Segregation during crystal growth from melt under two conditions is studied by using crystal mass,which can be measured easily,as an independent variable,and a method to determine the effective segregation coefficient...Segregation during crystal growth from melt under two conditions is studied by using crystal mass,which can be measured easily,as an independent variable,and a method to determine the effective segregation coefficient and absorption cross section of optical dopant is given.When the segregated solute disperses into the whole or just a part of melt homogenously,the concentration CS in solid interface will change by different formulas.If the crystal growth interface is conical and segregated solute disperses into melt in total or part,the solute concentration at r=2/3R,where r is the distance from the growth cross section center and R the crystal radius,is independent on the shape of the crystal growth interface,and its variation at r=2/3R can be regarded as the result from crystal growth in flat interface.With CS variation formula in solid and absorption cross section σ for optical dopant,the absorption coefficients along the crystal growth direction can be calculated,and the corresponding experimental value can be obtained through the crystal optical absorption spectra.By minimizing the half sum,whose independent variables are k,ΔW or σ,of the difference square between the calculated and experimental absorp-tion coefficients from one or more absorption peaks along the crystal growth di-rection,k and σ,or k and ΔW,can be determined at the same time through the Levenberg-Marquardt iteration method.Finally,the effective segregation coefficient k,ΔW and absorption cross sections of Nd:GGG were determined,the results fitted by two formula gave more closed effective segregation coefficient,and the value ΔW also indicates that the segregated dopant had nearly dispersed into the whole melt.Experimental results show that the method to determine effective segregation coefficient k,ΔW and absorption cross sections σ is convenient and reliable,and the two segregation formulas can describe the segregation during the crystal growth from melt relatively commendably.展开更多
Nanopowder of Cr:GGG and nanopowder of Cr,Nd:GGG with different concentrations of Cr3+ ranging from 0.1 at.% to 1.5 at.% were synthesized by the sol-gel method using acetic acid and ethylene glycol. Thermal gravime...Nanopowder of Cr:GGG and nanopowder of Cr,Nd:GGG with different concentrations of Cr3+ ranging from 0.1 at.% to 1.5 at.% were synthesized by the sol-gel method using acetic acid and ethylene glycol. Thermal gravimetric analysis and differential scanning calorimetry (TGA-DSC), X-ray diffraction (XRD) and photoluminescence spectroscopy were used to characterize the powder. The crystallite size was about 58 nm when treated at 1000 oC for 2 h. Cr3+ photoluminescence spectrum in GGG showed a broad band emission around 730 nm. The intensity of this band decreased when co-doped with Nd, indicating an efficient energy transfer from Cr3+ to Nd3+. Photoluminescence intensity of Nd in Cr,Nd:GGG at 1.06μm showed that the optimum concentration of Cr3+ was about 1 at.% (more or less) for 1 at.% Nd3+. This result was also confirmed by chromium fluorescence decay rate analysis. Energy transfer efficiency was found to be about 84% for 1 at.% concentration of each chromium and neodymium.展开更多
Neodymium doped gadolinium gallium oxide (Nd:GGG) nanopowders synthesized by microwave gel combustion using alanine as a fuel was reported. Metal nitrates solution with alanine fuel was combusted in microwave to gi...Neodymium doped gadolinium gallium oxide (Nd:GGG) nanopowders synthesized by microwave gel combustion using alanine as a fuel was reported. Metal nitrates solution with alanine fuel was combusted in microwave to give precursor. The micro-wave precursor powder was calcined at different temperatures from 800 to 1100 ℃. Phase pure Nd:GGG formation took place at 800 to 1100 ℃ as observed by X-ray diffraction (XRD) and Fourier transform infra-red (FTIR) spectroscopy. However particle size in-creased with calcinations temperature from 25 nm at 800 ℃ to 200 nm at 1100 ℃.Nd:GGG nanopowder obtained at different calci-nation temperatures were compacted and sintered at 1550 ℃ for 3 h in air. Most densified ceramic was obtained from Nd:GGG nanopowder calcined at 1100 ℃. Microstructure as observed from scanning electron microscopy (SEM) showed that the most densi-fied ceramic, obtained from nanopowder calcined at a higher calcination temperature, had a more uniform grain-size distribution, fewer pores and greater densification. XRD of sintered sample showed retention of phase purity.展开更多
