Fabrication and phase transition of uranium-doped LaxGd2−xZr2O7 transparent ceramics: A prospective neutron detection material

Via vacuum sintering, 2 mol% uranium-doped La_xGd_2−xZr₂O₇ (x = 2, 1.6, 1, and 0.4) transparent ceramics with Ca²⁺ as charge compensator was first fabricated by solid-state reaction. X-ray diffraction results of as-prepared powders and ceramic samples demonstrate that the phase transition from defective fluorite to pyrochlore occurs with the increase of x. Optical in-line transmittance spectrum shows that four ceramic samples have good in-line transmittance (nearly 80% from 700 to 2200 nm), especially the U:La_1.6Gd_0.4Zr₂O₇ ceramics. The cut-off wavelength of four U-doped transparent ceramics shifted from 250 to near 460 nm, and it is believed that such phenomenon is related to the stable existence form of uranium in ceramics lattices. Observing the excitation emission spectra, the main excitation peaks of four ceramic samples are located at 458 nm, and the main emission peaks are located around 513 nm. In addition, there are low-intensity emission peaks around 520, 537, and 566 nm, and the related explanation is given in combination with the U⁶⁺ ion energy level diagram. Thus, uranium-doped La_xGd_2−xZr₂O₇ transparent ceramics have potential for novel neutron detection materials.     

Effect of isovalent substitution on phase transition and negative thermal expansion of In2−xScxW3O12 ceramics

Solid solutions of In_2−xSc_xW₃O₁₂ (0≤x≤2) were successfully synthesized using the solid state reaction method. Effects of substituted scandium content on the phase composition, microstructure, phase transition temperatures and thermal expansion behaviors of the resulting In_2−xSc_xW₃O₁₂ (0≤x≤2) samples were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and thermal mechanical analyzer (TMA). Results indicate that the obtained In₂W₃O₁₂ ceramic undergoes a structure phase transition from monoclinic to orthorhombic at 248 °C. This phase transition temperature of In₂W₃O₁₂ can be easily shifted to a lower temperature by partly substituting the In³⁺ with Sc³⁺. When the x value increased from 0 to 1, the phase transition temperatures of In_2−xSc_xW₃O₁₂ (0≤x≤2) samples decreased from 248 to 47 °C. All the In_2−xSc_xW₃O₁₂ (0≤x≤2) ceramics show fine negative thermal expansion below their corresponding phase transition temperatures. The negative thermal expansion coefficients of the In_2−xSc_xW₃O₁₂ (0≤x≤2) ceramics change in the range from −1.08×10⁻⁶ °C⁻¹ to −7.13×10⁻⁶ °C⁻¹.     

Ferroelectric and dielectric properties of Nd2−xCexTi2O7 ceramics

The solid solution system Nd_2?xCe_xTi₂O₇ has been investigated. The solubility limit of Ce in Nd_2?xCe_xTi₂O₇ was found to be 0·5–0·75 according to X-ray diffraction and X-ray photoelectron spectroscopy results. Ce substitution increases the b and c axes and the volume of the unit cell due to its larger ionic radius. Nd_2?xCe_xTi₂O₇ (x?=?0·05, 0·25, 0·5, 0·75) textured ceramics were fabricated using spark plasma sintering. The ferroelectric and dielectric properties of the ceramics were studied. Ce substitution decreases the Curie point T_c of Nd_2?xCe_xTi₂O₇ compounds. The results suggest that the T_c of Ce₂Ti₂O₇ is <1445°C.     

