Energy transfer mechanism of GdPO4: RE3+ (RE = Tb, Tm) under VUV-UV excitation

Polycrystal of GdPO₄:RE³⁺ (RE=Tb, Tm) phosphors were prepared by solid-state method. Vacuum ultraviolet excitation and emission spectrum and the energy transfer mechanism between the host and dopants of Tb³⁺ and Tm³⁺ were investigated respectively. The emission of Gd³⁺ at 313 nm was enhanced by the strong absorption of CTS of Tm³⁺ at 180 nm in GdPO₄: Tm. It has also been concluded that the excitation of Gd³⁺ is transferred to Tb³⁺ and then emission peaks of ⁵D_J→⁷F_J of Tb³⁺ were observed.     

Synthesis and luminescence properties of red phosphor CaO: Eu3+

Ca(NO₃)₂·4H₂O, Eu(NO₃)₃ and H₂C₂O₄·2H₂O were adopted to synthesize CaO: Eu³⁺ with the chemical co-precipitation method, and the effects of the calcination temperature and Eu³⁺ doping concentration on the phosphor structure and its luminescent properties were investigated by TG-SDTA, XRD, and PL-PLE. The results confirm that the Eu³⁺ ions as luminescent centers substitutes Ca²⁺ sites without changing the crystal structure of cubic CaO. The optimum calcination temperature and the optimum concentration of Eu³⁺ are 1 100 ° and 1 mol%, respectively, under which the best crystallinity and highest PL intensity appeared. The maximum emission wavelength is 592 nm (⁵D₀→⁷F₁) which is excited by xenon lamp with the wavelength of 200–280 nm, indicating that the Eu³⁺ ion mainly locates in the symmetric position (O_h) in the crystal lattice of CaO. Funded by the Science and Technology Bureau of Sichun Province(No. 2006J13-059) and Education Bureau of Sichun Province (No. 2006A094)     

Preparation and structure characteristics of nano-Bi2O3 powders with mixed crystal structure

The nano-Bi2O3 powders were prepared by a chemical precipitation method with Bi(NO3)3, HNO3 and NaOH as reactants. The structural characteristics and morphology of nano-Bi2O3 powders were investigated by X-ray diffraction and transmission electron microscopy, respectively. The results show that under the optimum condition that 300g/L Bi(NO3)3 reacts at 90℃ for 2 h, the Bi203 powders with 60 nm on the average and 99.5% in purity are obtained. The prepared nano-Bi2O3 powders contain a mixed crystal structure of monoclinic and triclinic instead of traditional structure of monoclinic α-Bi2O3. And the mixed crystal structure is stable in air. The reason for the appearance of the mixed crystal structure may be that the ionic radius ratio of Bi^3 to O^2- changes easily during the formation of nano-Bi2O3 particles by a chemical precipitation method.     

Preparation of LiFePO4 for lithium ion battery using Fe2P2O7 as precursor

In order to obtain a new precursor for LiFePO4, Fe2P2O7 with high purity was prepared through solid phase reaction at 650 ℃ using starting materials of FeC2O4 and NH4H2PO4 in an argon atmosphere. Using the as-prepared Fe2P2O7, Li2CO3 and glucose as raw materials, pure LiFePO4 and LiFePO4/C composite materials were respectively synthesized by solid state reaction at 700 ℃ in an argon atmosphere. X-ray diffractometry and scanning electron microscopy（SEM） were employed to characterize the as-prepared Fe2P2O7, LiFePO4 and LiFePO4/C. The as-prepared Fe2P2O7 crystallizes in the Cl space group and belongs to β-Fe2P2O7 for crystal phase. The particle size distribution of Fe2P2O7 observed by SEM is 0.4-3.0 μm. During the Li^＋ ion chemical intercalation, radical P2O7^4- is disrupted into two PO4^3- ions in the presence of O^2-, thus providing a feasible technique to dispose this poor dissolvable pyrophosphate. LiFePO4/C composite exhibits initial charge and discharge capacities of 154 and 132 mA·h/g, respectively.     

