite) can be a good substitute as reinforcement in metal matrix composites. In the present study, the effect of the amount of 10Ce-TZP/Al₂O₃ particles on the microstructure and properties of Al/(10Ce-TZP/Al₂O₃) nanocomposites was investigated. For this purpose, aluminum powders with average size of 30 μm were ball-milled with 10Ce-TZP/Al₂O₃ nanocomposite powders (synthesized by aqueous combustion) in varying amounts of 1, 3, 5, 7, and 10 wt.%. Cylindrical-shape samples were prepared by pressing the powders at 600 MPa for 60 min while heating at 400–450 °C. The specimens were then characterized by scanning and transmission electron microscopy (SEM and TEM) in addition to different physical and mechanical testing methods in order to establish the optimal processing conditions. The highest compression strength was obtained in the composite with 7 wt.% (10Ce-TZP/Al₂O₃) sintered at 450 °C.     

Sinter-HIP of polymer-derived Al2O3–SiC composites with high SiC contents

Submicrometer Al₂O₃ composites with more than 20 vol.% of SiC particles were produced using a multiple infiltration of porous bodies with a liquid polymer SiC precursor. The fully dense composites were successfully densified using a sinter-HIP process. Parameters of sintering and HIP steps are discussed with respect to both densification and microstructure evolution of the composites. The initial pressure during the sintering step plays an important role for the preparation of fully dense composites with a submicrometer alumina matrix at 1750 °C. Optimized densification schedule of sinter-HIP represents a novel approach of densification at relatively mild conditions compared to previously reported or common densification methods of Al₂O₃–SiC composites with high SiC content, such as pressureless sintering, hot pressing and post-HIPing. The method expands the possibilities for preparation of alumina based composites with SiC volume fraction > 20 vol.%, filling the gap in available literature data.     

Dielectric behavior of TiO2 ceramic prepared by plasma activated sintering

Rutile-phase TiO₂ ceramic was rapidly fabricated by plasma activated sintering (PAS) at 650–850 °C for 3 min under 30 MPa. The temperature and frequency dependences of the dielectric properties (dielectric constant and dielectric loss) for the dense TiO₂ ceramic were investigated, and the dielectric behavior was briefly discussed. It was demonstrated that extraordinarily high dielectric constant (2–5 × 10⁴) was observed in the whole experimental ranges of ? 160 to 200 °C and 1 kHz–1 MHz. Moreover, the dielectric loss kept a relatively normal level, however, its temperature and frequency dependences were markedly different with those of the rutile-phase TiO₂ preforms. The unusual dielectric behavior was related with the particular dielectric polarizations of the TiO₂ ceramic and its dominant form of loss under different conditions.     

Structure and microwave dielectric characteristics of lithium-excess Ca0.6Nd0.8/3TiO3/(Li0.5Nd0.5)TiO3 ceramics

Compositions based on (1−x)Ca_0.6Nd_8/3TiO₃−x(Li_1/2Nd_1/2)TiO₃ + yLi (CNLNTx + yLi, x = 0.30–0.60, y = 0–0.05), suitable for microwave applications have been developed by systematically adding excess lithium in order to tune the microwave dielectric properties and lower sintering temperature. Addition of 0.03 excess-Li simultaneously reduced the sintering temperature and improved the relative density of sintered CNLNTx ceramics. The excess Li addition can compensate the evaporation of Li during sintering process and decrease the secondary phase content. The CNLNTx (x = 0.45) ceramics with 0.03 Li excess sintered at 1190 °C have single phase orthorhombic perovskite structure, together with the optimum combination of microwave dielectric properties of ɛ_r = 129, Q × f = 3600 GHz, τ_f = 38 ppm/°C. Obviously, excess-Li addition can efficiently decrease the sintering temperature and improve the microwave dielectric properties. The high permittivity and relatively low sintering temperatures of lithium-excess Ca_0.6Nd_0.8/3TiO₃/(Li_0.5Nd_0.5)TiO₃ ceramics are ideal for the development of low cost ultra-small dielectric loaded antenna.     

Effects of solids loading on microstructure and mechanical properties of HfB2–20 vol.% MoSi2 ultra high temperature ceramic composites through aqueous gelcasting route

HfB₂–20 vol.% MoSi₂ ultra high temperature ceramic composites were prepared through aqueous gelcasting route. The stability of HfB₂ and MoSi₂ suspensions were studied by zeta potential measurements, sedimentation tests and apparent viscosity measurements. The solids loading had significant effects on the green and sintered densities, microstructure and mechanical properties of HfB₂–MoSi₂ composites. The values of flexural strength of the green and sintered bodies ranged from 18.3 to 38.7, and 111.5 to 415.9 MPa, respectively, which were strongly dependent on the solids loading. The values of fracture toughness of the sintered bodies ranged from 2.18 to 4.24 MPa m^1/2. The highest relative density, mechanical properties and the most homogeneous microstructure was obtained when the solids loading was 45 vol.%. The highest green strength, flexural strength and fracture toughness were 38.7 ± 5.3 MPa, 415.9 ± 17.0 MPa and 4.24 ± 0.22 MPa m^1/2, respectively.     

