Microwave and conventional treatment of low-cement high-alumina castables with different water-to-cement ratio; Part II. Dehydration

Modern refractory castables contain between 3.5 and 5?wt.-% water that is necessary for sufficient flow during emplacement and for the formation of hydrate phases, necessary for the green strength of the material. Prior to the high temperature use of this material, it must be dried very carefully to avoid explosive spalling.This paper will demonstrate that beside conventional drying of pre-shaped materials in resistance furnaces microwave radiation is an energy saving and rapid method to remove pore water as well as hydrate bond water from the castable. In comparison to resistance furnaces, the use of microwave radiation does not affect the castable properties as there are mechanical strength (MOR, CCS), open porosity and pore size distribution. This study proved microwave radiation as valuable alternative with a series of tabular alumina based low cement castables (LLC) in which the water-to-cement-ratio (wcr = 0.64, 0.75, 0.82 and 1.13) was systematically altered by changing the cement concentration at constant mixing water concentration of 4.5%.     

The effect of nano-size additives on the electrical conductivity of matrix suspension and properties of self-flowing low-cement high alumina refractory castables

Influence of two nano-size additives on electrical properties of suspension matrix of self-flowing low-cement high alumina refractory castable is investigated. For this purpose, castament FS 10 and FS 20 on the basis of polycarboxylate ether were considered. The self-flow value, workability and mechanical strength of the castable are evaluated and their relations with electrical conductivity are determined. Using these relations, the type and optimum amount of proper additive for these refractory castables are determined. It was shown that if the electrical conductivity of matrix suspension is less than 0.71 mS/cm, high alumina low-cement self-flowing refractory castable can be obtained. The best self-flow, sufficient working time and adequate mechanical strength in the castables are obtained with 0.08 wt.% FS 20.     

The relationship between electric processing condition and microstructure in the solidification of multicomponent oxides

Systematic characterization for the relationship between the electropulse processing conditions and microstructure has been carried out in the present work. It is found that electropulse can refine porosity and alter grain morphology considerably. A processing with optimum electropulsing parameters can reduce over 75% volume fraction of porosity and more than 70% average pore diameter in comparison with that of reference sample without electric treatment. Electric current treatment promotes the growth of dendrites with smaller thickness of primary arms dendrite and prevents the liquid entrapping between the growing solid grains. The former is caused by the effect of electricity-enhanced kinetic mobility on the radius of curvature at the tip of dendrite. The latter is attributed to the effects of electric thermodynamics on the microstructural formation such as the enhanced connectivity of conductive phase. The microstructures obtained by electropulsing treatment are favourable for heat conduction, structural strength and crack prohibition. However, the excess pulse frequency and pulse width can generate unwanted heat to counteract electric effect. The research reveals the relationship between electric processing conditions and microstructure in a perfectly controversial solidification condition in oxide materials to that of the metals and alloys. The results confirm from the opposite side the validity of the pulsated solidification.     

Investigations of SiC aggregates oxidation: Influence on SiC castables refractories life time at high temperature

This paper reports results concerning the oxidation of silicon carbide of pure SiC powder (98% of SiC) and the oxidation of two silicon carbide castables, used in waste-to-energy plants (WTE), and containing 60% and 85% of SiC, respectively. The investigated temperature range (800-1200 °C) corresponds to typical service conditions in WTE. Thanks to thermogravimetric and thermal expansion tests, kinetics of oxidation of the powder of SiC and of the castables has been investigated. According to these tests, several important points have been underlined. Firstly, the oxidation of SiC aggregates has a high influence on the thermal expansion and on the weight gain of SiC castables. Secondly, the grain size distribution of SiC aggregates within castables plays a dominating role (especially enhanced for the fine particles) in castable expansion behaviour induced by oxidation. This is a key point according to the evolution of thermal expansion of such materials.     

