Crystalline morphology and photoluminescent properties of YInGe2O7:Eu phosphors prepared from microwave and conventional sintering

This paper describes an investigation of the crystalline morphology and photoluminescent properties of YInGe₂O₇:Eu³⁺ powders using microwave assisted sintering. For comparison, the properties of YInGe₂O₇:Eu³⁺ powders sintered at 1200 °C in conventional furnace for 10 h were also investigated. X-ray powder diffraction analysis confirmed the formation of monoclinic YInGe₂O₇ without second phase or phases of starting materials as YInGe₂O₇:50 mol% Eu powders sintered at 1200 °C in microwave furnace for 1 h. Scanning electron microscopy showed smaller particle size and more uniform grain size distributions are obtained by microwave assisted sintering. In the PL studies, both microwave sintered and conventionally sintered powders emitted a maximum luminescence centered at 620 nm under excitation of 393 nm with similar luminescent intensity. The results show that microwave processing has the potential to reduce the time and required energy input for the production of YInGe₂O₇:Eu³⁺ phosphors without sacrificing the photoluminescence.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

Mechanochemical synthesis of MgTa2O6 ceramic

MgTa₂O₆ powders were prepared by mechanochemical synthesis from MgO and Ta₂O₅ in a planetary ball mill in air atmosphere using steel vial and steel balls. High-energy ball milling gave nearly single-phase MgTa₂O₆ after 8 h of milling time. Annealing of high-energy milled powder at various temperatures (700–1200 °C) indicated that high-energy milling speed up the formation and crystallization of MgTa₂O₆ from the amorphous mixture. The powder derived from 8 h of mechanical activation gave a particle size of around 28 nm. Although at low-annealing temperatures the grain size was almost the same as-milled powder, the grain size increased with annealing temperature reaching to around 1–2 μm after annealing at 1200 °C for 8 h.     

Synthesis of lanthanum beta-alumina powders by the polymeric precursor technique

La-β-Al₂O₃ (LaAl₁₁O₁₈) powders were synthesized by the polymeric precursor technique using lanthanum nitrate and aluminum nitrate. The transformations during thermal treatment of the precursor solution with ethylene glycol and citric acid were evaluated by thermal analysis. Fourier transform infrared spectroscopy analysis was performed after calcinations of the polymeric resin for determination of residual carbon. The specific surface area was evaluated by the BET method. Fine powders with ∼121 m²/g specific surface area and 20 nm average particle size were obtained and observed by scanning and transmission electron microscopy. Nearly single phase LaAl₁₁O₁₈ was obtained after pressing and sintering these powders at 1600 °C with small additions of MgO. The sintered pellets were characterized by X-ray diffraction and scanning electron microscopy. Impedance spectroscopy measurements carried out in the 1000–1200 °C range show the electrolytic behavior of the La-β-Al₂O₃ pellets, suggesting their application as solid electrolytes in high temperature potentiometric oxygen sensors.     

Synthesis of LiNi1/3Co1/3Mn1/3O2 cathode materials by using a supercritical water method in a batch reactor

LiNi_1/3Co_1/3Mn_1/3O₂ and LiCoO₂ cathode materials were synthesized by using a supercritical water (SCW) method with a metal salt solution in a batch reactor. Stoichiometric LiNi_1/3Co_1/3Mn_1/3O₂ was successfully synthesized in a 10-min reaction without calcination, while overlithiated LiCoO₂ (Li_1.15CoO₂) was synthesized using the batch SCW method. The physical properties and electrochemical performances of LiNi_1/3Co_1/3Mn_1/3O₂ were compared to those of Li_1.15CoO₂ by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and charge/discharge cycling tests. The XRD pattern of LiNi_1/3Co_1/3Mn_1/3O₂ was found to be similar to that of Li_1.15CoO₂, showing clear splitting of the (0 0 6)/(1 0 2) and (1 0 8)/(1 1 0) peak pairs as particular characteristics of the layered structure. In addition, both cathode powders showed good crystallinity and phase purity, even though a short reaction time without calcination was applied to the SCW method. The initial specific discharge capacities of the Li_1.15CoO₂ and LiNi_1/3Co_1/3Mn_1/3O₂ powders at a current density of 0.24 mA/cm² in 2.5-4.5 V were 149 and 180 mAh/g, and their irreversible capacity loss was 20 and 17 mAh/g, respectively. The discharge capacities of the Li_1.15CoO₂ and LiNi_1/3Co_1/3Mn_1/3O₂ powders decreased with cycling and remained at 108 and 154 mAh/g after 30 cycles, which are 79% and 89% of the initial capacities. Compared to the overlithiated LiCoO₂ cathode powders, the LiNi_1/3Co_1/3Mn_1/3O₂ cathode powders synthesized by SCW method had better electrochemical performances.     