Modification of LiCo1/3Ni1/3Mn1/3O2 cathode material by CeO2-coating

LiCo_1/3Ni_1/3Mn_1/3O₂ was coated by a layer of 1.0 wt% CeO₂ via sol-gel method. The bared and coated LiMn_1/3Co_1/3Ni_1/3O₂ was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammogram (CV) and galvanotactic charge-discharge test. The results show that the coating layer has no effect on the crystal structure, only coating on the surface; the 1.0 wt% CeO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ exhibits better discharge capacity and cycling performance than the bared LiCo_1/3Ni_1/3Mn_1/3O₂. The discharge capacity of 1.0 wt% CeO₂-coated cathode is 182.5 mAh·g⁻¹ at a current density of 20 mA·g⁻¹, in contrast to 165.8 mAh·g⁻¹of the bared sample. The discharge capacity retention of 1.0 wt% CeO₂-coated sample after 12 cycles reaches 93.2%, in comparison with 86.6% of the bared sample. CV results show that the CeO₂ coating could suppress phase transitions and prevent the surface of cathode material from direct contact with the electrolyte, thus enhance the electrochemical performance of the coated material.     

Synthesis of γ-Al2O3 nanoparticles by chemical precipitation method

Highly pure active γ-Al₂O₃ nanoparticles were synthesized from aluminum nitrate and ammonium carbonate with a little surfactant by chemical precipitation method. The factors affecting the synthesis process were studied. The properties of γ-Al₂O₃ nanoparticles were characterized by DTA, XRD, BET, TEM, laser granularity analysis and impurity content analysis. The results show that the amorphous precursor Al(OH)₃ sols are produced by using 0.1 mol/L Al(NO₃)₃ · 9H₂O and 0.16 mol/L (NH₄)₂CO₃ · H₂O reaction solutions, according to the volume ratio 1.33, adding 0.024% (volume fraction) surfactant PEG600, and reacting at 40 °C, 1 000 r/min stirring rate for 15 min. Then, after stabilizing for 24 h, the precursors were extracted and filtrated by vacuum, washed thoroughly with deionized water and dehydrated ethanol, dried in vacuum at 80°C for 8 h, final calcined at 800 °C for 1 h in the air, and high purity active γ-Al₂O₃ nanoparticles can be prepared with cubic in crystal system, O _H ⁷ -FD3M in space group, about 9 nm in crystal grain size, about 20 nm in particle size and uniform size distribution, 131. 35 m²/g in BET specific surface area, 7 – 11 nm in pore diameter, and not lower than 99.93% in purity. Foundation item: Project(03JJY3015) supported by the Natural Science Foundation of Hunan Province     

Antibiotic properties of Al2O3 doping silver

The preparation technique and properties of Ag-type inorganic antibiotic material carried by Al₂O₃ were studied. The results show that the material has good antibiotic and safety properties, the acute toxicity taken by stomata is LD ₅₀>8 000 mg/kg (little and big white rats), and the normal quantity in subacute toxicity test is 80 mg/(kg · d). The better mass fraction of doping Ag₂O in antibiotic material carried by Al₂O₃ is 4%–8%, and the optimal sintering temperature is from 1 000 °C to 1 100 °C. Foundation item: Project (2002AA327090) supported by National High Technology Research and Development Program of China     

Kinetics of Fe3O4 formation by air oxidation

The kinetics of Fe₃O₄ formation by air oxidation of slightly acidic suspension of Fe(OH)₂ was studied. The effects of initial concentration of Fe(II), temperature, partial pressure of oxygen, air flow rate and stirring rate on the oxidation rate were investigated. The results show that Fe₃O₄ formation is composed of two-step reaction, the first step is the formation of Fe(OH) ₂ ⁺ by oxidation of Fe(OH)⁺ complex ions, the second step is the formation of magnetite by dehydration and deprotonation of Fe(OH)⁺ and Fe(OH) ₂ ⁺ . The oxidation reaction is zero-order with respect to the concentration of Fe(II) and around 0.5-order with respect to partial pressure of oxygen, and oxygen transfer process is rate-limiting step of oxidation reaction with apparent activation energy of 2.74 kJ · mol⁻¹.     