Relaxor transition and properties of Mn-doped SrxBa1−xNb2O6 ferroelectric ceramics

Pure and Mn-doped Sr_xBa_1−xNb₂O₆ (SBN) ceramics were prepared by the conventional solid state reaction method and their morphology, phase structure, relaxor behavior, ferroelectric and energy storage properties were investigated to reveal the effects of Mn doping on their properties. The results show that all the samples are single tetragonal tungsten bronze structure and the Mn element having few influences on their microstructures. The dielectric constant of pure SBN samples shows a classical relaxor behavior with obvious frequency dispersion and fits well with the Vogel–Fulcher law. However, the dielectric constant of Mn-doped samples does not show frequency dispersion. Then the relaxor behaviors of both pure and doped samples were further investigated by fitting the dielectric constant to the modified Curie-Weiss law. It was found that the diffusion parameters (γ) of Mn-doped samples are close to those pure samples, which indicates that the Mn-doped samples still have relaxor characteristics even without the classical dielectric frequency dispersion. Compared with the pure samples, the Mn-doped samples have a slimmer polarization-electric field (P-E) hysteresis loop, which can also prove their relaxor behaviors. The investigations of energy storage properties reveal that the Mn-doped samples have a higher energy storage density and efficiency.     

Dielectric properties and diffuse phase transition behavior of CuO-doped lead-free Ba(ZrxTi1−x)O3 ceramics

CuO-doped lead-free Ba(Zr_0.2Ti_0.8)O₃ (BZT20) ceramics were prepared through a solid state processing technique, and the effects of CuO on microstructure, dielectric properties and diffuse phase transition behavior were investigated. The average grain sizes were increased by CuO doping. The temperature and frequency dependences of the dielectric constant revealed that CuO-doped BZT20 ceramics exhibited broad diffuse phase transition behavior. The dielectric constant increased with increasing CuO concentration. The value of T_m and degree of diffusion (γ) changed regularly in the studied compositional ranges. The BZT20 samples with 1.0 mol% CuO doping, sintered at 1310 °C, showed excellent dielectric property and lower diffusivity with ε_m=21,371 and γ=1.87. These results can be explained by the disordered distribution of Cu ions in the B sites and the weakened bonding force with oxygen ions in Cu substituted BZT20 structure.     

Fabrication of transparent electro-optic (K0.5Na0.5)1−xLixNb1−xBixO3 lead-free ceramics

Lead-free transparent electro-optic ceramics (K_0.5Na_0.5)_1?xLi_xNb_1?xBi_xO₃ have been fabricated by hot-press sintering. Owing to the effective suppression of grain growth, the Li and Bi co-modified ceramics generally possess a dense and fine-grained structure. The co-modification also causes the ceramics to transform into a nearly cubic structure with minimal optical anisotropy. A diffuse phase transformation is also induced, causing the ceramics to become more relaxor-like and contain more polar nano-regions. These would reduce the light scattering by the grains, at the grain boundaries and at the domain walls, respectively, and thus making the ceramics become optically transparent. For the ceramic modified with 5 mol% Li⁺ and Bi⁵⁺, the optical transmittance reaches a high value of 60% in the near-IR region. The ceramics also exhibit a strong linear EO response, giving a large effective linear EO coefficient in the range of 120–200 pm/V.     

Fabrication of Er:Y2O3 transparent ceramics for 2.7 μm mid-infrared solid-state lasers

Laser grade 7 at.% Er:Y₂O₃ transparent ceramics with submicron grain size were fabricated by using one-step vacuum sintering followed by hot isostatic pressing (HIPing) technique. Through studying the sintering trajectory of Er:Y₂O₃ ceramics, the sintering temperature zone where sufficient relative density (>96%), no pore-boundary separation, and sub-micron grain size (<1 μm) ceramic samples could be identified. The samples pre-sintered in this zone were readily densified by HIPing. To maximum the densification and achieve high transparency, it is critical to suppress the final-stage grain growth. After HIPing at 1520 °C, the Er:Y₂O₃ ceramics were fully densified without further grain growth, and exhibited in-line transmission of about 81.6% at 2000 nm. Continuous wave (CW) room temperature laser operation of the Er:Y₂O₃ transparent ceramic at 2.7 μm was demonstrated.     