Characterization of IT-SOFC non-symmetrical anode sintered through conventional furnace and microwave

This work investigated the mechanical and electrical properties of NiO–SDC/SDC anode sintered by two different methods: in a microwave at about 1200 °C for 1 h and in a conventional furnace at 1200 °C with a holding time of 1 h (total sintering time of 21 h). Nano-powders Sm_0.2Ce_0.8O_1.9 (SDC) and NiO were mixed using a high-energy ball mill, followed by the co-pressing technique at a compaction pressure of 400 MPa. No binder was used between the layers. The electrical behaviors of all sintered samples were studied using electrochemical impedance spectra in the frequency range of 0.01–10⁵ Hz under 97% H₂–3% H₂O, an amplitude of 10 mV, and at high temperature range of 600–800 °C. Results indicate that the non-symmetrical NiO–SDC/SDC anode achieved through microwave sintering has finer grain size and higher electrochemical performance. However, hardness and Young׳s modulus increased in the samples sintered through a conventional furnace.     

TBA-based freeze/gel casting of porous hydroxyapatite scaffolds

Porous ceramic scaffolds with a controlled “designer” pore structure have been prepared by the freeze/gel casting route using a TBA-based hydroxyapatite slurry system with 20–40 wt.% solid content. The products were characterized in terms of sintered microstructure, together with physical and mechanical properties. After sintering at 1050–1250 °C, the advantages of freeze casting and gel casting appeared in the pore structure and compressive strength of the ceramics, i.e., unidirectional aligned macro-pore channels developed by controlling the solidification direction of the TBA solvent used in the freeze casting together with small diameter (micron sized) isolated pores formed in the dense outer walls of the pore channels when processed by gel casting. The sintered porosity and pore size generally resulted in a high solid loading giving low porosity and small pore size, this leading to higher compressive strengths. The scaffolds obtained exhibited an average porosity and compressive strength in the range 41.9–79.3% and 35.1–2.7 MPa, respectively, depending on the processing conditions used.     

Pore-forming agent induced microstructure evolution of freeze casted hydroxyapatite

There were interconnected small lamellar pores, big spherical pores and ceramic walls in the hydroxyapatite (HAP) ceramics fabricated by a freeze casting/pore-former method. As keeping the content of polymethyl methacrylate (PMMA) constant and decreasing the size of PMMA, the size of spherical pores and length of ceramic wall both decreased, and the compressive strength increased. As keeping the size of PMMA and decreasing the content of PMMA, the open porosity decreased and compressive strength increased. The shapes of pores caused by ice crystals were reticular, lamellar and treelike, in turn. The HAP ceramics with the spherical pores of 150-250 μm, open porosity of 62.13% and compressive strength of 7.01 MPa are prospective to have biomedical application.     

Enhanced performance of Fe2O3 doped yttria stabilized zirconia hollow fiber membranes for water treatment

Fe₂O₃ powders were introduced as sintering aid to fabricate yttria-stabilized zirconia (YSZ) hollow fiber membranes using a combined wet-spinning and post-sintering method. The obtained Fe₂O₃-YSZ hollow fiber membranes show enhanced performance for water treatment with fine crystal structure in terms of bending strength and pure water permeability. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and thermogravimetric analysis (TGA) along with mechanical tests were employed to investigate the structural evolution in the sintering process and the effect of Fe₂O₃. It is suggested that the Fe₂O₃ dopants dissolve into YSZ at elevated temperatures, providing defect sites and vacancies for fast ion migration, favoring for densification and grain growth of the YSZ, which yields dense microstructures of fine crystallites at relatively low sintering temperature. The Fe₂O₃-YSZ hollow fiber membranes sintered at 1150 °C show a 3-fold increase of the permeate flux of pure water (F) (743 L m⁻² h⁻¹) along with comparable bending strength (152 MPa) compared to pure YSZ membranes. This modified method can reduce sintering costs and therefore fabrication costs which should pave the way for scale-up production for ceramic hollow fiber membranes.     

