Aims As a unique geographical unit of the earth,the tibetan Plateau is extensively covered by various Stipa communities.However,their vegetation features have not been reported systematically till now,especially in so...Aims As a unique geographical unit of the earth,the tibetan Plateau is extensively covered by various Stipa communities.However,their vegetation features have not been reported systematically till now,especially in some scantily explored regions.In this study,we endeavor to reveal the community types,quantitative charac-teristics and climatic distribution patterns of Stipa steppes in these areas based on primary relevés obtained from fieldwork.Methods We collected a total of 223 plots in 79 study sites in the Changthang Plateau and the Yarlung Zangbo Valley,ranging from 79°E to 91°E.the categories of Stipa formations were identified according to the classification scheme in Vegetation of China and then verified by Nonmetric Multidimensional Scaling.We performed detrended correspondence analysis and detrended canonical correspondence analysis to hunt for the alteration of Stipa communities along the precipitation gradient.Quantitative characteristics including species richness,coverage,biomass as well as importance values(IV)of dominant species were calcu-lated and visualized,respectively.Important Findings Stipa steppes in scantily explored regions of the tibetan Plateau are classified into 11 formations but major formations are rather limited in number.Formation(form.)Stipa purpurea is the most widespread Stipa assemblages not only in scantily explored regions but also across the whole tibetan Plateau.the characteristics of Stipa com-munities,including coverage,species richness,productivity and IV of dominant species,demonstrate the features of typical alpine steppes on the tibetan Plateau.Precipitation proves to be the prime climatic factor controlling the distribution patterns of Stipa assemblages.Form.Stipa subsessiliflora var.basiplumosa and form.Stipa glareosa normally distribute in arid habitats,but rainfall for the former is of greater variance.Form.Stipa roborowskyi and form.Stipa capillacea favor moderately moist environment.Form.Stipa purpurea and form.Stipa roborowskyi can tolerate a fairly broad range of precipitation.展开更多
The in situ nano Ta_(4)HfC_(5) reinforced SiBCN-Ta_(4)HfC_(5) composite ceramics were prepared by a combination of two-step mechanical alloying and reactive hot-pressing sintering.The microstructural evolution and mec...The in situ nano Ta_(4)HfC_(5) reinforced SiBCN-Ta_(4)HfC_(5) composite ceramics were prepared by a combination of two-step mechanical alloying and reactive hot-pressing sintering.The microstructural evolution and mechanical properties of the resulting SiBCN-Ta_(4)HfC_(5) were studied.After the first-step milling of 30 h,the raw materials of TaC and HfC underwent crushing,cold sintering,and short-range interdiffusion to finally obtain the high pure nano Ta_(4)HfC_(5).A hybrid structure of amorphous SiBCN and nano Ta_(4)HfC_(5) was obtained by adopting a second-step ball-milling.After reactive hot-pressing sintering,amorphous SiBCN has crystallized to 3C-SiC,6H-SiC,and turbostratic BN(C)phases and Ta_(4)HfC_(5) retained the form of the nanostructure.With the in situ generations of 2.5 wt% Ta_(4)HfC_(5),Ta_(4)HfC_(5) is preferentially distributed within the turbostratic BN(C);however,as Ta_(4)HfC_(5) content further raised to 10 wt%,it mainly distributed in the grain-boundary of BN(C) and SiC.The introduction of Ta_(4)HfC_(5) nanocrystals can effectively improve the flexural strength and fracture toughness of SiBCN ceramics,reaching to 344.1 MPa and 4.52 MPa·m^(1/2),respectively.This work has solved the problems of uneven distribution of ultra-high temperature phases in the ceramic matrix,which is beneficial to the real applications of SiBCN ceramics.展开更多
基金Key Program of National Fundamental Scientific Research funded by the Ministry of Science and Technology of China:Integrated Scientific Investigation in Data-deficient Regions of the Tibetan Plateau(2012FY111400)Redaction of Vegetation of China(2015FY210200).