Fabrication of 4 at.% La3+ ion doped (TbxLu0.96−x)2O3 transparent ceramics by using NC-PLSH technology and characterisation of their magneto-optical properties

Nanocrystalline powders and pressureless sintering (NC-PLSH) were used in a H₂ atmosphere to fabricate 4 at.% La³⁺ doped (Tb_xLu_0.96?x)₂O₃ magneto-optical transparent ceramics. La³⁺ was utilised as a sintering aid, and Lu³⁺ was used as a phase stabiliser. A component of the (Tb_xLu_0.96?xLa_0.04)₂O₃ magneto-optical material was designed and investigated based on powder sinterability and green compact sintering. Transparent (Tb_xLu_0.96?xLa_0.04)₂O₃ ceramics were obtained by using a one-step sintering process in H₂ atmosphere, without the need for subsequent hot isostatic pressing. The in-line transmittance of the as-polished 4 at.% La³⁺:(Tb_0.8Lu_0.16)₂O₃ ceramic was 79% at 1400 nm. The measured Verdet constant at wavelengths of 633 nm and 1064 nm was 354 rad/(T·m) and 100.2 rad/(T·m), respectively. The required length of the (Tb_0.8Lu_0.16La_0.04)₂O₃ ceramics was 63% less than that of Tb₃Ga₅O₁₂ ceramics.     

Diffuse phase transition and high electric field properties of BaCeyTi1−yO3 relaxor ferroelectric ceramics

Pure perovskite BaCe_yTi_1−yO₃ ceramics with compositions y=0.10, 0.20 and 0.30 have been prepared by solid state reaction. The temperature and frequency dependence of dielectric properties (permittivity, dielectric loss and dielectric modulus) of the ceramics has been investigated. A diffuse phase transition is typical for all the compositions, with a reduction of the Curie temperature with increasing Ce addition. If in the case of the sample with 0.10 Ce content, no frequency shift of the phase transition temperature (T_m) is noticed, a relaxor-like ferroelectric character become predominant for concentrations 0.20 and 0.30. The ceramic with y=0.20 presents higher tunability, a reduced hysteretic behavior and reasonable low dielectric losses at room temperature, which makes this composition a very good candidate for tunable capacitors applications.     

Fabrication of Al2O3–ZrO2 ceramics with high porosity and strength

This study describes the synthesis of ceramics, in which a micrometre-sized Al₂O₃–ZrO₂ nanopowder was used as an oxide base for the hardening of the materials. To a suspension of this mixed metal oxide, the pore-forming crystallisation additives camphor and carbamide were added to produce ceramics with thin permeable pores. Camphor crystallised in the oxide suspension in the form of single pentagonal stars and сarbamide crystallised in the form of thin elongated needles. The use of the different crystallisation additives allowed the formation of ceramics after sintering that have both permeable and complex pore morphologies, where anisotropic properties were observed using carbamide as an additive but not when camphor was used. The total porosity of the resulting ceramics was 51.3%, with a compressive strength in the range of 17.3–92.3 MPa.     

Studies on electrical and dielectric properties of co-precipitated Aurivillius phase Ca1−xBaxBi2Nb2O9 ceramics

Polycrystalline samples of lead free Ca_1−xBa_xBi₂Nb₂O₉ ceramics has been synthesized by simple chemical co-precipitation method. Single phase layered perovskite structure was confirmed by X-ray diffraction with average crystallite size in the range of 46–57 nm. Randomly oriented grains with inhomogeneous morphology was observed by scanning electron microscopy. Study of dielectric constant and corresponding tangent loss was carried out in the frequency range of 1 Hz to 5 MHz at room temperature. Ca_1−xBa_xBi₂Nb₂O₉ shows maximum value of dielectric constant of 535 at 1 Hz. The highest polarization of 0.026 μC/cm² was obtained at an applied electric field of 10 kV/cm using PE loop measurement.     

Temperature stability and phase transition of Li0.02(KxNa1−x)0.98NbO3 ceramics

Li_0.02(K_xNa_1?x)_0.98NbO₃(x = 0.35–0.55) ceramics were prepared using the conventional solid state sintering method. The thermal behaviors of Li-modified (K_xNa_1?x)NbO₃ ceramics were investigated from ?30 to 150 °C, and the effect of Na/K ratio in (K_xNa_1?x)NbO₃ ceramics on thermal behavior and electrical properties was also studied. In the case of Li_0.02(K_xNa_1?x)_0.98NbO₃ ceramics with 0.5 wt.% ZnO, the transition temperature was sharply decreased because of a phase transition as the composition range of x was 0.425–0.475. From the results of the temperature dependence of piezoelectric properties, it is assumed that the Na-rich phase is less stable than the K-rich phase for temperature change.     

