ABSTRACT
In this paper, we reported on the lattice distortion, surface morphologies, vacancy defects and electrochemical performance that had been observed in Na3V2(PO4)2F3 prepared at different annealing temperatures. X-ray diffraction indicated that all the samples were single phase materials with tetragonal structure and exhibited lattice distortion with the increase of annealing temperatures. A possible mechanism causing the strain-induced lattice distortion had been discussed. Moreover, scanning electron microscopy and positron annihilation techniques were used to study the grain size and vacancy defects as a function of annealing temperatures. The superior electrochemical performance of Na3V2(PO4)2F3 electrode was obtained at the annealing temperature of 350 degrees C with 167.73 F center dot g-1 specific capacitance and 85% capacitance retention. The better electrochemical performance was due to the synergistic effects of grain size and vacancy defect regulated by the annealing temperatures. These results could provide experimental basis for enhancing electrochemical performance of Na3V2(PO4)2F3 in sodium-ion battery area applications. (c) 2023 Elsevier B.V. All rights reserved.
ABSTRACT
Objective: To describe the COVID-19 epidemic and its characteristics in Heilongjiang province, and provide evidence for the further prevention and control of COVID-19 in the province. Methods: The information of COVID-19 cases and clusters were collected from national notifiable disease report system and management information system for reporting public health emergencies of China CDC. The Software's of Excel 2010 and SPSS 23.0 were applied for data cleaning and statistical analysis on the population, time and area distributions of COVID-19 cases. Results: On January 22, 2020, the first confirmed case of COVID-19 was reported in Heilongjiang. By March 11, 2020, a total of 482 cases domestic case of COVID-19, The incidence rate was 1.28/100 000, the mortality rate was 2.70% (13/482) in 13 municipalities in Heilongjiang. There were 81 clusters of COVID-19, The number of confirmed cases accounted for 79.25% (382/482) of the total confirmed cases and 12 cases of deaths. The family clusters accounted for 86.42% (70/81). Compared with the sporadic cases, the mortality rate, proportion of elderly cases aged 60 or above and severe or critical cases of clinical classification were all higher in the clusters especially the family clusters, but the differences were not significant (P>0.05). There were 34 clusters involving more than 5 confirmed cases accounted for 41.98% (34/81) of the total clusters, the involved cases accounted for 68.31% (261/382) of the total cases of clusters. There were significant differences in age distribution of the cases among the case clusters with different case numbers. In the clusters involving 6-9 cases, the proportion of cases aged 65 years or above was more (26.53%, 39/147). Conclusions: The incidence rate of COVID-19 was relatively high and the early epidemic was serious in Heilongjiang, The number of cases was large in clusters especially family clusters.