The effect of nano-Cr2O3 on solid-solution assisted sintering of MgO refractories

The effect of Cr₂O₃ particle size on the densification of magnesia refractories was investigated. Magnesia grains (<45 μm) were mixed with 2 wt% of micro-Cr₂O₃ (2 μm) and nano-Cr₂O₃ particles (10–20 nm) and sintered at 850–1450 °C, for 5 h in air. The progress of the densification and phase evolution of samples was studied with the support of X-ray diffraction phase analysis (XRD), Fourier transformer infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). It was shown that the densification of magnesia was enhanced by reducing the particle size of the added chromia to the range of 20 nm. According to the phase analysis results, the higher dissolution rate of Cr₂O₃ in MgO in the MgO–Cr₂O₃ system was responsible for the faster densification of nano-Cr₂O₃ containing mixes.     

Synthesis of Zn3Nb2O8 ceramics using a simple and effective process

Synthesis of Zn₃Nb₂O₈ ceramics using a simple and effective reaction-sintering process was investigated. The mixture of ZnO and Nb₂O₅ was pressed and sintered directly without any prior calcination. Single-phase Zn₃Nb₂O₈ ceramics could be obtained. Density of these ceramics increased with soaking time and sintering temperature. A maximum density 5.72 g/cm³ (99.7% of the theoretical density) was found for pellets sintered at 1170 °C for 2 h. Pores were not found and grain sizes >20 μm were observed in pellets sintered at 1170 °C. Abnormal grain growth occurred and grains >50 μm could be seen in Zn₃Nb₂O₈ ceramics sintered at 1200 °C for 2 h and 1200 °C for 4 h. Reaction-sintering process is then a simple and effective method to produce Zn₃Nb₂O₈ ceramics for applications in microwave dielectric resonators.     

Synthesis and characterization of high-density non-spherical Li(Ni1/3Co1/3Mn1/3)O2 cathode material for lithium ion batteries by two-step drying method

Non-spherical Li(Ni_1/3Co_1/3Mn_1/3)O₂ powders have been synthesized using a two-step drying method with 5% excess LiOH at 800 °C for 20 h. The tap-density of the powder obtained is 2.95 g cm⁻³. This value is remarkably higher than that of the Li(Ni_1/3Co_1/3Mn_1/3)O₂ powders obtained by other methods, which range from 1.50 g cm⁻³ to 2.40 g cm⁻³. The precursor and Li(Ni_1/3Co_1/3Mn_1/3)O₂ are characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscope (SEM). XPS studies show that the predominant oxidation states of Ni, Co and Mn in the precursor are 2⁺, 3⁺ and 4⁺, respectively. XRD results show that the Li(Ni_1/3Co_1/3Mn_1/3)O₂ material obtained by the two-step drying method has a well-layered structure with a small amount of cation mixing. SEM confirms that the Li(Ni_1/3Co_1/3Mn_1/3)O₂ particles obtained by this method are uniform. The initial discharge capacity of 167 mAh g⁻¹ is obtained between 3 V and 4.3 V at a current of 0.2 C rate. The capacity of 159 mAh g⁻¹ is retained at the end of 30 charge-discharge cycle with a capacity retention of 95%.     