Fabrication and properties of porous mullite ceramics from calcined carbonaceous kaolin and α-Al2O3

High purity calcined carbonaceous kaolin and α-Al₂O₃ powders were employed to prepare porous mullite ceramics (Sample A) using graphite as pore former with the reaction sintering method. For the purpose of comparison, porous mullite ceramics (Sample B) was also fabricated from the uncalcined carbonaceous clay incorporated with α-Al₂O₃ powders. Mullitization in the two samples was both nearly complete at 1500 °C, despite the fact that calcination of the clay remarkably depressed mullitization and promoted the formation of glass phase. The Sample A sintered at 1500 °C fractured mainly in an intergranular way, while the Sample B mainly underwent transgranular fracture. The experimental results revealed that densification behavior/open porosity of the Sample A was far more sensitive to sintering temperature. The pore size of the Sample A as well as the Sample B sintered at 1500 °C was in a narrower range of 0.3–5 μm.     

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.     

Structure and microwave dielectric properties of Ba1−xSrxMg2V2O8 ceramics

The Ba_1−xSr_xMg₂V₂O₈ (0≤x≤0.4) microwave dielectric ceramics were fabricated by a standard solid-state reaction method. The formation of a continuous solid solution within the whole composition range was identified. The ceramic samples could be well densified in the temperature range of 885–975 °C in air for 4 h. The permittivity ε_r was found to increase with increasing ionic polarizabilities. The Q×f values were believed to be closely related with packing fraction and grain refinement. The Sr²⁺ substitution contributed to a monotonous increase of the A-site bond valence, such that the τ_f value experienced a considerable variation from negative to positive values. The optimum microwave dielectric properties of an ε_r of 13.3, a high Qxf of 86,640 GHz (9.6 Hz) and a near-zero τ_f of −6 ppm/°C could be yielded in the x=0.15 sample when sintered at 915 °C for 4 h.     

Fabrication and dielectric properties of transparent PZN–BT ceramic

Transparent ceramic of 0.85Pb(Mg_1/3Nb_2/3)O₃–0.15BaTiO₃ has been successfully prepared by a two-stage sintering method using conventional raw materials. The ceramics exhibited an excellent crystallinity, high density and clean grain boundary. The transmittance keeps about4 0% from visible to near infrared regions. The frequency dependence of T_m and relaxor behavior has also been investigated using Vogel–Fulcher model and Power model.     

Electrical properties of La2-xPrxTi2O7 (x = 0–0.3) ceramics

The electrical properties of La₂Ti₂O₇ (LTO) ceramics have been enhanced through the substitution of La³⁺ ions by Pr³⁺ ions. Almost all doped Pr³⁺ ions will get at A - site without causing a change on monoclinic phase of LTO. The average grain size is 17.8 μm for La_1.9Pr_0.1Ti₂O₇ ceramics. The relaxation activation energy which is contributed by defect dipoles that are formed from TiO₆ oxygen octahedrons’ distortions in grains is 1.6 eV for La_1.8Pr_0.2Ti₂O₇ ceramics. This kind of defects will be activated from 520 °C and completely be activated until 650 °C. The piezoelectric coefficient d₃₃ = 3.0 pC/N of La_2-xPr_xTi₂O₇ ceramics maintains stable when the Pr³⁺ doping content x ranging from 0.1 to 0.3.     

Combustion Synthesis of PrBa2Cu3O7−δ and YBa2Cu3O7−δ cuprate materials

PrBaCuO and YBaCuO cuprate materials were prepared from cooper, barium peroxide, and yttrium/praseodymium oxide by SHS and standard solid-state synthesis. SHS reactions were carried out using relatively large cooper particles (< 63 im) to obtain small product samples (13 mm in diameter). High ambient temperature was used to stabilize a combustion front in the ignited pellets. Explored was the effect of cooper particles size, starting density, and ambient temperature on phase evolutions in synthesized materials.     

