Influence of Gd2O3 on the microstructure and properties of transparent MgAl2O4: Cr3+ ceramic

In this work, MgAl₂O₄: Cr³⁺ transparent ceramics have been synthesized by the hot press sintering techniques, and the effect of the sintering aid Gd₂O₃ and its content on the densification, microstructure, and optical, photoluminescence was studied and discussed. The relative density reached 99.29% with 0.8 wt% Gd₂O₃ as a sintering aid, and the optical transmittance at 686 nm and 1446 nm were approximately 76%. As Gd₂O₃ content continued to increase, the grain size of the ceramics became smaller and uniform, accompanied by some pores with the size of ～1 μm. The ceramics with 4.0 wt% Gd₂O₃ showed a higher transmittance, of 82% at 1446 nm. Additionally, Gd₂O₃ was helpful for Cr³⁺ in the sites of octahedral symmetry, which increased the quantum yield. The quantum yield of MgAl₂O₄: Cr³⁺ with 0.8 wt% Gd₂O₃ was about 0.175, which was 36% higher than that of ceramic without Gd₂O₃. In short, the sintering aid Gd₂O₃ not only contributed to improving the densification, homogenizing the grain size, and heightening the optical transmittance but also enhanced the quantum yield of Cr³⁺.     

Influence of Al2O3 on the electrical conductivity and structure of calcium-silicate-based melts

Electrical conductivity and structure of the CaO-SiO₂-based mold flux melts with various Al₂O₃ contents were investigated. The results show that the electrical conductivity increases with the addition of Al₂O₃ from 2 wt% to 4 wt%, but decreases with the further increase of Al₂O₃ from 4 wt% to 8 wt%. Correspondingly, the apparent activation energy reduces firstly from 55.12 ± 1.20 kJ mol to 41.09± 0.38 kJ mol, and then increases from 41.09 ± 0.38 kJ mol to 98.99 ± 1.42 kJ mol. The structure analyses suggest that complex structural units, such as Si-O-Al, Al-O⁰, Si-O-Si and Q³(Si), reduce first, but increase with the further addition of Al₂O₃. Conversely, these simple structural units, such as Al-O^-, Q⁰(Si), Q¹(Si) and Q²(Si) vary in the opposite way with the change of Al₂O₃ content. From the variations of electrical conductivity, activation energy and structural units, it can be found that when Al₂O₃ works as network breaker to simplify the melt structure, the energy barrier for transportation of conducting ions/ionic reduce, which results in the increase of electrical conductivity; while when Al₂O₃ becomes into network former, the conductivity increases, correspondingly.     

Effect of Ca2+ site substitution on structural,optical, electrical and magnetic properties in Nd3+ and Mn2+-co-doped calcium molybdato-tungstates

Ca_1-3x-yMn_y[]_xNd_2x(MoO₄)_1-3x(WO₄)_3x molybdato-tungstates (? denotes vacant sites) were successfully synthesized by high-temperature solid-state reaction. New materials crystallize in scheelite-type structure within whole homogeneity range of solid solution (x ≤ 0.2000 and y = 0.0200). Morphological features and particle size distribution were investigated by SEM and laser diffraction methods, respectively. Spectroscopic measurements in the UV–vis range was carried out to determine optical direct band gap (E_g), Urbach energy (E_U) and confirmation of structural disorder. Refractive index (n) was calculated using four different models. Magnetic studies revealed paramagnetic behavior with long-range ferrimagnetic and short-range antiferromagnetic interactions. New materials showed weak n-type electrical conductivity and thermoelectric power factor (S²σ) that strongly depends on Nd³⁺ ions content. Dielectric parameters, i.e. relative permittivity (ε_r) and energy loss (tanδ) are insignificantly dependent on Nd³⁺ ions concentration. These effects were considered in terms of structural defects, thermal activation of charge carriers, and the Maxwell–Wagner polarization.     