Combustion synthesis and luminescent properties of CaO: Eu<Superscript>3+</Superscript>, M<Superscript>+</Superscript> (M=Li,Na, K) phosphor

Li⁺, Na⁺, or K⁺ co-doped CaO: Eu³⁺ phosphors were prepared by the combustion synthesis method and characterized by X-ray diffraction (XRD), photoluminescence and photoluminescence excitation (PL-PLE) spectra. The experimental results show that, upon excitation with 250 nm xenon light, the emission spectrum of the CaO: Eu³⁺ consists of 4f-4f emission transitions from the ⁵ D ₀ excited level to the ⁷ F _J (J=1, 2, 3) levels with the mainly electric dipole transition ⁵ D ₀→⁷ F ₂ of Eu³⁺, indicating that the Eu³⁺ occupies a low symmetry. The charge-transfer band (CTB) shows somewhat red shift with the decreasing ionic radii of co-doped alkali metal ions. The PL and PLE intensities are significantly enhanced, especially the strongest intensity of luminescent is CaO: Eu³⁺, Li⁺ phosphor, when alkali metal ions are incorporated. The strongest peak of emission is slightly shifted from 614 to 593 nm, indicating that the Eu³⁺ ion locates in a symmetric position (O _h ) when alkali metal ions are incorporated.     

The Effect of Bi2O3 on the Structure and Microwave Dielectric Properties of Ba6-3xNd8＋2xTi18O54 System（x=2/3）

The structures and dielectric properties of Ba6-3xNd8＋2xTi18O54 system（x=2/3） doped with different contents of Bi2O3, whose final molecular formula is Ba6-3x（Nd1-yBiy）8＋2xTi18O54 were investigated. It is indicated that the dielectric constant increases greatly whereas Q value（f0=4 GHz） decreases with the increase of Bi2O3 content. However, the temperature coefficient could be controlled below 0±30×10^-6/℃ in the experiment. These phenomena are related to the appearance of a new phase, Bi4Ti3O12, which has high dielectric constant. Also, that Bi^3＋（0.13 nm） substitutes for Nd^3＋（0.099 5 nm） will increase the unit cell volume, which will lead to the enlargement of the octahedron B site occupied by Ti^4＋. So the spontaneous polarization of Ti^4＋ ions will be strengthened. Besides, Bi^3＋ will fill up some vacancies which Ba^2＋ or Nd^3＋ ions leave in two A1 sites and four A2 sites. More positive ions polarize, which also contributes to higher dielectric constant. The samples got with the optimium properties are sintered at 1 200 ℃ for 4 h, when y=0.25, ε≈110, Q≈5 400（f0=4 GHz）, TCC=-4.7×10^-6/℃; When y=0.3, ε≈120, Q≈5 000（f0=4 GHz）, TCC=-24×10^-6/℃.     

Molar heat capacities of La2Mo2O9 and La1.9Sr0.1MO209-δ

The molar heat capacities of La2Mo209 and La1.9Sr0.1MO209-δ were obtained using the differential scanning calorimetry （DSC） technique in a temperature range from 298 to 1473 K. The DSC curve of La2Mo209 showed an endothermal peak around 834 K corresponding to a first-order monoclinic-cubic phase transition, and the enthalpy change accompanying this phase transition is 5.99 kJ/mol. No evident endothermal peak existed in the DSC curve of La1.9Sr0.1MO209-δ, but a broad thermal anomaly existed in its heat capacity curve at around 832 K. In addition, the heat capacity values of La2Mo209 and La1.9Sr0.1MO209-δ began to decrease at 1196 and 1330 K, respectively. The non-transitional heat capacity values of La2Mo209 and La1.9Sr0.1MO209-δ were formulated using multiple regression analysis in two temperature ranges.     

Effects of In2O3 doping and sintering temperature on the electrical properties of ZnO varistors

ZnO varistors are prepared using the 0.1–0.3 mm ZnO powders. The effects of the sintering temperature, contents of In₂O₃ doping on the non-linear properties of ZnO varistors have been investigated. The results show that this kind of ZnO powder has a high sintering activity. It is suitable for making the low voltage varistors. The V _c decreases with the increase of sintered temperature, when the In₂O₃ content is fixed (0.98%, mass fraction), and increases with the increase of In₂O₃ contents when the temperature is steady. Project supported by the Nonferrous Metals Industry Corporation of China Synopsis of the first anthor Zhao Ruirong, born in Jan. 1935, majoring in nonferrous metal metallurgy and products exploition.     

