Sintering behaviour of a ZnO waste powder obtained as by-product from brass smelting

The effect of two sintering methods (conventional sintering and two-step sintering) and of alumina addition on the sintering behaviour of a ZnO-rich waste powder (ZnO > 95 wt%), a by-product from brass smelting industry, was studied aiming to improve the sintered density and grain size. Both conventional sintering and two-step sintering methods did not lead to fully dense powder compacts, as densification was conditioned by abnormal grain growth and the particle size of the ZnO-rich residue. When two-step sintering was used the grain growth was reduced comparatively to conventional sintering method. The highest relative sintered density (about 90%) was achieved when samples of ZnO waste and samples of ZnO waste with 2 wt% added Al₂O₃ were processed by two-step sintering and corresponded to a mean grain size of around 18 µm and 7 µm, respectively. XRD and SEM results indicated that alumina addition helped to inhibit grain growth due to the formation of gahnite spinel (ZnAl₂O₄) precipitates in the grain boundaries of zincite (ZnO) grains.     

Synthesis of rod-like β-Si3N4 seed crystals with tailored morphology

β-Si₃N₄ seed crystals were synthesized by sintering (α+β)-Si₃N₄ powders with Y₂O₃+MgO additives at 1800 °C. Full α- to β-phase transformation was achievable at 1800 °C for 1 h. The pre-existing β-Si₃N₄ particles acted as nuclei during a sintering process. The length and mean aspect ratio of β-Si₃N₄ seed grains could be tailored by careful control of α/β-Si₃N₄ ratio, which resulted in various nuclei and driving force. The sample A95B5 with 5% β-nuclei shows a bimodal size distribution containing large amount of abnormal elongated β-Si₃N₄ grains with remarkable large diameter. With increasing the β-phase content from 5 wt% to 100 wt%, the average diameter and aspect ratio of the β-Si₃N₄ single crystals decreased from 1.43 µm to 0.92 µm and from 4.36 to 2.79, respectively.     

Investigation of optical,mechanical, and thermal properties of ZrO2-doped Y2O3 transparent ceramics fabricated by HIP

Highly transparent ZrO₂-doped Y₂O₃ ceramics were successfully synthesized using the hot isostatic pressing (HIP) process. The effects of the ZrO₂ content on the sintering behaviors, optical transmission spectra, Vickers hardness, grain size, size distribution, and Raman spectra were determined. The results indicated that decreased ZrO₂ content could promote increased transmittance, red-shifted infrared cutoff wavelength, increased thermal conductivity, decreased Vickers hardness, and increased lattice ordering. According to the optical transmission spectra, the optimized ZrO₂ content was 0.50 at%, at which point the ceramic exhibited a larger pre-sintering temperature range of 1650–1750 °C and the average grain size of 3.35 µm at 1750 °C. The grain size was significantly decreased at lower pre-sintering temperatures. Furthermore, a moderate Vickers hardness of 8.42 GPa and high thermal conductivity of 10.85 W/m K at room temperature were obtained for the optimized ceramic.     

Preparation of high-quality transparent Al-rich spinel ceramics by reactive sintering

Transparent Al-rich spinel ceramics (MgO·nAl₂O₃, n = 1.05–2.5) were prepared by reactive sintering in air followed by the hot isostatic press (HIP). Commercial MgO and γ-Al₂O₃ powders were used as the raw materials, and the effects of composition and HIP temperature on the transmittance and microstructure of resulting samples were investigated. To obtain the high optical quality, extra alumina (n ≥ 1.1) was used to help eliminate residual pores and suppress abnormal grain growth during the sintering process. The appropriate HIP temperature was also critical to realize the single-phase formation and prevent the generation of second-phase precipitates. The resulting samples with n = 1.1 and 1.3 exhibited excellent optical quality and fine grains below 5 µm after HIPed at 1550 °C.     

Structural,photocatalytic, biological and catalytic properties of SnO2/TiO2 nanoparticles