Effect of Sr and Ca substitution of Ba on the photoluminescence properties of the Eu2+ activated Ba2MgSi2O7 phosphor

The effects of Sr and Ca substitution of Ba on the Ba_1.98-xSr_x(Ca_x)MgSi₂O₇:Eu²⁺ photoluminescence properties have been investigated. The physical mechanisms for the photoluminescence variations are discussed. With Rietveld refinement method, the crystal structure of Ba_1.98MgSi₂O₇:0.02Eu²⁺ and the lattice parameters of Sr and Ca substituted phosphors were refined. The emission band shift, the photoluminescence intensity variation, the phosphor chromaticity evolution, the Eu²⁺ lifetime distribution and the thermal stability elevation were investigated. With Sr and Ca substitution, the cell is shrinks. The cell shrinkage is resulting in the increase of the Eu²⁺ 5d electron crystal field splitting intensity, which is the reason for the emission band shift towards the long wavelength band. The photoluminescence intensity is increased firstly and then decreased. The intensity variation is the competitive result between the increase of the crystal structure rigidity and the rise of the lattice defect. The correlated color temperature can be cut down and the color purity can be adjusted. The photoluminescence life time of Eu²⁺ is raised firstly and then decreased. For Sr and Ca substitution, the thermal stability can be elevated. With the forbidden band gap calculation, the reason for the thermal stability elevation was investigated that for the substituted phosphors the forbidden band gap is enlarged and then limits the Eu²⁺ 5d self-ionization from the splitting levels to the conduction band. This work reveals that the Sr and Ca substitution of Ba can elevate the Ba_1.98-xSr_x(Ca_x)MgSi₂O₇:Eu²⁺ photoluminescence properties and improve the applications for the White Light Emitting Diode.     

Molten salt synthesis and optical properties of Eu3+, Dy3+or Nd3+ doped NiNb2O6 columbite-type phosphors

Pure, Eu³⁺, Dy³⁺ or Nd³⁺-doped NiNb₂O₆ powders have been prepared by a molten salt synthesis method by using Li₂SO₄–Na₂SO₄ salt mixture as a flux at relatively low temperatures as compared to the solid state reaction method. X-ray diffraction patterns of pure NiNb₂O₆ samples indicated an orthorhombic single phase. For Eu³⁺-doped NiNb₂O₆ samples, the luminescence of Eu³⁺ was observed at 615 nm as red emission while Dy³⁺-doped NiNb₂O₆ showed yellow emission at 577 nm and Nd³⁺ doped sample exhibited a typical emission at 1064 nm varying with the Eu³⁺ or Nd³⁺ doping concentrations. These luminescence characteristics of the doped samples may be attributed to the energy transfer between rare earth ions and NiO₆ octahedral groups in the columbite structure.     

Preparation and study of red phosphor pewder Gd2-xMo3O9:Eu3+x in white light LED

用高温固相法成功制备了G2-x Mo3O9:Eu3+x,用XRD荧光光谱仪对其物相以及粉体的激发和发射光谱进行表征和研究;结果表明:在395和464nm两主激发峰均可得到616nm处红光发射峰,属于Eu3+典型的5D0-7F2的跃迁所致.由464nm激发得到的发射峰为单峰,峰宽较窄且发射强度较强.     

Effects of B2O3 substitution for Al2O3 on the crystallization and properties of translucent mica glass-ceramics

The effects of substituting Al₂O₃ with B₂O₃ on the structure, crystallization, mechanical properties, thermal properties and optical properties of translucent mica glass-ceramics were thoroughly investigated. The results demonstrated that the addition of 0.5 wt% B₂O₃ was optimum for glass precipitation, which increases the crystallinity of glass-ceramics and provides good translucency. When the content of B₂O₃ was greater than 0.5 wt%, both crystallinity and translucency decreased noticeably. The replacement of B₂O₃ for Al₂O₃ had no influence on the type of crystal phase and the precipitation of tetrasilicic fluoromica with non-stoichiometric ratio, while it did have an effect on the crystallinity and structure. The crystal sizes of glass-ceramics were in the nanoscale range and the transmittance test results indicated that they exhibit excellent translucency.     

TiN含量对TiN-Al2O3复合材料力学性能及导电性能的影响

以纯度(质量分数)均为99%的微米级TiN和α-Al2O3粉体为原料,采用湿法球磨混合5h制备了TiN体积分数分别为0、10%、20%、30%和40%的TiN-Al2O3复合粉体。将复合粉体压制成型,在200℃干燥12h,通过热压炉在pN2=-0.098MPa条件下于1800℃保温3h烧结得到TiN-Al2O3复合材料,并研究了微米级TiN含量对TiN-Al2O3材料力学性能和导电性能的影响。结果表明:随着TiN含量的增加,复合材料的烧结性能和力学性能不断提高,电阻率不断下降。TiN含量为40%(体积分数)时,材料的抗弯强度498MPa,断裂韧性4.285MPa·m1/2,电阻率1.34×10-3Ω·cm;材料的导电性能符合渗流理论,其渗流阈值Vc=17.24%,这与试验结果一致。而显微结构分析表明:TiN颗粒主要分布在Al2O3晶界处,晶粒细小,它们相互交织在一起形成网络,起到了抑制Al2O3晶粒长大和增韧补强作用,提高了材料的力学性能。     

