Phase and melting relationships of β, α and α′-Ca3(PO4)2 polymorphs in the Ca3(PO4)2-Zn3(PO4)2 system

In order to provide an exact knowledge of the phase transitions and melting relationships of Ca₃(PO₄)₂ (TCP) in the presence of zinc, a revisited version of the rich-Ca₃(PO₄)₂ region of the phase diagram of the system Ca₃(PO₄)₂-Zn₃(PO₄)₂ has been established in the present work. Experimental determination of this diagram was carried out by solid-state reactions of samples prepared from pure NH₄H₂PO₄, CaCO₃ and ZnO raw materials. X-ray Diffraction, Differential Thermal Analyses and Field Emission Scanning Electron Microscopy studies allowed to revise the α, β, α + β-TCP phase stability fields, delimitating for the first time the biphasic α + α′-TCP field and the melting relationships in the high temperature region of the system. The results allowed to determine two peritectic invariant points, at ≈1400 °C for 95 mol% Ca₃(PO₄)₂ and at ≈1490 °C for ≈99.5 mol% Ca₃(PO₄)₂.     

Role of dopants (B,P and S) on the stabilization of β-Ca2SiO4

The interest in belite (Ca₂SiO₄ + impurities or dopants) phase increases significantly since new types of clinkers based essentially on this phase become promising alternatives to Portland clinker for reducing CO₂ emission. Belite is also of interest as a biomedical cement. For the cement industry, stabilization of the β-polymorph is essential since the γ-polymorph has no hydraulic activity. In order to understand the mechanism of β-polymorph stabilization, this paper explores the addition of three dopants, namely P, B and S. It turns out that these dopants can modify the lattice parameters of Ca₂SiO₄ and induce specific twinning morphologies and surface relief, as well as grain boundaries deformations. A link between the addition of dopant leading to significant microstructural changes and the stabilization of β-polymorph has been established.     

New vacancied and Dy3+-doped molybdates – Their structure,thermal stability,electrical and magnetic properties

Microcrystalline samples of new Cd_1–3xDy_2x⌷_xMoO₄ solid solution with limited homogeneity (0<x≤0.2222) and cationic vacancies (denoted as ⌷) were prepared by a high-temperature solid state reaction. The XRD data and SEM analysis showed that as-prepared ceramics crystallize in the tetragonal scheelite type symmetry (space group I4₁/a) with the crystallite size varying between ~2 and ~20 µm. A systematic change in lattice constants, a and c, as well as in lattice parameter ratio c/a with an increase of Dy content was observed. Dy-doped molybdates are paramagnets with the antiferromagnetic short-range interaction and spin-orbit coupling. Optical and electrical investigations proved Cd_1–3xDy_2x⌷_xMoO₄ solid solution to be in the insulating state of E_g>3 eV at room temperature and the thermally activated conduction of the Arrhenius-type above 350 K. Moreover, the I-V characteristics provided the evidence of symmetrical and non-linear behavior typical of charge carrier emission weakly induced by the temperature. Relative dielectric permittivity ε_r below 10 as well as loss tangent tanδ below 0.15 do not substantially depend both on the temperature in the range of 76–400 K and the frequency in the range of 5·10²–1·10⁶ Hz. These results are interpreted in the framework of the acceptor and donor vacancy centers.     

Structural,optical, and spectroscopic properties of Er3+-doped TeO2–ZnO–ZnF2 glass-ceramics

Transparent fluorotellurite glass-ceramics have been obtained by heat treatment of precursor Er-doped TeO₂–ZnO–ZnF₂ glasses. ErF₃ nanocrystals nucleated in the glass-ceramics have a typical size of 45 ± 10 nm. Based on the Judd-Ofelt theory, the main radiative parameters for the ⁴I_13/2 → ⁴I_15/2 transition have been obtained. The split of the absorption and emission bands and the reduction of the Ω₂ parameter, as compared to the glass, confirm the presence of Er³⁺ ions in a crystalline environment in glass-ceramic samples. The analysis of the ⁴I_13/2 decays suggests that a fraction of Er³⁺ ions remains in a glass environment while the rest forms nanocrystals. For the glass-ceramics, intense red and green upconversion emissions were observed with an enhancement of the ⁴F_9/2 → ⁴I_15/2 red one compared to the glass sample. The temporal evolution of the red emission together with the excitation upconversion spectra suggests that energy transfer processes are responsible for the enhancement of the red emission.     