TiO₂ and SnO₂/TiO₂ nanoparticles with different SnO₂ contents (0–20 wt%) were synthesized via surfactant-assisted sol-gel method using a cationic surfactant (cetyltrimethylammonium bromide, CTAB). The effects of SnO₂ content on the structural, optical, and catalytic activity of TiO₂ have been studied by X-ray diffraction (XRD), Transmission electron microscope (TEM), Scanning electron microscope (SEM), Fourier transformer infrared (FTIR) and UV–vis diffuse reflection spectroscopy (DRS). The total surface acidity of the prepared samples was measured by nonaqueous titration of n-butylamine in acetonitrile and the types of Brönsted and Lewis acid sites were distinguish using FTIR spectra of chemisorbed pyridine. XRD patterns analysis indicates that the crystallite size reduced remarkably and the transformation of anatase-to-rutile phase accelerated greatly with increasing the SnO₂ content. TEM images exhibit a spherical shape with an average particle size varying in the range 10–24 nm and high-resolution TEM images (HRTEM) show lattice fringes with interplanar spacing 0.35 nm and 0.32 nm which corresponding to anatase and rutile phases, respectively. SEM images show the amount of SnO₂ on the TiO₂ surface increases with increasing the SnO₂ content and the particles of SnO₂ were aggregated on TiO₂ surface with increasing SnO₂ content to 20% wt. The catalytic activity was tested by various applications: Photodegradation of Methylene Blue (MB) and Rhodamine B (RhB) under UV–vis irradiations and synthesis of xanthene (14-phenyl-14H-dibenzo [a,j] xanthene). Antibacterial and antioxidant activities were also studied. The antibacterial property test was carried out via agar disc diffusion method, and the results indicated that the prepared catalysts showed moderate antibacterial activity.     

Electric field-assisted flash sintering of tin dioxide

SnO₂ green pellets were submitted to ac electric fields at temperatures below 1350 °C. Electric current pulses occurred and a substantial modification was found in the microstructure of the pellets after application of 80 V cm⁻¹ at 900, 1100 and 1300 °C. Similar experiments were carried out in SnO₂ mixed to 2 wt.% MnO₂. The linear shrinkage of the pellets was monitored with a dilatometer during the application of the electric field. Scanning electron microscopy micrographs of the pellets show the grain structure evolution after the electric current pulses. The larger is the electric current flow through the SnO₂ pellet, the larger are the shrinkage and the average grain size. Even though sintering occurs without significant densification in SnO₂, the welding of the grains is evident. The apparent density of green pellets of SnO₂ with MnO₂ addition sintered at 1100 °C increased 110% with the application of 80 V cm⁻¹, 5 A.     

Fabrication of porous fibrous alumina ceramics by direct coagulation casting combined with 3D printing

A novel forming method for preparing porous alumina ceramics using alumina fibers as raw materials by direct coagulation casting (DCC) combined with 3D printing was proposed. Porous fibrous alumina ceramics were fabricated through temperature induced coagulation of aqueous-based DCC process using sodium tripolyphosphate (STPP) as dispersant and adding K₂SO₄ as removable sintering additives. The sacrificial coated sand molds was fabricated by 3D printing technology, followed by the infiltration of silica sol solution for the subsequent suspension casting. Stable alumina suspension of 40 vol% solid loading was obtained by adding 2.0 wt% STPP and 40 wt% K₂SO₄. The controlled coagulation of the suspension could be realized after heating at 90 °C for about 35 min. The ceramic sample sintered at 1450 °C for 2 h showed the highest compressive strength of 24.33 MPa with porosity of 57.38%. All samples sintered at 1300–1450 °C had uniform pore size distributions with average pore size of 7.2 µm, which indicated the good structure stability when sintered at high temperature.     

Effect of temperature and holding time on the densification of alumina obtained by two-step sintering

The two-step sintering technique is a process of controlling the sintering curve, which provides materials with higher density and smaller grain size when compared to conventional sintering. This technique was evaluated by optical dilatometry with three commercial alumina powders of different purity (92, 96 and 99 wt% of Al₂O₃) and particle size (between 0.73 and 2.16 µm). Different sintering conditions in the first (temperature, T1) and second (temperature, T2, and holding time, t2) steps were studied in order to evaluate the effect of these variables on densification and grain growth. Considering T1 as the temperature at which a relative density (D_rel) of 83% was achieved, and for the range of conditions tested, it was found that higher D_rel values and lower grain size of alumina were obtained with higher T2 and lower t2. Alumina with 99 wt% purity sintered at T1 of 1550 °C for 5 min and T2 of 1500 °C for 4 h showed the best relationship between higher densification (~96% relative density) and reduced grain size (0.94±0.15 µm). Thus, this work demonstrated that suppression of grain growth can also be obtained for commercial alumina.     

