Structural and electrical properties of magnetic ceramics of Co1−xCuxFe2O4 spinel nanoferrites

Abstract

Magnetic ceramics of the type of spinel nanoferrites of Co_1?xCu_xFe₂O₄ with Cu concentrations of x?=?0·00, 0·25, 0·50, 0·75 and 1·00 were prepared by chemical co-precipitation method. X-ray diffraction results confirmed the formation of a single spinel ferrite structure with crystallite size in the range of 20–63 nm. Scanning electron microscopy and EDS were used to study the morphological and compositional changes taking place with varying Cu concentration. DC electrical resistivity, activation energy and drift mobility are strongly influenced by both Cu concentration and temperature. Resistivities of the prepared magnetic ceramics were found to decrease with the increase in Cu concentration to follow Verwey mechanism. The semiconductor behaviour of the prepared nanoparticles was confirmed from the standard Arrhenius relation of resistivity versus temperature. The dielectric constants were measured in the frequency range of 100 Hz–3·0 MHz and are explained by the Maxwell–Wagner interfacial type of polarisation.     

Blends of polylactic acid and arylated soy protein isolate with triacetine as plasticiser

Abstract

Blends of polylactic acid (PLA) and arylated soy protein isolate (ASPI) were successfully prepared by the extrusion process followed by injection moulding. To improve the interfacial adhesion between PLA and ASPI powder, mandelic acid as an amphiphilic additive was incorporated. Rheological and thermal characterisations of PLA/ASPI blends were performed on rheometer, differential scanning calorimeter and thermogravimetric analyser. Thermomechanical characterisation of PLA/ASPI blends was carried out on a dynamic mechanical analyser. Tensile and flexural modulus of PLA/ASPI blends increased compared to neat PLA. Different amounts (5–15 wt-% wrt PLA/ASPI blends) of plasticiser was added to PLA/ASPI blends containing 2·5 wt-% of ASPI. Results indicated that at 10 wt-% of plasticiser, PLA/ASPI blends showed maximum tensile strength of ～8·8 MPa as well as appreciable elongation at break. Morphological studies of PLA/ASPI blends at different amounts of plasticiser were also carried out by scanning electron microscope.     

Elaboration and characterisation of low cost ceramic support membrane

Abstract

A porous tubular ceramic membrane was prepared from low cost Tunisian clay. The characterisation of the raw material and the effect of the sintering temperature on the morphology, pores size distribution and the mechanical properties of the ceramic membrane were studied. A ceramic membrane fired at 1000°C for 1?h presented a mean pore diameter of ～1·04?μm. The porosity was equal to 38?vol.-%. The filtration of a 0·5?g?L^?1 bovine serum albumin solution indicated that the limiting flux of permeate was 245?L?h^?1?m^?2?bar^?1, which corresponded to a retention rate of about 13%.     

Cylindrical Al2O3 /TZP functionally graded materials by EPD

Abstract

Cylindrical Al₂O₃/Ce–TZP functionally graded com posites were fabricated by electrophoretic deposition and pressureless sintering in air. Negatively charged Ce–TZP and Al₂O₃ powders were deposited on a cylindrical electrode from an acetone based suspension. A continuous composition change is realised by chang ing the composition of the suspension. Sintering of the FGM at different temperatures, showed that a temper ature higher than 1550°C was necessary for densification. The resultant FGM cylinder with 5·6 mm dia. had an ideal mechanical structure: a central open channel, a tough Ce–ZrO₂ core (K_Ic > 10 MPa m^1/2), a continuous gradient interlayer, and a hard surface layer with HV higher than 15 GPa.     

New BST–silica suspension coating material for dielectric thin films fabricated at low temperatures

Abstract

A new Ba_0·7Sr_0·3TiO₃ (BST)–silica suspension has been developed as a coating material for dielectric thin films. Using a spin on glass (SOG) wet process, a thin layer consisting of nanoparticles of BST and silica was formed on an Al/SiO₂/Si wafer. The BST–silica suspension was made by mixing a BST nanoparticle dispersion with methyl siloxane oligomer. A silicon wafer was coated with the BST–silica suspension using a spin coater and heat treated. The sample was then coated with methyl siloxane oligomer and heat treated again, and finally, a top electrode was applied. The Al/SiO₂/BST–SiO₂/Al/SiO₂/Si stack thus prepared is a metal insulator metal (MIM) capacitor. The electrical properties of the MIM capacitor were evaluated and its capacitance, dissipation factor and voltage coefficient of capacitance were determined to be 1054 pF mm^?2, <0·1 and <100 ppm V^?1 respectively.     