Microstructure and microwave dielectric properties of lead borosilicate glass ceramics with Al2O3

The effects of Al₂O₃ addition on both the sintering behavior and microwave dielectric properties of PbO-B₂O₃-SiO₂ glass ceramics were investigated by Fourier transform infrared spectroscope (FTIR), differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that with the increase of Al₂O₃ content the bands assigned to [SiO₄] nearly disappear. Aluminum replaces silicon in the glass network, which is helpful for the formation of boron-oxygen rings. The increase of the transition temperature T _g and softening temperature T _f of PbO-B₂O₃-SiO₂ glass ceramics leads to the increase of liquid phase precipitation temperature and promotes the structure stability in the glasses, and consequently contributes to the decreasing trend of crystallization. Densification and dielectric constants increase with the increase of Al₂O₃ content, but the dielectric loss is worsened. By contrast, the 3% (mass fraction) Al₂O₃-doped glass ceramics sintered at 725 °C have better properties of density ρ=2.72 g/cm³, dielectric constant ɛ _r =6.78, dielectric loss tan δ=2.6×10⁻³ (measured at 9.8 GHz), which suggest that the glass ceramics can be applied in multilayer microwave devices requiring low sintering temperatures.     

Study on the isothermal oxidation behavior in air of Ti3AlC2 sintered by hot pressing

The isothermal oxidation behavior at 900–1300°C for 20 h in air of bulk Ti₃AlC₂ with 2.8 wt% TiC sintered by means of hot pressing was investigated in the work. The isothermal oxidation behavior generally followed a parabolic rate law. The parabolic rate constants increased from 1.39×10⁻¹⁰ kg²·m⁻⁴·s⁻¹ at 900°C to 5.56×10⁻⁹ kg²·m⁻⁴·s⁻¹ at 1300°C. The calculated activation energy was 136.45 kJ/mol. It was demonstrated that Ti₃AlC₂ had excellent oxidation resistance due to the continuous, dense and adhesive protect scales consisted of a mass of α-Al₂O₃ and a little of TiO₂ and/or Al₂TiO₅. In principle, the oxide scale was grown by the inward diffusion of O²⁻ and the outward diffusion of Ti⁴⁺ and Al³⁺. The rapid outward diffusion of cations usually resulted in the formation of cracks, gaps, and holes.     

Synthesis and luminescence of nano fluorescent powder of Y<Subscript>4</Subscript>Al<Subscript>2</Subscript>O<Subscript>9</Subscript>: Eu<Superscript>3+</Superscript>

Nano fluorescent powder of Y₄Al₂O₉: Eu³⁺ was synthesized by sol-gel method. The XRD shows that the product prepared at 900°C is pure-phase Y₄Al₂O₉: Eu³⁺. The Y₄Al₂O₉ powder is nano-size crystal testified by BF and ED analysis of TEM. The grain diameter of Y₄Al₂O₉ is in the range between 20 and 50nm, and its average is 30 nm. The luminescent spectra show that Eu³⁺ ious occupy two kinds of sites in Y₄Al₂O₉ crystal lattice. One is in the strict inversion center, and the other is in off lying inversion center. When excited with UV light (λ=254nm), Y₄Al₂O₉: Eu³⁺ exhibits an orange emission bond at λ=590 nm due to the⁵D_o→⁷F₁ transition and a red emission band at λ=610 nm due to⁵D_o→⁷F₂ transition. YUAN Xi-ming: Born in 1951 Funded by Key Scientific and Technological Project of Hubei Province (2001 AA102A03)     

In situ prepared Al-Si alloy matrix composites reinforced by γ-Al2O3p

A γ-Al2O3 particles reinforced Al-Si alloy matrix composite was fabricated by adding NH4Al（SO4）2 to molten aluminum alloy. TEM observation shows that in-situ γ-Al2O3 particles are generally spherical and uniformly distributed in the matrix. The results of dry sliding wear tests show that the wear resistance of the composites increases with increasing mass fraction, and the volume loss is considerably lesser than that of the matrix and is lesser than that of the composites by adding γ-Al2O3 particles directly.     