Phase transition mechanisms involved in the formation of structurally stable β-Ca3(PO4)2-α-Al2O3 composites

Composite powders comprising various proportions of β-Tricalcium phosphate [β-Ca₃(PO₄)₂] and α-Alumina (α-Al₂O₃) were synthesized by wet precipitation and then heat treated for drying and crystalline phase development. The phase formation mechanism was assessed through a set of characterization techniques including XRD, FT-IR and Raman spectra, and quantitative Rietveld refinement analysis. Al₂O₃ additions delayed the transformation kinetics from calcium deficient apatite to β-Ca₃(PO₄)₂ and preserved the thermal stability of β-Ca₃(PO₄)₂ − α-Al₂O₃ composites till 1400 °C. Such enhancement of thermal stability was due to the occupancy of Al³⁺ at both Ca²⁺(4) and Ca²⁺(5) lattice sites of β-Ca₃(PO₄)₂. Beyond the occupancy saturation limit for Al³⁺, the excess of aluminium crystallized as α-Al₂O₃. Morphological analysis revealed the growth of rod-like α-Al₂O₃ platelets on the surface of micron sized β-Ca₃(PO₄)₂ grains. The mechanical data obtained from indentation of bulk composites displayed enhanced hardness and Young’s modulus with increasing α-Al₂O₃ content in the composites.     

Structural and magnetic studies of A site doped LaRh1−xCuxO3(A=Ca2+, Sr2+, Pb2+ and Bi3+)

A site doped Rh perovskites with general formula La_0.75A_0.25Rh_0.7Cu_0.3O₃ and La_0.75A_0.25Rh_0.5Cu_0.5O₃ (A=Ca²⁺, Sr²⁺, Pb²⁺and Bi³⁺) were synthesized by solid-state methods and their crystallographic, magnetic, and electric properties investigated. Doping by divalent cations resulted in much lower cell volumes and octahedral distortions than doping with a trivalent oxide. The Pb²⁺ and Bi³⁺ (6s²) doped oxides exhibited the lowest magnetic moments and the highest activation energies. Magnetization curves are indicative of antiferromagnetic behavior. The addition of Ca²⁺, Sr²⁺, Pb²⁺and Bi³⁺cations to the A-site decreases the conductivity.     

Influences of gallium substitution on the phase stability,mechanical strength and cellular response of β-tricalcium phosphate bioceramics

β-tricalcium phosphate (β-TCP), a well-accepted synthetic bone grafting biomaterial, is confronted with limitations of poor phase stability and lacking the capacity to mediate the biological functions. In the current study, gallium (Ga) was substituted for calcium in the β-TCP, and the influences of Ga substitution on the phase stability, compressive strength and cellular response of β-TCP bioceramics were investigated. The results indicated that substitution of at least 2.5 mol% Ga for calcium prevented the β-TCP from transforming into α-TCP at 1250 °C. The β-TCP bioceramics substituted with 2.5 mol% Ga attained the highest compressive strength. The β-TCP bioceramics substituted with 2.5 and 5 mol% Ga showed good cytocompatibility, and suppressed in vitro osteoclastic activity as well as osteoblastic differentiation. Considering the favorable mechanical strength and the inhibitory effect on the osteoclastic activity, the β-TCP bioceramics substituted with 2.5 mol% Ga are promising for treating the bone defect in the pathological state of excessively rapid bone resorption.     

Preparation and study of the mechanical and optical properties of infrared transparent Y2O3–MgO composite ceramics

In this study, fine Y₂O₃–MgO composite nanopowders were synthesized via the sol–gel method. Dense Y₂O₃–MgO composite ceramics were fabricated by pre-sintering the green body in air at different temperatures for 1 h and then subjecting the sintered bodies to hot isostatic pressing at 1300°C for 1 h. The effects of pre-sintering temperature on the microstructural, mechanical, and optical properties of the resulting ceramics were studied. The average grain size of the ceramics was increased, whereas their hardness and fracture toughness were decreased with increasing pre-sintering temperature. A maximum fracture toughness of 1.42 MPa·m^1/2 and Vickers hardness of 10.4 GPa were obtained. The average flexural strength of the ceramics was 411 MPa at room temperature and reached 361 MPa at 600°C. A transmittance of 84% in the 3–5 µm region was obtained when the composite ceramics were sintered at 1400°C. Moreover, a transmittance of 76% in the 3–5 µm region was obtained at 500°C.     