High pressure FAST of nanocrystalline barium titanate

This work studies the microstructural evolution of nanocrystalline (<1 µm) barium titanate (BaTiO₃), and presents high pressure in field-assisted sintering (FAST) as a robust methodology to obtain >100 nm BaTiO₃ compacts. Using FAST, two commercial ~50 nm powders were consolidated into compacts of varying densities and grain sizes. Microstructural inhomogeneities were investigated for each case, and an interpretation is developed using a modified Monte Carlo Potts (MCP) simulation. Two recurrent microstructural inhomogeneities are highlighted, heterogeneous grain growth and low-density regions, both ubiqutously present in all samples to varying degrees. In the worst cases, HGG presents an area coverage of 52%. Because HGG is sporadic but homogenous throughout a sample, the catalyst (e.g., the local segregation of species) must be, correspondingly, distributed in a homogenous manner. MCP demonstrates that in such a case, a large distance between nucleating abnormal grains is required—otherwise abnormal grains prematurely impinge on each other, and their size is not distinguishable from that of normal grains. Compacts sintered with a pressure of 300 MPa and temperatures of 900 °C, were 99.5% dense and had a grain size of 90±24 nm. These are unprecedented results for commercial BaTiO₃ powders or any starting powder of 50 nm particle size—other authors have used 16 nm lab-produced powder to obtain similar results.     

Influence of Pd-loading on gas sensing characteristics of SnO2 thick films

Nanocrystalline pristine and 0.5, 1.5 and 3.0 wt% Pd loaded SnO₂ were synthesized by a facile co-precipitation route. These powders were screen-printed on alumina substrates to form thick films to investigate their gas sensing properties. The crystal structure and morphology of different samples were characterized by using X-ray diffraction, scanning electron microscopy and transmission electron microscopy techniques. The 3.0 wt% Pd:SnO₂ showed response of 85% toward 100 ppm of LPG at operating temperature of 250 °C with fast response (8 s) and quick recovery time (24 s). The high response toward LPG on Pd loading can be attributed to lowering of crystallite size (9 nm) as well as the role of Pd particles in exhibiting spill-over mechanism on the SnO₂ surface. Also selectivity of 3.0 wt% Pd:SnO₂ toward LPG was confirmed by measuring its response to other reducing gases like acetone (CH₃COCH₃), ethanol (C₂H₅OH) and ammonia (NH₃) at optimum operating temperature.     

Densification behaviour and microstructural evolution of TiO2/CAS-incorporated alumina

Densification behaviour and microstructural evolution of TiO₂/CAS (CaO-Al₂O₃-SiO₂)-incorporated alumina has been investigated systematically. The experimental results show that TiO₂ can greatly enhance the densification rates of alumina but trigger severely abnormal grain growth (AGG) at 0.60 wt%. On the other hand, small amounts of CAS can effectively inhibit abnormal grain growth, but provoke an abnormal densification owing to the formations of in situ pores left by liquid phase. With additional 0.50 wt% of CAS incorporated into 0.60 wt% TiO₂-doped sample, the aspect-ratios of anisotropic growth grains increase by reduced growth rates of flat boundaries, accomplished by basal planes much smoother and straighter. This microstructure subsequently disappears when CAS incorporation is up to 4.0 wt%, substituted by that with low aspect-ratio and fine-equiaxed grains. Such occurrences are discussed in term of levels of liquid phase and 2-dimension (2-D) nucleation processes.     

Microstructure and high temperature 4-point bending creep of sol–gel derived mullite ceramics

Four-point bending creep behavior of mullite ceramics with monomodal and bimodal distribution of grain sizes was studied in the temperature range of 1320–1400 °C under the stresses between 40 and 160 MPa. Mullite ceramic with bimodal grain size distribution was prepared using aluminum nitrate nonahydrate as alumina precursor. When γ-Al₂O₃ or boehmite were used as alumina precursors, mullite grains are equiaxial with mean particle size of 0.6 μm for the former and 1.3 μm for the latter alumina precursor. The highest creep rate exhibited the sample with monomodal morphology and grains in size of 0.6 μm, which is about one order of magnitude greater than that for the monomodal morphology but with grains in size of 1.3 μm. The highest activation energy for creep (Q = 742 ± 33 kJ/mol) exhibits mullite with equiaxial grains of 1.3 μm, whereas for sample with smaller equiaxial grains the activation energy is much smaller and similar to mullite ceramics with bimodal grain morphology. Intergranular fracture is predominant near the tension surface, while transgranular more planar fracture is predominant near the compression surface zone.     

Preparation of meso-porous SnO2 fibers with enhanced sensitivity for n-butanol

Meso-porous SnO₂ fibers were synthesized using a solvothermal method with metaplexis fruit as the bio-template. The products were characterized by powder X-ray diffraction, high resolution scanning electron microscopy, transmission electron microscopy and nitrogen adsorption/desorption measurements. Results show that SnO₂ fibers present a high specific surface area of 73.665 m²/g and a meso-porous structure with the pore size of 7.821 nm, and the crystal size of SnO₂ is about 6.5 ± 0.5 nm. The gas sensing performance of the prepared SnO₂ fibers toward several volatile organic compounds was investigated. The results show that the meso-porous SnO₂ fibers were highly sensitive and selective to n-butanol.     