Acidic and basic binders for magnesite based aggregate in plaster of tundish

Binders are generally inorganic, organic or organomineral and have an important influence on the performance and corrosion resistance of slag line and deskulling. Since silicate and phosphate binders have some side effects, in this work sulphate binders such as sulphamic acid, H₂NSO₃H; aluminum sulphate, Al₂(SO₄)₃; ammonium sulphate, (NH₄)₂SO₄; magnesium sulphate, MgSO₄; calcium sulphate, CaSO₄; sodium sulphate, Na₂SO₄; and potassium sulphate, K₂SO₄, are investigated. Cold crushing strength at different heat treatments of room temperature, 110 °C, 1100 °C, 1400 °C is measured. Apparent porosity of samples without pulp and bulk density together with pH of the binder solution is evaluated and XRD and SEM studies are performed. Among these sulphate binders MgSO₄ was found to be the best. It is acidic in nature and develops strong bonds to the basic aggregate, MgO, at low temperatures. At high temperatures it dissociates from MgO(s) and SO₃(g) and the remained portion of MgO is the same as host oxide, with no corrosion and easy deskulling. Basic binders such as calcium sulphate, sodium sulphate and potassium sulphate could not strongly bond the MgO aggregates.     

Processing and in vitro bioactivity of high-strength 45S5 glass-ceramic scaffolds for bone regeneration

In this paper, the compressive strength and in vitro bioactivity of sintered 45S5 bioactive glass scaffolds produced by powder technology and polymer foaming were investigated. The sintering temperature of scaffolds was 975 °C. The characterization of scaffolds before immersion in SBF was performed by scanning electron microscopy (SEM) and microtomography (μCT). The scaffolds were also tested for compression, and their density and porosity were measured. After immersion, the samples were observed through SEM and analyzed using EDS, X-ray diffraction (XRD), and infrared spectroscopy (FT-IR). Mass variation was also estimated. The glass-ceramic scaffolds showed a 61.44±3.13% interconnected porosity and an average compressive strength of 13.78±2.43 MPa. They also showed the formation of a hydroxyapatite layer after seven days of immersion in SBF, demonstrating that partial crystallization during sintering did not suppress their bioactivity.     

Microwave hydrothermal synthesis of Sr doped ZnO crystallites with enhanced photocatalytic properties

Pure and Sr²⁺ doped ZnO crystallites were successfully synthesized via a microwave hydrothermal method using Zn(NO₃)₂·6H₂O and Sr(NO₃)₂·6H₂O as source materials. The phase and microstructure of the as-prepared Zn_1−xSr_xO crystallites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Ultraviolet–visible spectrum (UV–vis) and photochemical reaction instrument were used to analyze the photocatalytic properties of the particles. XRD results show that the diffraction peaks of the as-prepared Zn_1−xSr_xO crystallites shifted slightly toward lower 2θ angle with the increasing of Sr²⁺ doping concentration from 0% to 0.3%. The pure ZnO crystallites with lamellar structure are found transforming to a hexagonal columnar morphology with the increase of Sr²⁺ doping concentration. UV–vis analysis shows that the particles have a higher absorption in UV region with a slightly decreased of optical band (E_g) gap. The photocatalytic activity of Sr²⁺ doped ZnO crystallites was evaluated by the Rhodamine B (RhB) degradation in aqueous solution under visible-light irradiation. Compared with the pure ZnO particles, the photocatalytic properties of the Sr²⁺ doped ZnO crystallites are obviously improved. The photocatalysis experiment results demonstrate that the 0.1% Sr²⁺ doped ZnO exhibits the best photocatalytic activity for the degradation of Rhodamine B.     

Heat treatment-improved bond strength of resin cement to lithium disilicate dental glass-ceramic

This study investigated the influence of different hydrofluoric acid (HF) concentrations and heat treatments applied to a lithium disilicate dental glass-ceramic (EMX) on surface morphology and micro-shear bond strength (μSBS) to resin cement. Five HF concentrations (1%, 2.5%, 5%, 7.5% and 10%) and four different heat treatments applied before etching were assessed: 1. etching at room temperature with no previous heat treatment (control group); 2. HF stored at 70 °C for 1 min applied to the ceramic surface at room temperature; 3. HF at room temperature applied after a hot air stream is applied perpendicularly to the ceramic surface for 1 min; 4. the combination of previously heated HF and heated EMX surface. The etching time was fixed for 20 s for all groups. Etched EMX specimens were analyzed on field-emission scanning electron microscope (FE-SEM) and the μSBS was carried out on a universal testing machine at a crosshead speed of 1 mm/min until fracture. For the control groups, FE-SEM images showed greater glassy matrix dissolution and higher μSBS for 7.5% and 10% HF concentrations. The previous heat treatments enhanced the glassy matrix dissolution more evidently for 1%, 2.5% and 5% and yielded increased μSBS values, which were not statistically different for 7.5% and 10% HF concentrations (control group). HF concentrations and previous heat treatments did show to have an influence on the etching/bonding characteristics to lithium disilicate dental glass-ceramic.     