Evaluation and comparison of the microstructure and mechanical properties of fibrous Al2O3–(m-ZrO2)/t-ZrO2 composites after multiple extrusion steps

Microstructural variations in the Al₂O₃–(monoclinic-ZrO₂)/tetragonal-ZrO₂ core/shell composites (Al₂O₃–(m-ZrO₂)/t-ZrO₂) were tailored using different volume fractions of the core and shell by the multi-pass co-extrusion process. To introduce continuously fibrous microtextures with varying dimensions of microstructure, various volume fractions of the Al₂O₃–(m-ZrO₂) core and t-ZrO₂ shell phases were used (40/60, 50/50 and 60/40). The effect of the number of extrusion passes on the control of microstructure and the corresponding composite's mechanical properties were investigated. The bending strength and fracture toughness values were shown to increase as the t-ZrO₂ shell volume fraction increased, which was unlike the Vickers hardness value. However, the fracture toughness value was higher for 4th pass composites compared to 5th pass composites due to the presence of the fibrous core/shell microstructure. In addition, the bending strength was higher for 5th pass composites. Detailed microstructural analysis was carried out by SEM.     

Structural,dielectric and optical analysis of Li1.1−yNb0.9-3yTi0.1+4yO3 M-phase solid solutions

The present paper reports the structural, dielectric and optical analysis of the M-phase Li_1.1−yNb_0.9-3yTi_0.1+4yO₃ (y=0.00, 0.045, 0.09, 0.11, 0.135) solid solutions in the Li₂O–Nb₂O₅–TiO₂ ternary system prepared by conventional solid state reaction method. High density (relative density>91%) ceramics were obtained by sintering the solid solutions at 1100 °C. XRD spectra revealed these ceramics to exhibit hexagonal structure and the superstructure formation was observed for higher Ti content (≥28 mol%) samples. Plate-like shape and closely packed grains were formed with low porosity (≤10%). Particle size was obtained in the range of 392–235 nm using TEM images. Relative permittivity changed from 46 to 31 while temperature coefficient of resonant frequency ranged from −93.5 to 57.5 at 1 MHz frequency. The maximum photoluminous intensity of 203.13 for the emission peak at 453 nm (blue emission) was observed for the sample y=0.00.     

Structural,morphological, optical,cation distribution and Mössbauer analysis of Bi3+ substituted strontium hexaferrite

Single-phase M-type hexagonal ferrites, SrBi_xFe_12−xO₁₉ (0.0≤x≤1.0), were prepared by a co-precipitation assisted ceramic route. The influence of the Bi³⁺ substitution on the crystallization of ferrite phase has been examined using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and Mössbauer spectroscopy. The XRD data show that the nanoparticles crystallize in the single hexagonal magnetoplumbite phase with the crystallite size varying between 65 and 82 nm. A systematic change in the lattice constants, a=b and c, was observed because of the ionic radius of Bi³⁺ (1.17 Å) being larger than that of Fe³⁺ ion (0.64 Å). SEM analysis indicated the hexagonal shape morphology of products. From ⁵⁷Fe Mössbauer spectroscopy data, the variation in line width, isomer shift, quadrupole splitting and hyperfine magnetic field values on Bi substitutions have been determined.     

Structural,optical, electrical,dielectric, molecular vibrational and magnetic properties of La3+ doped Mg–Cd–Cu ferrites prepared by Co-precipitation technique

Ferrites are among the most frequently investigated materials mainly due to interesting and practically different properties. Therefore, easily and cost-effective lanthanum doped Mg_0.5Cd_0.25Cu_0.25Fe_2-xLa_xO₄ (x = 0.0, 0.0125, 0.025, 0.0375 and 0.05) ferrites were synthesized by a co-precipitation route, a comprehensive characterisation of their structural, optical, electric, dielectric, molecular vibrational, and magnetic properties were carried out. X-ray diffraction analysis confirmed the formation of a cubic spinel structure. Variations in frequency bands were also observed with amplification in optical band gap energy (2.95 – 3.38 eV) due to La³⁺ ions insertion. The electric resistivity had opposite trends at low and high temperatures with increasing La³⁺ content. The Curie temperature, activation energy, and drift mobility were also determined to have values consistent with the semiconducting behavior of the soft ferrites. The saturation magnetization (M_S) has a maximum value 49.385 emu/g with remanent magnetization (M_r) was 34.928 emu/g and coercivity 661.4 Oe for La³⁺ concentration x = 0.05. The minimum dielectric loss was observed for La³⁺ concentration x = 0.025. Moreover, the resistivity (ρ) has a maximum value of 7.95 × 10⁴ Ω cm for La³⁺ concentration x = 0.025. The calculated frequency range of La³⁺ doped Mg–Cd–Cu ferrites was detected in the microwave range (3.36 – 10.80 GHz), suggesting the potential application of the materials in longitudinal recording media and microwave absorbance.     