Surfactant-free microwave hydrothermal synthesis of SnO2 nanosheets as an anode material for lithium battery applications

SnO₂ nanosheets were synthesized using microwave hydrothermal method without using a surfactant and organic solvents. Formation of pure nanocrystalline rutile phase of SnO₂ sample was confirmed by X-ray diffraction (XRD) results and the average crystallite size of SnO₂ sample calculated using Scherrer's formula and XRD data is found to be 6 nm. HR-TEM, SAED and EDX results showed the formation of agglomerated nanosize sheets like morphology with high porous structured SnO₂ powder. Further, the formation of high porous structured SnO₂ powder was confirmed from BET surface area results (59.28 m² g^?1). The electrochemical performance of the lithium-ion battery made up of SnO₂ nanosheets, as an anode, was tested through the cyclic voltammetry and galvanostatic charge-discharge measurements. The galvanostatic charge-discharge results of the lithium-ion battery showed good discharge capacity of 257.8 mAh g^?1 after 50 cycles at a current density of 100 mA g^?1. The improved electrochemical properties may be due to the formation of a unique nanosize sheets type morphology with high porous structured SnO₂ powder. High porous structured nanosize sheets type morphology of SnO₂ can help to reduce the diffusion length and sustain the volume changes during the charging-discharging process.Hence, high porous structured nanosize sheets morphology of SnO₂ prepared using the microwave hydrothermal method without using a surfactant and organic solvents can be a better anode material for lithium ion battery applications.     

Nanocomposites of SnO2 and g-C3N4: Preparation,characterization and photocatalysis under visible LED irradiation

Tin dioxide nanoparticles were prepared in the presence of graphitized carbon nitride (g-C₃N₄) forming nanocomposites with different contents of SnO₂ up to 40 %. G-C₃N₄ was synthetized by heating of melamine at 550 °C in the open air and Sn²⁺ ions were precipitated by sodium hydroxide in g-C₃N₄ aqueous dispersions. Resulting mixtures were dried by freezing at ?20 °C and calcined at 450 °C to obtain SnO₂/g-C₃N₄ nanocomposites.The nanocomposites were characterized by common characterization methods in solid state and in their aqueous dispersions using dynamic light scattering (DLS) analysis and photocatalysis. SnO₂ nanoparticles in the nanocomposites were found to have an average size of 4 nm, however, those precipitated without g-C₃N₄ had an average size of 14 nm. Separation of photoinduced electron and holes via heterojunction between SnO₂ and g-C₃N₄ was demonstrated by photocatalytic decomposition of Rhodamine B (RhB) under LED visible irradiation (416 nm) and photocurrent measurements. The most photocatalytically active nanocomposite contained 10 % of SnO₂. Graphitized carbon nitride was assumed to serve as a template structure for the preparation of SnO₂ nanoparticles with a narrow size distribution without using any stabilizing additives.     

Texturing behaviours of (K0.47Na0.51Li0.02)(Nb0.8Ta0.2)O3 piezoelectric ceramics produced using NaNb1-xTaxO3 templates

Textured (K_0.47Na_0.51Li_0.02)(Nb_0.8Ta_0.2)O₃ (KNLNT20) piezoelectric ceramics were prepared using NaNb_1?xTa_xO₃ templates. The highest degree of grain orientation (97%) and piezoelectric constant (342 pC/N) were obtained upon adding 3 wt% of the NaNb_0.8Ta_0.2O₃ (NNT20) template and sintering at 1150 °C for 1 h. Back-scattered scanning electron micrographs of the textured KNLNT20 samples sintered at 1150 °C for 1 h indicated the presence of templates similar in size to the original ones within the cores of the textured grains. The peak value of the dielectric constant corresponding to the NNT20 core decreased after prolonged holding at 1150 °C, owing to a decrease in the size of the NNT20 core because of the interdiffusion of K, Na, and Li ions between the NNT20 core and KNLNT20 shell. This interdiffusion also decreased the piezoelectric constant, d₃₃ value of the textured KNLNT20 samples by inducing a change in the chemical composition of the shell region.     

Sub-micrometre grained UO2 pellets consolidated from sol gel beads using spark plasma sintering (SPS)

UO₂ beads from the sol supported precipitation method were calcined at a low temperature in order to obtain porous micro-beads, composed of nanometric particles. The sintering behaviour of the beads in spark plasma sintering was investigated. The powder had a good sinterability and the final grain size of the pellets could be tailored by varying the processing conditions, in order to resemble the microstructure of the traditionally fabricated UO₂ pellets (i.e. grains of several µm size), or to achieve sub-micrometre size as observed in the high burnup structure. Dense UO₂ pellets with a grain size as small as 300 nm were obtained by sintering at 835 °C without dwell time, whereas 3 µm grained pellets were obtained at 1000 °C and a 5 min dwell time.     