Fabrication and characterization of La2O3–Fe2O3–Bi2O3 nanopowders: Effects of La2O3 addition on structure,optical, and radiation-absorption properties

In this study, we fabricated and characterized six new nanopowders representing variations of La₂O₃–Fe₂O₃–Bi₂O₃, i.e., 100Bi₂O₃, 30Fe₂O₃–70Bi₂O₃, 3La₂O₃–27Fe₂O₃–70Bi₂O₃, 7La₂O₃–23Fe₂O₃–70Bi₂O₃, 10La₂O₃–20Fe₂O₃–70Bi₂O₃, and 20La₂O₃–10Fe₂O₃–70Bi₂O₃ (represented by 100B, 30F70B, 3L27F70B, 10L20F70B, and 20L10F70B, respectively). These nanopowders were prepared by the microwave-assisted hydrothermal synthesis method. Saponin extract from soapnuts was used as the nanoparticle capping agent. The structural, optical, and gamma radiation characteristics were measured, calculated, and analysed, respectively. The chemical structures of the nanocomposites influenced their optical and radiation shielding characteristics. The optical bandgaps of the 100B, 30F70B, 3L27F70B, 7L23F70B, 10L20F70B, and 20L10F70B nanopowders were 3.16, 3.13, 3.43, 3.45, 3.46, and 3.58 eV, respectively. The ranges of the mass attenuation coefficients of the nanopowders were computed, using XCOM, to be 0.0412–5.1624, 0.0401–4.5406, 0.0401–4.5285, 0.0401–4.5129, 0.0401–0.5015, and 0.0400–4.4156 cm²/g, respectively, and the ranges of mass energy absorption coefficients were found to be 0.0232–1.7525, 0.0228–1.5484, 0.0228–1.5598, 0.0288–1.5746, 0.0228–1.5853, and 0.0227–1.6192 cm²/g, respectively, for photon energies in the range of 0.1–10 MeV. The order of the dose rate trend was as follows: 30F70B < L27F70B < 7L23F70B < 10L20F70B < 20L10F70B. Analysis of the photon interaction parameters showed that the synthesized nanopowders could function well as fillers in radiation-shielding matrices.     

Fabrication and properties of ZrB2–SiC–BN machinable ceramics

ZrB₂–SiC–BN ceramics were fabricated by hot-pressing under argon at 1800 °C and 23 MPa pressure. The microstructure, mechanical and oxidation resistance properties of the composite were investigated. The flexural strength and fracture toughness of ZrB₂–SiC–BN (40 vol%ZrB₂–25 vol%SiC–35 vol%BN) composite were 378 MPa and 4.1 MPa m^1/2, respectively. The former increased by 34% and the latter decreased by 15% compared to those of the conventional ZrB₂–SiC (80 vol%ZrB₂–20 vol%SiC). Noticeably, the hardness decreased tremendously by about 67% and the machinability improved noticeably compared to the relative property of the ZrB₂–SiC ceramic. The anisothermal and isothermal oxidation behaviors of ZrB₂–SiC–BN composites from 1100 to 1500 °C in air atmosphere showed that the weight gain of the 80 vol%ZrB₂–20 vol%SiC and 43.1 vol%ZrB₂–26.9 vol%SiC–30 vol%BN composites after oxidation at 1500 °C for 5 h were 0.0714 and 0.0268 g/cm², respectively, which indicates that the addition of the BN enhances oxidation resistance of ZrB₂–SiC composite. The improved oxidation resistance is attributed to the formation of ample liquid borosilicate film below 1300 °C and a compact film of zirconium silicate above 1300 °C. The formed borosilicate and zirconium silicate on the surface of ZrB₂–SiC–BN ceramics act as an effective barriers for further diffusion of oxygen into the fresh interface of ZrB₂–SiC–BN.