Effect of mechanochemical synthesis on the structure,magnetic and optical behavior of Ni1−xZnxFe2O4 spinel ferrites

Ni_1−xZn_xFe₂O₄ (0≤x≤1) nanocrystals were prepared by a soft mechanochemical approach. The structure and morphology were assessed via X-ray powder diffractometery (XRD), infrared spectroscopy (FTIR), Raman spectroscopy, transmission electron microscopy (TEM) and Energy dispersive spectroscopy (EDS). The magnetic characteristics have been evaluated using vibrating sample magnetometer (VSM). The optical properties were explored by diffuse reflectance UV–visible spectrophotometry (DRS). The substitution of Zn into the Ni_1−xZn_xFe₂O₄ nanocrystals increased the mean nanocrystal size from 4 to 19 nm. The FTIR and Raman spectroscopies showed that the substitution with Zn up to x=0.5 in Ni_1−xZn_xFe₂O₄ nanocrystals results in a migration of Fe ions from tetrahedral to octahedral sites, leading to an improvement of the saturation magnetization value to 33.8 emu/g. At the same time, the optical band gap decreased from 2.6 to 1.93 eV due to the increase of the Zn content from x=0 to x=1. These promising characteristics of Ni_1−xZn_xFe₂O₄ nanocrystals make them suitable for the use in the field of magnetically recoverable catalysts including those for energy applications.     

Structural,optical, and thermodynamic properties of an LiCa2Mg2As3xV3−3xO12 solid-solution garnet

A series of LiCa₂Mg₂As_3xV_3−3xO₁₂ garnet powders (x = 0, 0.25, 0.50, 0.667, 0.75, and 1) were synthesized using solid-state reaction from mixed precursor powders. A complete solid-solution series was found between the endmembers. Energy-dispersive spectroscopy confirmed the homogeneity of the synthesized garnets. The compositions reversibly melted between 1000 and 1160°C and had low sintering temperatures between 650 and 900°C. Thermal diffusivity, heat capacity, thermal conductivity, density, lattice parameter, thermal expansion, index of refraction, and dispersion were measured from room temperature to 800°C.     

Interrelations between composition,structure, thermal stability,and chromatic characteristics of new NASICON-related solid solutions of Li1+xCrxZr2−x(PO4)3

In this contribution, formation and properties of full series of solid solutions of Li_1+xCr_xZr_2−x(PO₄)₃ are discussed. Special attention is paid to interrelations between substitution degree x, resulting crystal structure, crystal field parameters, and colour hue of the samples. It is shown, that within the studied synthesis conditions, an increase of the substitution degree x in the solid solutions leads to structural change from triclinic (new NASICON-based modification) to orthorhombic SW type, while both end-members of the solid solutions are rhombohedral. Interrelations between the substitution degree x, thermal stability and particle size distribution are also discussed. The colour of the samples is explained by means of crystal field theory. It is shown, that the crystal field strength is controlled by the substitution degree x: with an increase of the substitution degree x crystal field strength shifts to higher energies and the resulting colouration changes from light pink via brown to bright green. Because of the excellent thermal stability of the samples as well as the wide range of colours, the Li_1+xCr_xZr_2−x(PO₄)₃ series can be considered as attractive candidates for application as ceramic pigments.     

Preparation,thermal stability and magnetic properties of new AgY1−xGdx(WO4)2 ceramic materials

Polycrystalline samples of α-AgY_1−xGd_x(WO₄)₂ with x=0, 0.005, 0.01, 0.025, 0.05, 0.1, 0.2, and 1 have been prepared by a solid state reaction method and the influence of Gd³⁺ substitution for Y³⁺ on microstructure, thermal and magnetic properties was investigated. The X-ray diffraction analysis showed the phases to crystallize in the monoclinic symmetry, space group C2/m. A reversible monoclinic to tetragonal phase transition occurs in AgY_1−xGd_x(WO₄)₂ and strongly depends on Gd³⁺ ion concentration. Electron paramagnetic resonance (EPR) spectra of Gd³⁺ ions showed non-monotonous dependence of interaction strength on gadolinium concentration. Magnetic measurements showed paramagnetic behavior and strong increase of magnetic moment as the yttrium content decreases.     