Effect of Al2O3 particle size on preparation and properties of ZTA ceramics formed by gelcasting

Zirconia-toughened alumina (ZTA) ceramics were prepared using three different kinds of Al₂O₃ powders (marked PW-A average particle size: 7.53 μm, marked PW-B average particle size: 1.76 μm, marked PW-C average particle size: 0.61 μm) by gelcasting. Effect of Al₂O₃ particle size on zeta potential, dispersant dosage and solid volume fractions of ZTA suspensions as well as the mechanical properties of ZTA green bodies and ceramics were investigated. The optimum dosages of dispersant for ZTA suspensions prepared by PW-A, PW-B and PW-C are 0.4 wt%, 0.5 wt% and 0.7 wt%, respectively. The highest solid volume fractions of ZTA suspensions can reach 62 vol% (SP-A), 60 vol% (SP-B) and 52 vol% (SP-C), respectively. The green bodies show a bending strength as high as 20 MPa, which can meet the requirement of machining. The Al₂O₃ powder with fine particle size is beneficial to the improvement of mechanical properties. The ZTA ceramics prepared by PW-B Al₂O₃ powder show the highest bending strength (680 MPa) and toughness (7.49 MPa m^1/2).     

Nanorod-assembled spinel Li1.05Mn1.95O4 rods with a central tunnel along the rod-axis for high rate capability of rechargeable lithium-ion batteries

Nanorod-assembled spinel Li_1.05Mn_1.95O₄ rods with a central tunnel along the rod-axis were synthesized using highly crystalline β-MnO₂ rods as self-templates. The synthesized spinel Li_1.05Mn_1.95O₄ is an assembly of several single crystal-like nanorods with an average diameter and length of 100 and 400 nm, respectively, which was determined by microstructural Rietveld refinement using the synchrotron powder XRD data. Galvanostatic battery testing showed that central-tunneled and nanorod-assembled Li_1.05Mn_1.95O₄ rods have a high charge storage capacity at high current densities in comparison with those of the spinel rods without a tunnel structure and commercial powders. Moreover, a capacity retention value of ∼81% was observed at the end of 100 cycles at a current of 250 mAh g⁻¹.     

Molten salt synthesis of spherical LiNi0.5Mn1.5O4 cathode materials

This paper describes a novel simple redox process for synthesizing monodispersed MnO₂ powders and preparation of spherical LiNi_0.5Mn_1.5O₄ cathode materials by molten salt synthesis (MSS) method. Monodispersed MnO₂ powders have been synthesized by using potassium permanganate and manganese sulfate as the starting materials. By using this redox method, it was found that monodispersed MnO₂ powders with average particle size ∼5 μm can be easily obtained. Resultant MnO₂ and LiOH, Ni(OH)₂ was then used to synthesis LiNi_0.5Mn_1.5O₄ cathode materials with retention of spherical particle shape by MSS method. The discharge capacity was 129 mAh g⁻¹ in the first cycle and 127 mAh g⁻¹ after 50 cycles under an optimal synthesis condition for 12 h at 800 °C.     

Preparation of nanocrystalline Sr3Al2O6 powders via citric acid precursor

Sr₃Al₂O₆ was synthesized via citric acid precursor. The effects of the molar ratio of citric acid to total metal cations concentration (CA/M) on the formation of Sr₃Al₂O₆ were investigated. Increasing the CA/M promoted the formation of Sr₃Al₂O₆. Single-phase and well-crystallized Sr₃Al₂O₆ was obtained from the CA/M = 1, CA/M = 2 and CA/M = 4 precursor at temperature 1200 °C, 1100 °C and 900 °C, respectively. Differential thermal analysis and thermogravimetric (DTA/TG), X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM) were used to characterize the precursors and the derived oxide powders. Sr₃Al₂O₆ nanoparticles with a diameter of about 50-70 nm were obtained.     