Fabrication of corrosion resistant, bioactive and antibacterial silver substituted hydroxyapatite/titania composite coating on Cp Ti

The present work is aimed at developing a bioactive, corrosion resistant and anti bacterial nanostructured silver substituted hydroxyapatite/titania (AgHA/TiO₂) composite coating in a single step on commercially pure titanium (Cp Ti) by plasma electrolytic processing (PEP) technique. For this purpose 2.5 wt% silver substituted hydroxyapatite (AgHA) nanoparticles were prepared by microwave processing technique and were characterized by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy and transmission electron microscopy (TEM) methods. The as-synthesized AgHA particles with particle length ranging from 60 to 70 nm and width ranging from 15 to 20 nm were used for the subsequent development of coating on Cp Ti. The PEP treated Cp Ti showed both titania and AgHA in its coating and exhibited an improved corrosion resistance in 7.4 pH simulated body fluid (SBF) and 4.5 pH osteoclast bioresorbable conditions compared to untreated Cp Ti. The in vitro bioactivity test conducted under Kokubo SBF conditions indicated an enhanced apatite forming ability of PEP treated Cp Ti surface compared to that of the untreated Cp Ti. The Kirby-Bauer disc diffusion method or antibiotic sensitivity test conducted with the test organisms of Escherichia coli (E. coli) for 24 h showed a significant zone of inhibition for PEP treated Cp Ti compared to untreated Cp Ti.     

Microwave assisted sintering of CaCu3Ti4O12

CaCu₃Ti₄O₁₂ electroceramic was prepared by a microwave assisted solid-state reaction technique from CaCO₃, CuO and TiO₂ powders. Processing involved the preparation of raw material, mixing and milling, calcination, pellet forming and sintering processes. Conventional furnace and microwave assisted sintering processes were employed in order to improve phase structures, morphology and dielectric properties of CaCu₃Ti₄O₁₂ ceramics. Surface and fracture FESEM analysis showed that the microwave assisted sintered CaCu₃Ti₄O₁₂ produced better densification and more uniform grain size compared to the conventional sintered sample.     

XPS and optical studies of different morphologies of ZnO nanostructures prepared by microwave methods

Zinc oxide (ZnO) nanostructures of various morphologies were prepared using a microwave-assisted aqueous solution method. Herein, a comparative study between three different morphologies of ZnO nanostructures, namely nanoparticles (NPs), nanoflowers (NFs) and nanorods (NRs) has been reviewed and presented. The morphologies of the prepared powders have been studied using field effect scanning electron microscopy (FESEM). X-ray diffraction (XRD) results prove that ZnO nanorods have biggest crystallite size compared with nanoflowers and nanoparticles. The texture coefficient (T_c) of three morphologies has been calculated. The T_c changed with varying morphology. A comparative study of surfaces of NPs, NFs and NRs were investigated using X-ray photoelectron spectroscopy (XPS). The possible growth mechanisms of ZnO NPs, NFs and NRs have been described. The optical properties of the ZnO nanostructures of various morphologies have been investigated and showed that the biggest crystallite size of ZnO nanostructures has lowest band gap energy. The obtained results are in agreement with experimental and theoretical data of other researchers.     

Microwave-induced small size effect of (Ba,Sr)3MgSi2O8:0.06Eu, 0.1Mn phosphor for 660 nm-featured bio-lighting

The 660 nm-featured (Ba, Sr) ₃MgSi₂O₈:0.06Eu^{2 +}, 0.1Mn²⁺(AMS-EM) phosphor in violet for red/blue bio-lighting LEDs was prepared by 2.45 GHz microwave (MW) high temperature firing procedure. The phase-pure host phase, (Ba, Sr) ₃MgSi₂O₈, was formed to be responsible for simultaneous red band emission from Mn ion and blue band emission from Eu ion, while the formation of an impurity phase of Sr₂SiO₄ responsible for 505 nm-peaked green band emission for Eu ion was effectively suppressed owing to MW fast-heating procedure. Small sized and agglomeration-free phosphor particles were either observed, which was probably resulted from suppressing the grain growth in as-formed host particles, compared with conventional high-temp solid state (SS) reaction firing procedure. These results indicate that high-temp MW firing procedure is suitable for preparing this simultaneously red- and blue-emitting AMS-EM phosphor in the application of bio-lighting for plant cultivation.     