Structural,magnetic and magnetocaloric properties of CoFe2−xMoxO4 (0.0≤x≤0.3) ferrites

We have investigated structural, magnetic and magnetocaloric properties of CoFe_2-xMo_xO₄ (0.0≤x≤0.3) ferrites. Polycrystalline samples were prepared by the sol gel method and characterized by the powder X-ray diffraction and scanning electron microscopy. X-ray diffraction patterns show that all samples have a cubic spinel structure and the lattice parameter, a, decreases monotonically with increase in Mo concentration. Scanning electron micrographs indicate that most of the particles are in the range of 400–850 nm size. Magnetic measurements, performed by using a cryogen free vibrating sample magnetometer, show that these samples are soft ferromagnets in the measured temperature range. The saturation magnetization, M_s, values of Mo-doped samples are larger than the parent compound with a maximum value of ~106 emu/g for x=0.2 sample. The magnetic entropy change (−ΔS) increases with increase in applied magnetic field and shows a peak in the vicinity of blocking temperature. A maximum value of 0.56 J kg⁻¹ K⁻¹ at 5 T field has been observed for x=0.2 sample.     

Effect of lattice structure evolution on the thermal and mechanical properties of Cu–Al2O3/GNPs nanocomposites

In this study, high-energy ball milling accompanied by compaction and sintering were employed for manufacturing Cu-based hybrid nanocomposite reinforced by Al₂O₃ and GNPs. This hybrid nanocomposite is proposed to meet the specification of heat sink applications, where excellent mechanical and thermal performance is demanding. Different processing parameters were experimentally considered such as sintering temperature and weight percentage of GNPs, 0, 0.25, 0.50, 0.75, and 1 wt %. The weight percentage of Al₂O₃ was fixed at 10%. The results demonstrated that the mechanical and thermal performance of the fabricated nanocomposites were superior for nanocomposite containing 0.5% GNPs and sintered at 1000 °C. The hardness, the thermal conductivity and the coefficient of thermal expansion (CTE) were improved by 21%, 16.7%, and 55.2%, respectively, compared to composite without GNPs addition. The improved mechanical and thermal properties were attributed to the low stacking fault energy, small crystallite size, high dislocation density, and low lattice strain of the composite prepared at this composition. Moreover, the better dispersion of the nano-particles of GNPs and Al₂O₃ inside the matrix helped for the strength and thermal conductivity improvement while maintaining low CTE.     

Effect of Tb3+ doping and self-generated pressure on the crystallographic/morphological features and thermal stability of LaPO4·nH2O single-crystal nanorods obtained by microwave-assisted hydrothermal synthesis

The effect of Tb³⁺-doping content (in the range 0–20 mol%) and autogeneous/self-generated pressure (in the range 2.3–3.4±0.1 bar) on the crystallographic/morphological features and thermal stability of single-crystal nanorods of LaPO₄·nH₂O obtained by a simple and fast microwave-assisted hydrothermal synthesis was investigated. It is shown that high-quality (that is, well-shaped and linear/planar defect free) rhabdophane-type single-crystal La_1-xTb_xPO₄·nH₂O (x=0–0.20) nanorods, with a high morphological uniformity, are obtained in all cases. In addition, it is shown that the Tb³⁺ solutes are incorporated into the LaPO₄·nH₂O host, forming substitutional solid solutions (i.e., partial substitution of La³⁺ by Tb³⁺) with progressively smaller unit-cell volume but with identical thermal stability. Morphologically Tb³⁺ doping however results in the formation of nanorods with lower aspect ratio. With respect to the self-generated pressure during the microwave synthesis, it is shown that its increase does not affect to crystallographic aspects, but however results again in nanorods with lower aspect ratio. It is also demonstrated that monazite-type single-crystal La_1-xTb_xPO₄ nanorods can be obtained by calcining their rhabdophane-type La_1-xTb_xPO₄·nH₂O counterparts at ~700 °C in air. Consequently, in the present study important guidelines have been identified for the controlled synthesis of functional La_1-xTb_xPO₄·nH₂O nanorods with tailored morphology.