文摘Aims As a unique geographical unit of the earth,the tibetan Plateau is extensively covered by various Stipa communities.However,their vegetation features have not been reported systematically till now,especially in some scantily explored regions.In this study,we endeavor to reveal the community types,quantitative charac-teristics and climatic distribution patterns of Stipa steppes in these areas based on primary relevés obtained from fieldwork.Methods We collected a total of 223 plots in 79 study sites in the Changthang Plateau and the Yarlung Zangbo Valley,ranging from 79°E to 91°E.the categories of Stipa formations were identified according to the classification scheme in Vegetation of China and then verified by Nonmetric Multidimensional Scaling.We performed detrended correspondence analysis and detrended canonical correspondence analysis to hunt for the alteration of Stipa communities along the precipitation gradient.Quantitative characteristics including species richness,coverage,biomass as well as importance values(IV)of dominant species were calcu-lated and visualized,respectively.Important Findings Stipa steppes in scantily explored regions of the tibetan Plateau are classified into 11 formations but major formations are rather limited in number.Formation(form.)Stipa purpurea is the most widespread Stipa assemblages not only in scantily explored regions but also across the whole tibetan Plateau.the characteristics of Stipa com-munities,including coverage,species richness,productivity and IV of dominant species,demonstrate the features of typical alpine steppes on the tibetan Plateau.Precipitation proves to be the prime climatic factor controlling the distribution patterns of Stipa assemblages.Form.Stipa subsessiliflora var.basiplumosa and form.Stipa glareosa normally distribute in arid habitats,but rainfall for the former is of greater variance.Form.Stipa roborowskyi and form.Stipa capillacea favor moderately moist environment.Form.Stipa purpurea and form.Stipa roborowskyi can tolerate a fairly broad range of precipitation.
基金the financial support from the National Natural Science Foundation of China(52002092,51621091,51472059,51225203,and 51272300)the National Key Research and Development Program(2017YFB0310400)+2 种基金the Postdoctoral Innovative Talents Support Program(BX20190095)the China Postdoctoral Science Foundation(Grant Nos.LBH-Z19141 and 2019M660072)funded by Advanced Space Propulsion Laboratory of BICE and Beijing Engineering Research center of Efficient and Green Aerospace Propulsion Technology(No.LabASP-2019-08).
文摘The in situ nano Ta_(4)HfC_(5) reinforced SiBCN-Ta_(4)HfC_(5) composite ceramics were prepared by a combination of two-step mechanical alloying and reactive hot-pressing sintering.The microstructural evolution and mechanical properties of the resulting SiBCN-Ta_(4)HfC_(5) were studied.After the first-step milling of 30 h,the raw materials of TaC and HfC underwent crushing,cold sintering,and short-range interdiffusion to finally obtain the high pure nano Ta_(4)HfC_(5).A hybrid structure of amorphous SiBCN and nano Ta_(4)HfC_(5) was obtained by adopting a second-step ball-milling.After reactive hot-pressing sintering,amorphous SiBCN has crystallized to 3C-SiC,6H-SiC,and turbostratic BN(C)phases and Ta_(4)HfC_(5) retained the form of the nanostructure.With the in situ generations of 2.5 wt% Ta_(4)HfC_(5),Ta_(4)HfC_(5) is preferentially distributed within the turbostratic BN(C);however,as Ta_(4)HfC_(5) content further raised to 10 wt%,it mainly distributed in the grain-boundary of BN(C) and SiC.The introduction of Ta_(4)HfC_(5) nanocrystals can effectively improve the flexural strength and fracture toughness of SiBCN ceramics,reaching to 344.1 MPa and 4.52 MPa·m^(1/2),respectively.This work has solved the problems of uneven distribution of ultra-high temperature phases in the ceramic matrix,which is beneficial to the real applications of SiBCN ceramics.