Dispersion studies of La substitution on dielectric and ferroelectric properties of multiferroic BiFeO3 ceramic

Lanthanum La-substituted multiferroic Bi_1−xLa_xFeO₃ ceramics with x = 0.0, 0.05, 0.10, 0.15, 0.20 and 0.25 have been prepared by solution combustion method. The effect of La substitution for the dispersion studies on dielectric and ferroelectric properties of Bi_1−xLa_xFeO₃ samples have been studied by performing x-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), density, dc resistivity and dielectric measurements as well as characterizing the polarization-field hysteresis loop. The results of prepared samples are compared with those of bismuth ferrite (BiFeO₃). In the measuring frequency of 10 KHz to 1 MHz, the dielectric constants and dielectric losses for samples x = 0.20, 0.25 are almost stable and exhibited lowest dielectric loss close to 0.1. The resistivity of Bi_1−xLa_xFeO₃ samples reaches a maximum value of 10⁹ ohm-cm, which is about three times higher than that for pure BiFeO₃. The results also show that stabilization of crystal structure and nonuniformity in spin cycloid structure by La substitution enhances the resistivity, dielectric and ferroelectric properties. Furthermore, the substitution of rare earth La for Bi helps to eliminate the impurity phase in BiFeO₃ ceramic.     

Microstructure and phase evolution of alumina-spinel self-flowing refractory castables containing nano-alumina particles

The microstructure and phase composition of alumina-spinel self-flowing refractory castables added with nano-alumina particles at different temperatures are investigated. The physical and mechanical properties of these refractory castables are studied. The results show that the addition of nano-alumina has a great effect on the physical and mechanical properties of these refractory castables. With the increase of nano-alumina content in the castable composition, the mechanical strength is considerably increased at various temperatures. It is shown that nano-alumina particles can affect formed phases after firing. The platy crystals of CA₆ are detected inside the grain boundaries of tabular alumina and spinel grains in samples fired at 1500 °C. CA₆ phase can be formed at lower temperatures (1300 °C) with the addition of nano-alumina particles. As a result of using nanometer-sized alumina particles with high surface area, the solid phase sintering of the nano-sized particles and CA₆ formation can occur at lower temperatures.     

Sinterability,microstructure and compressive strength of porous glass-ceramics from metallurgical silicon slag and waste glass

Porous glass-ceramics have been prepared by the direct sintering of powder mixtures of metallurgical silicon slag and waste glass. The thermal behavior of silicon slag was examined by differential thermal analysis and thermogravimetry to clarify the foaming mechanism of porous glass-ceramics. The mass loss of silicon slag below 700 °C was attributed to the oxidation of amorphous carbon from residual metallurgical coke in the silicon slag, and the mass gain above 800 °C to the passive oxidation of silicon carbide. The porosity of sintered glass-ceramics was characterized in terms of the apparent density and pore size. By simply adjusting the content of waste glass and sintering parameters (i.e. temperature, time and heating rate), the apparent density changed from 0.4 g/cm³ to 0.5 g/cm³, and the pore size from 0.7 mm to 1.4 mm. In addition to the existing crystalline phases in the silicon slag, the gehlenite phase appeared in the sintered glass-ceramics. The compressive strength of porous glass-ceramics firstly increased and then decreased with the sintering temperature, reaching a maximal value of 1.8 MPa at 750 °C. The mechanical strength was primarily influenced by the crystallinity of glass-ceramics and the interfaces between the crystalline phases and the glassy matrix. These sintered porous glass-ceramics exhibit superior properties such as light-weight, heat-insulation and sound-absorption, and could found their potential applications in the construction decoration.