Fe and Co porphyrins and phthalocyanines are excellent catalysts for the oxygen reduction reaction （ORR） and are promising alternatives to Pt in fuel cells. However, the stability of these molecular catalysts in aci...Fe and Co porphyrins and phthalocyanines are excellent catalysts for the oxygen reduction reaction （ORR） and are promising alternatives to Pt in fuel cells. However, the stability of these molecular catalysts in acidic media is poor. This study explores whether demetalation through proton ex- change causes these metal macrocyclic catalysts to be unstable in acidic media. We first present a theoretical scheme for investigating exchange reactions of metal ions in metal macrocyclic com- pounds with protons in acidic media. The equilibrium concentrations of metal ions in solution when various metalloporphyrins （MPs） and metallophthalocyanines （MPcs） are brought into contact with a strongly acidic solution （pH = 1） were then estimated using density functional theory calculations; these values were used to evaluate the stability of these metal macrocyclic compounds against demetalation in acidic media, The results show that Fe, Co, Ni, and Cu phthalocyanines and porphy- rins have considerable resistance to exchange with protons, whereas Cr, Mn, and Zn phthalocya- nines and porphyrins easily undergo demetalation through ion exchange with protons, This sug- gests that the degradation in the ORR activity of Fe and Co macrocyclic molecular catalysts and of carbon materials doped with Fe（Co） and nitrogen, which are believed to have metal-nitrogen coor- dination structures similar to those of macrocyclic molecules as ORR catalytic centers, is not the result of replacement of metal ions by protons. The calculation results show that electron-donating substituents could enhance the stability of Fe and Co phthalocyanines.展开更多
In this paper, an improved low-complexity sum-product decoding algorithm is presented for low-density parity-check （LDPC） codes. In the proposed algorithm, reduction in computational complexity is achieved by utiliz...In this paper, an improved low-complexity sum-product decoding algorithm is presented for low-density parity-check （LDPC） codes. In the proposed algorithm, reduction in computational complexity is achieved by utilizing fast Fourier transform （FFT） with time shift in the check node process. The improvement in the decoding performance is achieved by utilizing an op- timized integer constant in the variable node process. Simulation results show that the proposed algorithm achieves an overall coding gain improvement ranging from 0.04 to 0.46 dB. Moreover, when compared with the sum-product algorithm （SPA）, the proposed decoding algorithm can achieve a reduction of 42%-67% of the total number of arithmetic operations required for the decoding process.展开更多
Accurate calculation of circulating current is one of the key problems for stator transposition bars in the design of turbo-generators. Aimed at limitation that analytical algorithm of circulating current could not re...Accurate calculation of circulating current is one of the key problems for stator transposition bars in the design of turbo-generators. Aimed at limitation that analytical algorithm of circulating current could not reflect the local electromagnetic field distribution and difficulty that overlaps easily exist in solid modeling process of stator transposition bars, a simplified physical model of transposition bars is established. A three-dimensional(3-D) numerical method for circulating current in stator transposition bars of large water-cooled turbo-generators is investigated, which is combined with field-circuit coupling method. Taking stator bars less than 540° transposition with void model of a 600-MW water-cooled turbo-generator as the research object, the magnetic flux density distribution, current density distribution and circulating current distribution of transposition strands are obtained by numerical calculation. Compared with calculation results of the improved analytical algorithm, the correctness of the numerical calculation for circulating current is demonstrated, the calculation value difference for the maximum current of strands is obtained. The numerical calculation for circulating current will provide an appropriate basis for the reasonable calculation of local overheating of stator transposition bars and the design of safety margin for turbo-generators.展开更多
基金supported by the National Basic Research Program of China(973 program,2012CB215500 and 2012CB932800)~~
文摘Fe and Co porphyrins and phthalocyanines are excellent catalysts for the oxygen reduction reaction （ORR） and are promising alternatives to Pt in fuel cells. However, the stability of these molecular catalysts in acidic media is poor. This study explores whether demetalation through proton ex- change causes these metal macrocyclic catalysts to be unstable in acidic media. We first present a theoretical scheme for investigating exchange reactions of metal ions in metal macrocyclic com- pounds with protons in acidic media. The equilibrium concentrations of metal ions in solution when various metalloporphyrins （MPs） and metallophthalocyanines （MPcs） are brought into contact with a strongly acidic solution （pH = 1） were then estimated using density functional theory calculations; these values were used to evaluate the stability of these metal macrocyclic compounds against demetalation in acidic media, The results show that Fe, Co, Ni, and Cu phthalocyanines and porphy- rins have considerable resistance to exchange with protons, whereas Cr, Mn, and Zn phthalocya- nines and porphyrins easily undergo demetalation through ion exchange with protons, This sug- gests that the degradation in the ORR activity of Fe and Co macrocyclic molecular catalysts and of carbon materials doped with Fe（Co） and nitrogen, which are believed to have metal-nitrogen coor- dination structures similar to those of macrocyclic molecules as ORR catalytic centers, is not the result of replacement of metal ions by protons. The calculation results show that electron-donating substituents could enhance the stability of Fe and Co phthalocyanines.
文摘In this paper, an improved low-complexity sum-product decoding algorithm is presented for low-density parity-check （LDPC） codes. In the proposed algorithm, reduction in computational complexity is achieved by utilizing fast Fourier transform （FFT） with time shift in the check node process. The improvement in the decoding performance is achieved by utilizing an op- timized integer constant in the variable node process. Simulation results show that the proposed algorithm achieves an overall coding gain improvement ranging from 0.04 to 0.46 dB. Moreover, when compared with the sum-product algorithm （SPA）, the proposed decoding algorithm can achieve a reduction of 42%-67% of the total number of arithmetic operations required for the decoding process.
基金supported by the National Natural Science Foundation of China(Grant No.51477038)
文摘Accurate calculation of circulating current is one of the key problems for stator transposition bars in the design of turbo-generators. Aimed at limitation that analytical algorithm of circulating current could not reflect the local electromagnetic field distribution and difficulty that overlaps easily exist in solid modeling process of stator transposition bars, a simplified physical model of transposition bars is established. A three-dimensional(3-D) numerical method for circulating current in stator transposition bars of large water-cooled turbo-generators is investigated, which is combined with field-circuit coupling method. Taking stator bars less than 540° transposition with void model of a 600-MW water-cooled turbo-generator as the research object, the magnetic flux density distribution, current density distribution and circulating current distribution of transposition strands are obtained by numerical calculation. Compared with calculation results of the improved analytical algorithm, the correctness of the numerical calculation for circulating current is demonstrated, the calculation value difference for the maximum current of strands is obtained. The numerical calculation for circulating current will provide an appropriate basis for the reasonable calculation of local overheating of stator transposition bars and the design of safety margin for turbo-generators.