Effect of Gd3+ and Al3+ on optical and dielectric properties of ZnO nanoparticle prepared by two-step hydrothermal method

Gd³⁺ and Al³⁺ co-doped ZnO nanoparticles were prepared by hydrothermal method. The effects of dopants on the microstructural, morphological, optical and dielectric properties of host ZnO system were investigated in the present report. The microstructural and morphological investigations of all the samples were carried out by X-ray diffraction (XRD) and Field emission scanning electron microscopy (FESEM). To investigate the presence of various intrinsic defects inside the undoped and doped ZnO samples, the Raman spectroscopy and photoluminescence spectroscopy measurements were recorded. The dielectric spectroscopy was carried out as a function of frequency and temperature. The consequences of dielectric measurements suggest that the dielectric response of the 3% Gd³⁺ and Al³⁺ co-doped ZnO sample is significantly enhanced compared to that of undoped ZnO sample. The dielectric response enhances due to the presence of large amount of oxygen vacancies and grain boundaries in the nanostructure of the doped material. All the dielectric parameters confirm the presence of dielectric dispersion inside the doped ZnO samples. The values of dielectric constant as well as ac conductivities were found to decrease at higher concentration of Gd³⁺ and Al³⁺ in the doped ZnO sample and it occurs due to predominating acceptor effect of Al³⁺ at the interstitial site of ZnO nanostructure.     

Electrical conductivity and thermal properties of spark plasma sintered Al2O3-SiC-CNT hybrid nanocomposites

In this study, we report the electrical conductivity and thermal properties of Al₂O₃-SiC-CNT hybrid nanocomposites processed via ball milling (BM) and spark plasma sintering (SPS). The initial powders and consolidated samples were characterized using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM), respectively. A multifunction calibrator and a high-resolution digital multimeter were used to measure the electrical conductivity. The thermal properties were measured using a thermal constants analyser. The SiC and CNT-reinforced alumina hybrid nanocomposites exhibited a significant increase in their room-temperature electrical conductivity, which made them suitable for electrical discharge machining. The Al₂O₃-5SiC-2CNTs had a high electrical conductivity value of 8.85 S/m compared to a low value of 6.87×10⁻¹⁰ S/m for the monolithic alumina. The addition of SiC and CNTs to alumina decreased its room-temperature thermal properties. The increase in temperature resulted in a decrease in the thermal conductivity and thermal diffusivity but an increase in the specific heat of the monolithic alumina and the hybrid nanocomposites. These properties were correlated with the microstructure, and possible transport mechanisms were discussed.     

Impedance spectroscopic investigation on electrical conduction and relaxation in manganese substituted pyrochlore type semiconducting oxides

The effect of simultaneous substitutions of Mn in both A and B sites of the pyrochlore type semiconducting oxides: (CaCe_1−2xMn_2x)(Sn_1−xMn_xINb)O_7−δ (x=0, 0.1, 0.2, 0.3 and 0.4) on the electrical conduction and relaxation was studied in detail using impedance analysis as a function of frequency over a wide range of temperature. Impedance and modulus analysis clearly explain the relaxation in these materials and its dependence on Mn concentration. Grain boundary dominant electrical characteristics have been observed with progressive Mn substitution. Correlated barrier hopping model was successfully applied for explaining the conduction mechanism in these compounds. Variation of hopping parameters with Mn substitution in these materials indicates strong dependence on the grain and grain boundary contributions. This insight to the conduction mechanism of the system offers in tuning the electrical properties for desired applications such as NTC thermistors.     