Synthesis and sintering of Y2O3-doped ZrO2 powders using two Pechini-type gel routes

Yttria-tetragonal zirconia polycrystal (ZrO₂ + 4.5 mol% Y₂O₃) nanocrystalline powder was synthesized by two Pechini-type gel routes, the in situ polymerized complex (IPC) method and the PEG/AF method. FTIR spectra confirmed coordination of metal ions with the polymer by different routes, depending on the method used. The crystallite size of the powder increased from 5 nm to 8 nm when the temperature was increased from 450 °C to 600 °C and calcination times increased from 2 h to 24 h. The morphology of the powders differed only when the organic impurities were not completely eliminated. After calcination, the morphology of the powders produced by the two methods showed porous agglomerates composed of smaller particles. All the resulting microstructures were very similar, regardless of the method employed to obtain the powder or the calcination times and temperatures.     

Novel synthesis of layered LiNi1/2Mn1/2O2 as cathode material for lithium rechargeable cells

A new solution combustion synthesis of layered LiNi_0.5Mn_0.5O₂ involving the reactions of LiNO₃, Mn(NO₃)₂, NiNO₃, and glycine as starting materials is reported. TG/DTA studies were performed on the gel-precursor and suggest the formation of the layered LiNi_0.5Mn_0.5O₂ at low temperatures. The synthesized material was annealed at various temperatures, viz., 250, 400, 600, and 850 °C, characterized by means of X-ray diffraction (XRD) and reveals the formation of single phase crystalline LiNi_0.5Mn_0.5O₂ at 850 °C. The morphology of the synthesized material has been investigated by means of scanning electron microscopy (SEM) and suggests the formation of sub-micron particles. X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) studies on the synthesized LiNi_0.5Mn_0.5O₂ powders indicate that the oxidation states of nickel and manganese are +2 and +4, respectively. Electrochemical galvanostatic charge-discharge cycling behavior of Li//LiNi_0.5Mn_0.5O₂ cell using 1 M LiPF₆ in EC/DMC as electrolyte exhibited stable capacities of ∼125 mAh/g in the voltage ranges 2.8-4.3 V and 3.0-4.6 V and is comparable to literature reports using high temperature synthesis route. The capacity remains stable even after 20 cycles. The layered LiNi_0.5Mn_0.5O₂ powders synthesized by this novel route have several advantages as compared to its conventional synthesis techniques.     

Agglomeration of α-Al2O3 powders prepared by gel combustion

Ultrafine α-Al₂O₃ powders were prepared by a gel combustion method and the agglomeration characteristic of the resultant powders was studied. A variety of fine crystallite α-Al₂O₃ powders with different agglomeration structures could be obtained by altering the citrate-to-nitrate ratio γ and calcining the precursors at 1050 °C for 2 h. All the powders were of nearly equivalent crystallite size (60–80 nm) except for the P1 powder (113 nm) from the gel with γ = 0.033. The primary crystallites of the obtained α-Al₂O₃ powders were formed into large secondary particles with different degree of agglomeration. Except for the powder P1, the mean particle sizes from specific surface area and particle size distribution measurement increase with increasing citrate-to-nitrate ratio in the fuel-lean condition and decrease in the fuel-rich condition. Densities of alumina ceramics from powders P4 and P5 sintered at different temperatures were relatively low due to the wide particle size distribution.     

Growth of Sb2O3 submicron rods by the thermal evaporation of a mixture of Zn and Sb powders

We synthesized crystalline Sb₂O₃ rods by heating a mixture of Sb and Zn powders. Scanning electron microscopy indicated that the Zn powder as well as the growth temperature affected the morphology of the product. We discussed the possible growth mechanisms. Scanning electron microscopy, X-ray diffraction, selected area electron diffraction, Raman spectroscopy and transmission electron microscopy collectively revealed that the products consisted of Sb₂O₃ having a pure cubic structure with diameters in the range of 150-600 nm. The photoluminescence spectrum of the Sb₂O₃ submicron rods under excitation at 325 nm exhibited a visible light emission.     