Phase structure and electrical properties of lead free Na0·5Bi0·5TiO3–CaTiO3 ceramics

Abstract

Abstract

(1?x)Na_0·5Bi_0·5TiO_3?xCaTiO₃ ceramics with x?=?0–0·2 were prepared by solid state sintering method. Structural and morphology studies carried out by X-ray diffraction and scanning electron microscopy indicate the change in crystal structure from rhombohedral to orthorhombic symmetry (R3C to Pnma). The morphotropic phase boundary of this system was found to lie around x?=?0·08–0·14, where the orthorhombic and rhombohedral symmetries coexist. The orthorhombic phase is stabilised for x>0·14, indicating that the rhombohedral phase of Na_0·5Bi_0·5TiO₃ is susceptible to orthorhombic distortion brought about by Ca substitution. Calcium substitution in Na_0·5Bi_0·5TiO₃ caused an obvious decrease in peak temperature and a decrease in relative permittivity. The compositional variation of the fundamental dielectric behaviour is discussed in relation to the crystal chemistry of the system. The highest piezoelectric constant d₃₃ of 85 pC N^?1 is achieved for x?=?0·1, with the coercive field of 18 kV cm^?1 and the dielectric maximum temperature of 148°C.     

Properties,morphology and structure of BPDA/PPD/ODA polyimide fibres

Abstract

A family of random co-poly(amic acid)s containing 4,4′-oxydianiline (ODA) moiety were synthesised in N,N′-dimethylacetamide. The co-poly(amic acid) solutions were used as spinning dope for dry jet wet spinning process into as spun poly(amic acid) (PAA) fibres. The polyimide (PI) fibres were obtained from PAA fibres after being imidised and drawn in furnace. The processability and mechanical properties of the fibres were notably improved by incorporating ODA into 3,3′,4,4′-biphenyltetracarboxylic dianhydride/p-phenylenediamine (BPDA/PPD) backbone. The best strength and modulus of BPDA/PPD/ODA PI fibre (diamine mole ratio of PPD/ODA?=?85∶15) attained 2·25 and 96·5 GPa respectively, which were approximately three times the tenacity of the BPDA/PPD PI fibre. The SEM image showed that the cross-section of each stage fibres was round and void free. In addition, ‘skin–core’ and microfibrillar structure were not observed. The thermal properties of PI fibres were also investigated. The results showed that the PI fibres have excellent thermal stability; moreover, the dimensional stability and structural homogeneity of the fibres were significantly improved by heat drawn stage. T_g was found to be ～290°C by thermomechanical and dynamic mechanical analyses. The X-ray (wide angle X-ray diffraction and small angle X-ray scattering) experiments indicated that the ordering degree of longitudinal and lateral stacks, as well as the molecular orientation of PI fibre, was improved in the preparation process of fibres. Furthermore, the mechanical properties of fibres are profoundly affected by the heat drawn conditions.     

Impact of crystallisation processes on depth profile formation in sol–gel PbZr0·52Ti0·48O3 thin films

Abstract

Abstract

This study revealed the influence of crystallisation processes on the homogeneity of the sol–gel PbZr_0·52Ti_0·48O₃ thin films, allowing identification and further optimisation of thin film performance. Crystallisation processes determine the optical gradient appearance, irrespective of the chemical solvents used in this work. X-ray diffraction analysis showed that a refractive index gradient was apparent in the samples which had dominant (001)/(100) orientation and significant change of lattice parameters with thickness.     

Pyrolysis of polyborosilazane and its conversion into SiBN ceramic

Abstract

The polyborosilazane was prepared by coammonolysis of boron trichloride and methyldichlorosilane with hexamethyldisilazane. The decarburisation process of the polyborosilazane under ammonia was studied. The results suggested carbon was effectively removed under ammonia and the removal of carbon mainly occurred during the temperature range of 400–600°C. After pyrolysed at 900°C under ammonia, the carbon content was only 0·29 wt-%. Further heat treatment at 1500°C under argon yielded SiBN ceramic. The structure and morphological properties of SiBN ceramic were studied by solid ²⁹Si nuclear magnetic resonance, X-ray diffraction and scanning electron microscopy. The results showed that the SiBN ceramic was amorphous and possessed a smooth surface. The contents of boron, silicon and nitrogen in the SiBN ceramic are 10·9, 42·6 and 38·48 wt-% respectively.     