Influence of Ba2+ doping on the thermoelectric properties of BiCuSeO fabricated by spark plasma sintering

We studied the influence of Ba²⁺ doping on the thermoelectric properties of the p-type Bi_1–xBa_xCuSeO (0?≤?x?≤?0.21) fabricated by spark plasma sintering. The substitution of Ba²⁺ for Bi³⁺ gradually increased the electrical and thermal conductivities and decreased the Seebeck coefficient, which were due to the increased hole concentration. The largest value of dimensionless figure-of-merit (0.57) was obtained for the Bi_0.86Ba_0.14CuSeO at 500?°C, which was over three times greater than that of pristine BiCuSeO (0.18) at 500?°C. We believe that the thermoelectric properties of BiCuSeO were substantially enhanced through the partial substitution of Ba²⁺ for Bi³⁺.     

Color-tunable properties of Eu3+- and Dy3+-codoped Y2O3 phosphor particles

Rare-earth phosphors are commonly used in display panels, security printing, and fluorescent lamps, and have potential applications in lasers and bioimaging. In the present study, Eu³⁺- and Dy³⁺-codoped uniform-shaped Y₂O₃ submicron particles were prepared using the urea homogeneous precipitation method. The structure and morphology of the resulting particles were characterized by X-ray diffraction, field emission scanning electron microscope, and field emission transmission electron microscope, whereas their optical properties were monitored by photoluminescence spectroscopy. The room-temperature luminescence color emission of the synthesized particles can be tuned from red to yellow by switching the excitation wavelength from 254 to 350 nm. The luminescence intensities of red and yellow emissions could be altered by varying the dopant concentration. Strong quenching was observed at high Eu³⁺ and Dy³⁺ concentrations in the Y₂O₃ host lattice.     

Crystal structure,electrical and magnetic properties of anion-deficient perovskite-like Ba3SmFe2O7.5

Anion-deficient perovskite-like Ba₃SmFe₂O_7.5 was prepared using a glycerol–nitrate synthesis. Using high-temperature X-ray diffraction in situ a crystal structure transition temperature range 800 and 840 °C was established. These results were further confirmed by high-temperature dilatometric analysis. The average thermal expansion coefficient (TEC) of Ba₃SmFe₂O_7.5 is about 12.8 × 10⁻⁶ K⁻¹ between 25 °C and 800 °C. Magnetic experiments proved an excellent phase purity of the oxide and reveal that Fe³⁺ ions stay in high and intermediate spin states in a ratio of 75% and 25% respectively.     

Structural and spectral properties of red light emitting Eu3+ activated TiO2 nanophosphor for white LED application

The structural and luminescent properties of Eu³⁺ doped TiO₂ nanophosphors synthesized by low cost combustion method were investigated. The X-ray diffraction analysis revealed that crystallite size decreases with doping concentration. Lattice volume expansion occurred due to the substitution of Ti⁴⁺ ions by larger ionic radii ions Eu³⁺. FESEM images showed prepared phosphors to be nano size spherical shaped particles. Energy band gap of 3 mol% Eu³⁺ doped samples decreased to 3.15 eV due to doping effect. The Eu³⁺ doped TiO₂ nanophosphors exhibited main red emission peak centered at 616 nm under 395 nm UV light excitation. Concentration quenching was observed at 3 mol% doping, that has been ascribed to dipole-dipole interaction. The covalent nature of Eu-O bond and environment around Eu³⁺ ions were discussed using Judd-Ofelt (J-O) intensity parameters. Internal quantum efficiency was calculated using excited state lifetime ⁵D₀ state of Eu³⁺ ion and J-O theory. The CIE colour coordinates and colour purity were calculated using the spectral energy distribution function. Low excited state life time indicated that Eu³⁺ doped TiO₂ can be used as red emitting phosphor for white light emitting diode applications.     

Fine Ti‐dispersed Al2O3 composites and their mechanical and electrical properties

Al₂O₃/Ti composites containing 0‐30 vol% dispersed fine Ti particles were fabricated using a hot‐press sintering method at 1500°C from mixtures of Al₂O₃ and TiH₂ powders. During sintering, TiH₂ decomposed to form metallic Ti. The effects of the Ti content on the mechanical and electrical properties of the composites were then investigated. No Ti‐Al intermetallic compounds were detected by X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy indicated the presence of Al‐Ti‐O solid solution and Ti‐O phases. The composites showed enhanced densification; the measured densities were higher than the calculated theoretical values. Microstructural observation revealed homogeneously distributed fine Ti particles dispersed in the Al₂O₃ matrix. The Ti particle size ranged from submicrometer to a few micrometers depending on the Ti content. The fracture mode of the composites was primarily transgranular, in contrast to the intergranular fracture mode of monolithic Al₂O₃. Although the flexural strength was decreased with increase in Ti content, the composite containing 20 vol% Ti displayed the maximum fracture toughness of 4.3 MPa·cm^1/2, which was 37% greater than that of monolithic Al₂O₃. The composites containing more than 15 vol% Ti exhibited drastic decreases in resistivity (~10^?1 Ωcm), which were attributed to the formation of interconnected Ti networks at these Ti contents. The percolation threshold volume for electrical conduction in the present system was calculated to be 13.8 vol%. The results indicate that dispersing fine Ti particles into Al₂O₃ increased the fracture toughness and improved the conductivity of Al₂O₃.     