Synthesis and characterization of MgAl2O4–ZrO2 composites

Different types of dense 5–97% ZrO₂–MgAl₂O₄ composites have been prepared using a MgAl₂O₄ spinel obtained by calcining a stoichiometric mixture of aluminium tri-hydroxide and caustic MgO at 1300 °C for 1 h, and a commercial yttria partially stabilized zirconia (YPSZ) powder as starting raw materials by sintering at various temperatures ranging from 1500 to 1650 °C for 2 h. The characteristics of the MgAl₂O₄ spinel, the YPSZ powder and the various sintered products were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface area, particle size analysis, Archimedes principle, and Vickers indentation method. Characterization results revealed that the YPSZ addition increases the sintering ability, fracture toughness and hardness of MgAl₂O₄ spinel, whereas, the MgAl₂O₄ spinel hampered the sintering ability of YPSZ when sintered at elevated temperatures. A 20-wt.% YPSZ was found to be sufficient to increase the hardness and fracture toughness of MgAl₂O₄ spinel from 406 to 1314 Hv and 2.5 to 3.45 MPa m^1/2, respectively, when sintered at 1600 °C for 2 h.     

Formation of GdAlO3–Al2O3 composite having fine pseudo-eutectic microstructure

GdAlO₃ and Al₂O₃ powders were mixed and pulverized using ball mills. The prepared powder was sintered by SPS at 1450 °C without holding time. SEM observation of the sintered specimen showed a eutectic-like microstructure. This is called ‘pseudo-eutectic’ in this research. The microstructure formed from a powder pulverized by a tumbling ball mill for one week was much finer than that by a planetary ball mill for 5 and 10 h. The fine homogeneous eutectic-like (pseudo-eutectic) microstructures could be formed at both eutectic and off-eutectic compositions. In case of crystallization from a melt of eutectic components, homogeneous eutectic microstructures can be formed only at restricted compositions very close to the eutectic one. Coarse primary crystals generally exist in the eutectic microstructure at off-eutectic compositions. The pseudo-eutectic microstructures can be formed at any compositions because a mixing ratio of the starting powders can be varied.     

Influence of magnesia on the structure and properties of MgO-Al2O3-SiO2-F glass-ceramics

Effects of MgO content (13.4–31.4 mol%) on the structure and properties of MgO-Al₂O₃-SiO₂-F⁻ glass-ceramics were investigated by differential scanning calorimetry (DSC), X-ray diffractometry (XRD), infrared spectrophotometry (IR) and scanning electron microscopy (SEM). Results show that the main units of glass network structure are [SiO₄] and [AlO₄]. MgO contributes to the weakening of silica network and reduce the stability of glass structure. The main crystals of the MgO-Al₂O₃-SiO₂-F⁻ glass-ceramics are phlogopite, spinel, flur-pargrasite and forsterite. The increase of reheating temperature and MgO content are beneficial to the separation of phlogopite crystal, and cause an higher aspect ratio of the phlogopite phase, which improves the machinability of the glass-ceramics. Excellent machinability is obtained when MgO content is 31.4 mol% at the processing temperature of 1100 °C for 2 h.     

Co-precipitated ZnAl2O4 spinel precursor as potential sintering aid for pure alumina system

Ultra-fine ZnAl₂O₄ spinel hydrogel precursor synthesized from mixed salt solutions of Zn²⁺ and Al³⁺ ions using ammonium hydroxide–hexamethylenetetramine as basic media for co-precipitation was used as bonding material and sintering aid for pure alumina system. The hydrogel powder exhibited some well-defined ZnAl₂O₄ spinel phases at 800 °C. Alumina compacts were fabricated by incorporating small proportions of the precursor in alumina powder and firing at different temperatures (1350–1500 °C). The degree of densification was studied by measurement of fired shrinkage, apparent porosity, bulk density and cold crushing strength. Phase compositions and microstructural features of sintered samples were evaluated by XRD and SEM respectively. Addition of 0.2% hydrogel powder to alumina exhibited remarkable influence on development of high mechanical strength. The in situ formed ZnAl₂O₄ spinel dopant acted as a grain growth inhibitor in the alumina system.