Crystallisation kinetics of hydroxyapatite thin films prepared by sol-gel process

Abstract

Abstract

In this study, the crystallisation of nano hydroxyapatite (HA) films on stainless steel 316L was studied. The film was prepared by sol-gel technique. The process was started with preparation of an HA sol. After aging of the sol at room temperature, a stainless steel 316L substrate was dip coated and then was heat treated from 350 to 450°C at different periods of time in air. The crystallisation behaviour and the transformation-temperature-time diagram of HA films were achieved and analysed using the avrami equation. The results showed that the crystallisation of HA began at 250°C and was increased up to 450°C. The obtained HA film showed a nanostructure character with a suitable crystalinity after heat treatment.     

Preparation and properties of low friction silicon carbide ceramic seals by pore creation

Abstract

Low friction silicon carbide (SiC) ceramic seals were prepared by combining liquid phase sintering (LPS) with pore creation, the effects of polyethylene oxide (PEO) as pore former on the sintering and mechanical properties, microstructure, and friction properties of LPS–SiC ceramic seals were investigated. The PEO firstly forms cross-linking aggregates with loose structure in SiC granules, which uniformly creates macropores on the surface and inside of LPS–SiC ceramic seals during the sintering. The addition of PEO decreases the sintering and mechanical properties of LPS–SiC ceramic seals obviously, while significantly reduces the dry friction coefficient of LPS–SiC ceramic seals. When the PEO content increases from 0 to 15 wt-%, the dry friction coefficient drops from 0·403 to 0·126. The pore creation of PEO improves the friction performances of SiC ceramic seals in sliding wear applications.     

Fabrication of aluminium titanate castable

Abstract

A castable was made using an aggregate of aluminium titanate (AT) of composition 95%Al_1·57 Fe_0·43 TiO₅ , 2% corundum, and 3% aluminosilicate glass with 10% high alumina cement, deflocculant, and water. Cast blocks after firing at 1400°C for 6 h were used to determine physical and mechanical properties at ambient temperature. The castables had a crushing strength of 125 MPa, a bend strength of 14 MPa, and a Young's modulus of 9·6 GPa. They had a coefficient of thermal expansion (CTE) of 1·3 × 10^-6 K^-1 from ambient temperature to 1200°C. The low Young's modulus and CTE indicate that the castable should have excellent thermal shock resistance. The properties of the AT castable when compared with those of commercial high alumina castables showed it to have a similar strength, but much lower porosity (10%), Young—s modulus, and CTE. Phase analysis of the cast blocks showed that the major phase was iron aluminium titanate with minor phases (<10%) of corundum and a calcium aluminosilicate, which is probably a glass.     

Influence of phosphorus pentoxide on crystallisation,microstructure and mechanical properties of potassium fluorophlogopite glass ceramics

Abstract

Potassium fluorophlogopite glass ceramics were prepared. Differential thermal analysis showed that there were two exothermic peaks at ～750 and 950°C, corresponding to the Avrami exponent ～1·5 and 2·0. Spherical particles were found using a scanning electron microscope when the glass ceramics were heat treated at 750 and 800°C. With the increase in heat treatment temperature, a kind of lamellar crystals was gradually formed, and the shape of crystals changed from spherical to lamellar when the Avrami exponent changed from 1·5 to 2·0. Mechanical tests showed that material cutting and bending strength increased, while the Vickers hardness decreased with the increasing of P₂O₅ content and heat treatment temperature.     

Effects of sintering temperature on properties of Na0·5K0·5NbO3-LiSbO3-BiFeO3 lead free ceramics

Abstract

Abstract

Lead free 0·95K_0·5Na_0·5NbO₃-0·05LiSbO₃ (KNN-LS) ceramics doped with 0·4?mol.-% BiFeO₃ (BF) have been prepared by the conventional mixed oxide method with sintering temperature at 1065-1135°C in this paper. The samples are characterised by X-ray diffraction analysis and scanning electron microscopy. The dielectric and piezoelectric properties are also investigated. The results present that initially the increase in the sintering temperature is very effective in improving the density and electric properties. However, the properties of the samples would be deteriorated as they are sintered over the optimum temperature. The KNN-LS (doped with 0·4%BF) ceramics shows excellent properties with sintering temperature at 1100°C.     