Improvement of wetability,sinterability, mechanical and electrical properties of Al2O3-Ni nanocomposites prepared by mechanical alloying

The wetability improvement and particle size reduction of alumina/Ni composites through mechanical alloying were addressed. Their effect on the sinterability (at high temperature), mechanical and electrical properties were studied. Al₂O₃ matrix nanocomposites reinforced with different volume fractions of Ni up to 10 vol% were prepared by mechanical alloying. The milled powders were cold pressed and sintered at different firing temperatures up to 1600 °C. The morphology of powders and the microstructure of sintered bodies were investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), respectively. Furthermore, relative density, apparent porosity, mechanical properties and electrical resistivity of the sintered composites were investigated. The results revealed that Al₂O₃ matrix was successfully coated with Ni thin film through mechanical alloying; the thickness of coat was increased with increasing the Ni content. Moreover, the increasing of both Ni content and sintering temperature up 1600 °C, led to a remarkable increase in the relative density and facture toughness of the sintered specimen. On the other hand, microhardness and elastic modulus were decreased with increasing of Ni content, while they increased significantly with the increase of sintering temperature. The electrical resistivity was decreased with increasing Ni content and sintering temperature.     

Thermoelectric properties of La3+ and Ce3+ co-doped CaMnO3 prepared by tape casting

The structural and thermoelectric transport properties of Ca_0.9La_0.1–xCe_xMnO_3–δ (x = 0 – 0.1) prepared through tape casting process were investigated. The thermoelectric transport properties of CaMnO₃ were optimized by the La³⁺ and Ce³⁺ co-doping and tape casting process. The Ce³⁺ substitution substantially enhanced the dimensionless figure-of-merit (ZT) due to the increases in electrical conductivity and absolute Seebeck coefficient. Of the studied samples, Ca_0.9La_0.025Ce_0.075MnO_3–δ had the maximum ZT (0.17) at 800 °C, and this value was 42% and 70% larger than those of singly doped Ca_0.9La_0.1MnO_3–δ (0.12) and Ca_0.9Ce_0.1MnO_3–δ (0.10), respectively. We demonstrate that both the La³⁺ and Ce³⁺ co-doping and tape casting process are highly promising approaches in enhancing the high-temperature thermoelectric transport properties of CaMnO₃.     

Improvement on the phase stability,mechanical properties and thermal insulation of Y2O3-stabilized ZrO2 by Gd2O3 and Yb2O3 co-doping

Gd₂O₃ and Yb₂O₃ co-doped 3.5 mol% Y₂O₃–ZrO₂ and conventional 3.5 mol% Y₂O₃–ZrO₂ (YSZ) powders were synthesized by solid state reaction. The objective of this study was to improve the phase stability, mechanical properties and thermal insulation of YSZ. After heat treatment at 1500 °C for 10 h, 1 mol% Gd₂O₃–1 mol% Yb₂O₃ co-doped YSZ (1Gd1Yb-YSZ) had higher resistance to destabilization of metastable tetragonal phase than YSZ. The hardness of 5 mol% Gd₂O₃–1 mol% Yb₂O₃ co-doped YSZ (5Gd1Yb-YSZ) was higher than that of YSZ. Compared with YSZ, 1Gd1Yb-YSZ and 5Gd1Yb-YSZ exhibited lower thermal conductivity and shorter phonon mean free path. At 1300 °C, the thermal conductivity of 5Gd1Yb-YSZ was 1.23 W/m K, nearly 25% lower than that of YSZ (1.62 W/m K). Gd₂O₃ and Yb₂O₃ co-doped YSZ can be explored as a candidate material for thermal barrier coating applications.