Fabrication of Al2O3–ZrB2 in situ composite by SHS dynamic compaction: A novel approach

Al₂O₃–ZrB₂ in situ composites of 97% of theoretical density were successfully fabricated by a novel self-propagating high temperature synthesis (SHS) dynamic compaction, using less expensive raw materials zirconium oxide, boron oxide, and aluminium. The process is fast, energy efficient, where no furnace sintering is required. The process inhibits and controls the grain growth and microstructure. The densification behaviour and correlation with microstructure of the SHS dynamic compacts were compared with the furnace sintered composite samples where the composite powder was prepared by SHS process. The furnace sintered samples showed coarser grain growth and maximum density of 94.5% of theoretical density was achieved. The SHS dynamic compacted in situ composite had much finer grains in the range of 0.5–3 μm with density 95.5% of the theoretical value. The average grain size was found to decrease from 10 μm to 1.4 μm for alumina and from 5.4 μm to 1.0 μm for zirconium diboride from furnace sintering to SHS dynamic compaction, respectively. Addition of Al₂O₃ as a diluent during SHS reaction enhanced the density to 97%. During SHS dynamic compaction, the amount of liquid and the time interval at which the sample stays at high temperature are the controlling factor of the final microstructure and the densification of the composite.     

Microstructure control of TCP/TCP-(t-ZrO2)/t-ZrO2 composites for artificial cortical bone

In this study, bone like continuously porous TCP/TCP-(t-ZrO₂)/t-ZrO₂ composites with a central channel were fabricated using a multi-pass extrusion process and their mechanical properties and microstructure at different sintering temperatures were investigated. Hydroxyapatite (HAp) powder was used as the raw powder which undergoes a phase transformation into the α-tricalcium phosphate phase (α-TCP) at a sintering temperature of 1500 °C. The external diameter and inside cylindrical hollow core were approximately 10.3 mm and 4.8 mm, respectively. The frame region contained numerous microchannels that extended from one side of the fabricated body to the other. The channeled frame region had a multi-layer microstructure with a TCP/TCP-(t-ZrO₂)/t-ZrO₂ layer configuration. The inner layer consisted of TCP, which make the wall of the microchannel. The material properties were characterized and microstructural analysis was carried out. The maximum pore size, compressive strength, and relative density of the fabricated system were approximately 86 μm, 53 MPa, and 77% when sintered at 1500 °C. The composites exhibited excellent biocompatibility and cell proliferation behavior resulted in the MTT assay and cell adhesion test using osteoblast-like MG-63 cells.     

Al2TiO5–Al2O3–TiO2 nanocomposite: Structure,mechanical property and bioactivity studies

Novel biomaterials are of prime importance in tissue engineering. Here, we developed novel nanostructured Al₂TiO₅–Al₂O₃–TiO₂ composite as a biomaterial for bone repair. Initially, nanocrystalline Al₂O₃–TiO₂ composite powder was synthesized by a sol–gel process. The powder was cold compacted and sintered at 1300–1500 °C to develop nanostructured Al₂TiO₅–Al₂O₃–TiO₂ composite. Nano features were retained in the sintered structures while the grains showed irregular morphology. The grain-growth and microcracking were prominent at higher sintering temperatures. X-ray diffraction peak intensity of β-Al₂TiO₅ increased with increasing temperature. β-Al₂TiO₅ content increased from 91.67% at 1300 °C to 98.83% at 1500 °C, according to Rietveld refinement. The density of β-Al₂TiO₅ sintered at 1300 °C, 1400 °C and 1500 °C were computed to be 3.668 g cm^?3, 3.685 g cm^?3 and 3.664 g cm^?3, respectively.Nanocrystalline grains enhanced the flexural strength. The highest flexural strength of 43.2 MPa was achieved. Bioactivity and biomechanical properties were assessed in simulated body fluid. Electron microscopy confirmed the formation of apatite crystals on the surface of the nanocomposite. Spectroscopic analysis established the presence of Ca and P ions in the crystals. Results throw light on biocompatibility and bioactivity of β-Al₂TiO₅ phase, which has not been reported previously.     