基金the National Natural Science Foundation of China(Grant No.50772112)
文摘Segregation during crystal growth from melt under two conditions is studied by using crystal mass,which can be measured easily,as an independent variable,and a method to determine the effective segregation coefficient and absorption cross section of optical dopant is given.When the segregated solute disperses into the whole or just a part of melt homogenously,the concentration CS in solid interface will change by different formulas.If the crystal growth interface is conical and segregated solute disperses into melt in total or part,the solute concentration at r=2/3R,where r is the distance from the growth cross section center and R the crystal radius,is independent on the shape of the crystal growth interface,and its variation at r=2/3R can be regarded as the result from crystal growth in flat interface.With CS variation formula in solid and absorption cross section σ for optical dopant,the absorption coefficients along the crystal growth direction can be calculated,and the corresponding experimental value can be obtained through the crystal optical absorption spectra.By minimizing the half sum,whose independent variables are k,ΔW or σ,of the difference square between the calculated and experimental absorp-tion coefficients from one or more absorption peaks along the crystal growth di-rection,k and σ,or k and ΔW,can be determined at the same time through the Levenberg-Marquardt iteration method.Finally,the effective segregation coefficient k,ΔW and absorption cross sections of Nd:GGG were determined,the results fitted by two formula gave more closed effective segregation coefficient,and the value ΔW also indicates that the segregated dopant had nearly dispersed into the whole melt.Experimental results show that the method to determine effective segregation coefficient k,ΔW and absorption cross sections σ is convenient and reliable,and the two segregation formulas can describe the segregation during the crystal growth from melt relatively commendably.
文摘由于Nd3+离子半径0.112nm和Y3+离子半径0.101nm相差10.9%,使得Nd3+离子非常难于进入YAG晶体中。我们用温度梯度法生长了大尺寸高浓度(2.8 at%)的Nd:YAG晶体,同时与用提拉法Nd:GGG晶体进行了比较。分析了高浓度掺杂Nd:GGG和Nd:YAG晶体浓度猝灭问题。研究了不同浓度掺杂的猝灭效应。在同样的掺杂浓度下,我们发现它们的猝灭程度不同,其原因是两种晶体中ΔE(m is-)m和ΔE(m i s+)m不同。
文摘Nanopowder of Cr:GGG and nanopowder of Cr,Nd:GGG with different concentrations of Cr3+ ranging from 0.1 at.% to 1.5 at.% were synthesized by the sol-gel method using acetic acid and ethylene glycol. Thermal gravimetric analysis and differential scanning calorimetry (TGA-DSC), X-ray diffraction (XRD) and photoluminescence spectroscopy were used to characterize the powder. The crystallite size was about 58 nm when treated at 1000 oC for 2 h. Cr3+ photoluminescence spectrum in GGG showed a broad band emission around 730 nm. The intensity of this band decreased when co-doped with Nd, indicating an efficient energy transfer from Cr3+ to Nd3+. Photoluminescence intensity of Nd in Cr,Nd:GGG at 1.06μm showed that the optimum concentration of Cr3+ was about 1 at.% (more or less) for 1 at.% Nd3+. This result was also confirmed by chromium fluorescence decay rate analysis. Energy transfer efficiency was found to be about 84% for 1 at.% concentration of each chromium and neodymium.
基金Laser Science and Technology Centre, Defence Research and Development Organization (DRDO), Delhi for his encouragement and support to carry out this work
文摘Neodymium doped gadolinium gallium oxide (Nd:GGG) nanopowders synthesized by microwave gel combustion using alanine as a fuel was reported. Metal nitrates solution with alanine fuel was combusted in microwave to give precursor. The micro-wave precursor powder was calcined at different temperatures from 800 to 1100 ℃. Phase pure Nd:GGG formation took place at 800 to 1100 ℃ as observed by X-ray diffraction (XRD) and Fourier transform infra-red (FTIR) spectroscopy. However particle size in-creased with calcinations temperature from 25 nm at 800 ℃ to 200 nm at 1100 ℃.Nd:GGG nanopowder obtained at different calci-nation temperatures were compacted and sintered at 1550 ℃ for 3 h in air. Most densified ceramic was obtained from Nd:GGG nanopowder calcined at 1100 ℃. Microstructure as observed from scanning electron microscopy (SEM) showed that the most densi-fied ceramic, obtained from nanopowder calcined at a higher calcination temperature, had a more uniform grain-size distribution, fewer pores and greater densification. XRD of sintered sample showed retention of phase purity.