Influence of chemical disinfection on mechanical and structural properties of type III and IV dental stones

Abstract

Abstract

We report on the effect of chemical disinfection in aqueous 1% virkon, 0·525% hypochlorite and slurry solutions on the mechanical and structural properties of type III and IV dental stones. For both dental stones, compressive strength and diametral tensile strength decrease with the application of all types of disinfectants. Virkon disinfectant decreases the mechanical strength with the lowest magnitude. The X-ray diffraction patterns of samples show the existence of dihydrate and hemihydrates microcrystals along with traces of CaSO₄.xH₂O (x?=?0·02–0·48) in as prepared samples, but do reveal that hemihydrate phase is removed after spraying disinfectants. Fractography of the dental stones clearly indicates that the separation between the microcrystals is widened by the disinfection process as a consequence of the possible dissolution of last precipitated gypsum phases. The mechanical properties of the dental stones are effectively decreased after disinfection to different levels.     

Dielectric properties of CaCu3Ti4O12 ceramics with Zn substitution for Cu

Abstract

CaCu_3–xZn_xTi₄O₁₂ (x is from 0 to 1·0) polycrystalline samples were fabricated via a two-step solid state reaction process. The lattice parameter of the monophasic CaCu₃Ti₄O₁₂ phase increased as Zn content increased. Scanning electron microscopy (SEM) images of the CCTO ceramic show bimodal grain size distribution and the grain size decrease largely with the appearance of Zn₂TiO₄ second phase. The dielectric permittivity of pure CCTO ceramic is ～1·5×10⁴ at f?=?100 Hz. The dielectric constant of the sample largely increased with Zn substitution in the frequency range f<10⁴ Hz. The highest dielectric constant was 6·2×10⁴ at f?=?100 Hz with Zn substitution of x?=?0·8. The improved dielectric properties are believed to be related to the presence of a thin grain boundary barrier layer. The resistivity of the grain boundary decreased largely with Zn substitution as evidenced from the impedance plots.     

Self-propagating high temperature synthesis of intragranular TiC-Ti3AlC2 composites

Abstract

Abstract

TiC-Ti₃AlC₂ composites with a novel intragranular structure were fabricated by self-propagating high temperature synthesis technique using Ti, Al and C elemental powders. X-ray diffraction results indicated that the as synthesised composites were composed of TiC and Ti₃AlC₂, where the amounts of TiC were measured to be 5·4, 9·2 and 16·7?wt-%. Transmission electron microscopy observation showed that the globular TiC was located within the plate-like Ti₃AlC₂ grain, which confirms the ubiquitous existence of intragranular structure in the composites.     

Effect of ceramic binders on microwave dielectric loss of alumina

Abstract

Several binders have been evaluated to determine their effect on high frequency dielectric loss. Alumina ceramics have been powder pressed with different binder levels, sintered, and then the dielectric loss (tan δ = Q^-1) has been measured at approximately 9·5 GHz. The quality factor Q was determined and significant differences were observed between samples made with different binders. The differences are caused by the different levels of residual porosity left by the different binders. In addition, differences in levels of impurities in the binders may also influence Q. Selected samples were measured for green body strength as a function of binder content.     

Structure and dielectric behaviour of valence compensated perovskite oxide Sr1-x LaxTi1-x CrxO3 ceramics

Abstract

Attempts have been made to investigate, by sintering of powder components, the solid solution formation in the valence compensated perovskite oxide Sr_1-x La_xTi_1-x Cr_xO₃ system. Solid solutions, with cubic structure, form for all the compositions up to x ≤ 0·50. The unit cell volume decreases with increasing value of x. The grain size of sintered samples was in the range 2–4 µm. The dielectric constant was found to be temperature and frequency dependent. Orientational and space charge polarisation contribute to the dielectric behaviour.     

In situ synthesis and thermal shock resistance of mullite used for thermal storage ceramics in solar thermal power generation

Abstract

Mullite ceramic, as one of high performance thermal storage ceramics for solar thermal power generation systems, was in situ fabricated via semidry pressing and pressureless sintering in the air. Andalusite (57–68 wt-%) and calcined bauxite (24–29 wt-%) were used as the raw materials, with kaolin and a tiny of boric acid being added to promote the densification and improve the mechanical properties. The best physical properties and thermal shock resistance were obtained on an optimum A3 sample sintered at 1600°C for 3 h, i.e. a bending strength of 120·44 MPa and 30 cycles thermal shock cycling without cracking (wind cooling from 1000°C to room temperature) with a loss of bending strength of 8·7%.