Strengthening and toughening of TiN-based and TiB2-based ceramic tool materials with HfC additive

Effects of HfC addition on the microstructures and mechanical properties of TiN-based and TiB₂-based ceramic tool materials have been investigated. Their pore number decreased gradually and relative densities increased progressively when the HfC content increased from 15 wt% to 25 wt%. The achieved high relative densities to some extent derived from the high sintering pressure and the metal phases. HfC grains of about 1 µm evenly dispersed in these materials. Both TiN and TiB₂ grains become smaller with increasing HfC content from 15 wt% to 25 wt%, which indicated that HfC additive can inhibit TiN grain and TiB₂ grain growth, leading to the formation of a fine microstructure advantageous to improve flexural strength. Especially, TiB₂-HfC ceramics exhibited the typical core-rim structure that can enhance flexural strength and fracture toughness. The toughening mechanisms of TiB₂-HfC ceramics mainly included the pullout of HfC grain, crack deflection, crack bridging, transgranular fracture and the core-rim structure, while the toughening mechanisms of TiN-HfC ceramics mainly included pullout of HfC grain, fine grain, crack deflection and crack bridging. Besides, HfC hardness had an important influence on the hardness of these materials. Higher HfC content increased Vickers hardness of TiN-HfC composite, but lowered Vickers hardness of TiB₂-HfC composite, being HfC hardness higher than for TiN while HfC hardness is lower than for TiB₂. The decrease of fracture toughness of TiN-HfC ceramic tool materials with the increase of HfC content was attributed to the formation of a weaker interface strength.     

Effect of milling type on the microstructural and mechanical properties of W-Ni-ZrC-Y2O3 composites

This study reports the effect of milling type on the microstructural, physical and mechanical properties of the W-Ni-ZrC-Y₂O₃ composites. Powder blends having the composition of W-1 wt% Ni-2 wt% ZrC-1 wt% Y₂O₃ were milled at room temperature for 12 h using a Spex™ 8000D Mixer/Mill or cryomilled in the presence of externally circulated liquid nitrogen for 10 min using a Spex™ 6870 Freezer/Mill or sequentially milled at room temperature and cryogenic condition. Then, powders were compacted in a hydraulic press under a uniaxial pressure of 400 MPa and green bodies were sintered at 1400 °C for 1 h under Ar/H₂ atmosphere. Phase and microstructural characterization of the milled powders and sintered samples were performed using X-ray diffractometer (XRD), TOPAS software, scanning electron microscope/energy dispersive spectrometer (SEM/EDS), X-ray fluorescence (XRF) spectrometer and particle size analyzer (PSA). Archimedes density and Vickers microhardness measurements, and sliding wear tests were also conducted on the sintered samples. The results showed that sequential milling enables the lowest average particle size (214.90 nm) and it is effective in inhibiting W grain coarsening during sintering. The cryomilled and sintered composite yielded a lower hardness value (5.80±0.23 GPa) and higher wear volume loss value (149.42 µm³) than that of the sintered sample after room temperature milling (6.66±0.39 GPa; 102.50 µm³). However, the sequentially milled and sintered sample had the highest relative density and microhardness values of 95.09% and 7.16±0.59 GPa and the lowest wear volume loss value of 66.0 µm³.     

Ceramic nanoparticles addition in pure copper plate: FSP approach,microstructure evolution and texture study using EBSD

In this study, the effect of ceramic nanoparticles addition on the microstructure and texture of friction stir processed (FSP) copper has been investigated. For this purpose, two pure copper plates with and without Al₂O₃ nanoparticles were FSPed at rotational speed of 800 rpm and traverse speed of 100 mm min^?1. Electron back scattered diffraction (EBSD) technique was employed in order to study the microstructure and texture of the fabricated samples. Based on the obtained results, considerable grain refinement by dynamic recrystallization (DRX) mechanism was observed in both specimens. However, Al₂O₃ inset led to evolution of ultrafine grained (UFG) structure with an average grain size of 0.7 µm. In addition, Al₂O₃ addition caused formation of lower twin boundaries and stronger texture components compared with the sample without ceramic reinforcements. The presence of nanoparticles increased the proportion of the continuous DRX mechanism (CDRX) compared to the discontinuous mechanism (DDRX) during grain structure formation.