Enhanced densification and microwave dielectric properties of Mg2TiO4 ceramics added with CeO2 nanoparticles

We report the study of the effects of processing parameters and additive concentration on the structure, microstructure and microwave dielectric properties of MTO–CeO₂ (x wt.%) ceramics with x = 0, 0.5, 1.0 and 1.5 prepared by solid-state reaction method by adding CeO₂ nanoparticles as a sintering aid. The pure Mg₂TiO₄ ceramics were not densifiable below 1450 °C. However, when CeO₂ nanoparticles were added to MTO, the densification achieved at 1300 °C along with the increase in average grain size with the uniform microstructure and improved microwave dielectric properties. This is mainly driven by the large surface energy of CeO₂ nanoparticles and their defect energy during the sintering process. While the addition of CeO₂ nanoparticles in MTO ceramics does not change the dielectric constant (?_r), the unloaded quality factor (Q_u) was altered significantly. MTO–CeO₂ (1.5 wt.%) ceramics sintered at 1300 °C exhibit superior microwave dielectric properties (?_r ～ 14.6, Q × f₀ ～ 167 THz), as compared to the pure Mg₂TiO₄ ceramics. The observed results are correlated to the enhancement in density and the development of uniform microstructure with the enhanced grain size.     

Fabrication of conductive porous alumina (CPA) structurally modified with carbon nanotubes (CNT)

A novel binary porous composite nano-carbon networks (NCNs)/alumina, which is denoted as electrically conductive porous alumina (CPA), was structurally modified by carbon nanotubes (CNT) pre-treated with mixed concentrated acids at 60 °C for 6 h in this study. This conductive ceramics (CCs) was fabricated by combination of gelcasting and high temperature reductive sintering (HTRS) in novel atmosphere. CNT pre-treatment leading to the increased hydrophilicity makes it possible to make uniformly dispersed CNT/alumina slurry. And by HTRS in Ar at 1700 °C for 2 h, well-gelled polymer net-paths in green body prepared by gelcasting technology were totally converted to nano-carbon networks (NCNs) without destruction of CNT. NCN with graphitic crystal structure was evaluated by Raman spectroscopy in sintered ceramic body. Moreover, comparing with as-received CNT, the decreased surface defect of detected composite also supported the further graphitization of CNT via HTRS in Ar instead of burning out. With the aid of field-emission scanning electronic microscopy (FE-SEM) observation, the increased alumina grains in sintered ceramic body CNT/NCN/alumina was valid. Moreover, it was demonstrated that there were three components in this composite, which is carbon filler with two different forms (CNT and NCN) and alumina matrix. And these three components CNT covered with Al₂O₃ particles (Al₂O₃/CNT), NCN and alumina grains (alumina) co-exist in four different situations as follows: (a) Al₂O₃/CNT–alumina co-junction, (b) Al₂O₃/CNT–NCN co-junction, (c) Al₂O₃/CNT–alumina–NCN and (d) Al₂O₃/CNT mesh between alumina boundaries. Furthermore, by comparing with binary composite NCN/alumina (CPA), the increased flexural strength of ternary composite CNT/NCN/alumina (CNT/CPA) up to 38 MPa was attributed to the reinforcement CNT acting as elastic bridge in composite.     

Enhanced properties of alumina–aluminium titanate composites obtained by spark plasma reaction-sintering of slip cast green bodies

Full dense alumina + 40 vol.% aluminium titanate composites were obtained by colloidal filtration and fast reaction-sintering of alumina/titania green bodies by spark plasma sintering at low temperatures (1250–1400 °C). The composites obtained had near-to-theoretical density (>99%) with a bimodal grain size distribution. Phase development analysis demonstrated that aluminium titanate has already formed at 1300 °C. The mechanical properties such as Vickers hardness, flexural strength and fracture toughness of bulk composites are significantly higher than those reported elsewhere, e.g. the composite sintered at 1350 °C show values of about 24 GPa, 424 MPa and 5.4 MPa m^1/2, respectively. The improved mechanical properties of these composites are attributed to the enhanced densification and the finer and more uniform nanostructure achieved by non-conventional fast sintering of slip-cast dense green compacts.     

Electrochemical behavior of sintered YAG dispersed 316L stainless steel composites

The present work investigates the effect of second phase dispersoid addition and sintering temperature on the corrosion behavior of austenitic (316L) stainless steels. Yttrium aluminum garnet (YAG) was added as second phase to the austenitic stainless steels in varying amounts (1, 2.5 and 7.5 wt.%), and the compacts were sintered at 1200 and 1400 °C corresponding to solid-state and supersolidus sintering, respectively. The sintered samples were characterized for their corrosion resistance in 0.1N H₂SO₄ using potentiodynamic polarization. It is shown that YAG addition does not appreciably increase corrosion rate of 316L compacts. However, as compared to solid-state sintering, supersolidus sintering resulted in superior corrosion resistance. The electrochemical behavior of the 316L–YAG composites with sintering temperature is correlated to the densification response and microstructure.