Pressureless sintered 3Y-TZP/20%Al2O3 composite ceramic

Nanometer 3Y-TZP/20%Al₂O₃ (mass fraction) composite powders prepared by the chemical coprecipitation method were pressureless sintered at 1550 °C for 2 h. Effects of calcining temperatures at 800 °C, 1 000 °C, and 1 200 °C on phase structure, relative density, and Vicker’s hardness of the sintered bodies were studied. The results show that 1 000 °C was the optimal calcining temperature, and the powder calcined was composed of tetragonal zirconia with the Scherrer crystalline size of 6.3nm. The relative density was up to 98.5% under pressureless sintering, and the sintered body was t-ZrO₂ (without m-ZrO₂)+α-Al₂O₃ with the average size of 0.4 μm. Foundation item: State Key Laboratory for Powder Metallurgy(No.9706-36) Biography of the first author: YIN Bang-yao, born in 1966, majoring in advanced ceramic materials.     

Synthesis and photoluminescence property of red phosphors LiEu1?xYx(WO4)0.5(MoO4)1.5 for white LED

A series of novel red phosphors LiEu_1−x Y _x (WO₄)_0.5(MoO₄)_1.5 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8) were synthesized by conventional solid state reaction method with the starting materials: WO₃, MoO₃, Eu₂O₃, Li₂CO₃ and Y₂O₃. The spectrum and the crystal structure of the phosphors were characterized by F-4500 and XRD respectively. Meanwhile the effects of flux and Y³⁺ concentration on the crystal structure and luminescent properties of the phosphors were investigated. The results showed that the optimal content of flux () was 1 wt% and the optimal doping concentration of Y³⁺ was 0.5 mol. The emission spectrum showed the most intense peak was located at 615 nm, which corresponds to the ⁵ D ₀→⁷ F ₂ transition of Eu³⁺ and that Eu³⁺ occupied the lattice site of noncentrosymmetric environment in the scheelite phases. The excitation spectrum displayed that these phosphors could be effectively excited by ultraviolet (UV) (396 nm) and blue (466 nm) light, nicely in correspondence with the widely applied output wavelengths of ultraviolet or blue LED chips. The influence of flux on the luminescent properties of LiEu_0.5Y_0.5(WO₄)_0.5(MoO₄)_1.5 phosphor was analyzed. The XRD spectra indicated that the flux could help to crystallize the phosphor, and no other phases were formed except the tetragonal. When adding flux, the relative intensity of LiEu_0.5Y_0.5(WO₄)_0.5(MoO₄)_1.5 became much stronger and the average particle size of the phosphor decreased. Supported by the Key Programs for Science and Technology Development of Hubei Province (Grant No. 2005AA105A05)     

Study of GaN MOS-HEMT using ultrathin Al2O3 dielectric grown by atomic layer deposition

We report on a GaN metal-oxide-semiconductor high electron mobility transistor (MOS-HEMT) using atomic-layer deposited (ALD) Al₂O₃ as the gate dielectric. Through further decreasing the thickness of the gate oxide to 3.5 nm and optimizing the device fabrication process, a device with maximum transconductance of 150 mS/mm was produced. The drain current of this 0.8 μm gate-length MOS-HEMT could reach 800 mA/mm at +3.0 V gate bias. Compared to a conventional AlGaN/GaN HEMT of similar design, better interface property, lower leakage current, and smaller capacitance-voltage (C-V) hysteresis were obtained, and the superiority of this MOS-HEMT device structure with ALD Al₂O₃ gate dielectric was exhibited. Supported by the National Natural Science Foundation of China (Grant No. 60736033) and the National Basic Research Program of China (“973“) (Grant No. 51327020301)     

Preparation of uitrafine α-Al2O3 powders by catalytic sintering of ammonium aluminum carbonate hydroxide at low temperature

The precursor of ammonium aluminum carbonate hydroxide was synthesized by using aluminum sulfate （Al2 （SO4）3 ） and ammonium carbonate（（NH4 ）2CO3 ）. The effects of α-Al2 O3 seeds and mixture composed of α- Al2O3 and ammonium nitrate, as well as multiplex catalysts （AT） on phase transformation of alumina in sintering process were investigated respectively. The results show that the α-Al2 O3 seeds and the mixture of α-Al2 O3 and ammonium nitrate can lower the phase transformation temperature of α-Al2O3 to different extents while the particles obtained agglomerate heavily. AT has great potential synergistic effects on the phase transformation of alumina and reduces the phase transformation temperature of α-Al2O3 and the trends of necking-formation between particles. Therefore the dispersion of powder particles is improved significantly.