全文获取类型
收费全文 | 185篇 |
免费 | 25篇 |
国内免费 | 30篇 |
专业分类
系统科学 | 20篇 |
丛书文集 | 1篇 |
理论与方法论 | 1篇 |
现状及发展 | 42篇 |
研究方法 | 16篇 |
综合类 | 154篇 |
自然研究 | 6篇 |
出版年
2022年 | 3篇 |
2021年 | 6篇 |
2020年 | 7篇 |
2019年 | 5篇 |
2018年 | 9篇 |
2017年 | 10篇 |
2016年 | 9篇 |
2015年 | 6篇 |
2014年 | 7篇 |
2013年 | 14篇 |
2012年 | 18篇 |
2011年 | 19篇 |
2010年 | 5篇 |
2009年 | 8篇 |
2008年 | 9篇 |
2007年 | 7篇 |
2006年 | 15篇 |
2005年 | 10篇 |
2004年 | 7篇 |
2003年 | 12篇 |
2002年 | 6篇 |
2001年 | 3篇 |
2000年 | 5篇 |
1999年 | 1篇 |
1997年 | 1篇 |
1996年 | 4篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 3篇 |
1976年 | 4篇 |
1975年 | 1篇 |
1971年 | 3篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1966年 | 1篇 |
1965年 | 1篇 |
1964年 | 1篇 |
1961年 | 1篇 |
排序方式: 共有240条查询结果,搜索用时 15 毫秒
161.
Mokhtar Rejili DaM Telahigue Belgacem Lachiheb Abdessalem Mrabet Ali Ferchichi 《自然科学进展(英文版)》2008,18(9)
The perennial Medicago sativa cv.Gabès is widely grown on saline soils in Tunisian oases.Morphological and physiological analyses of two populations(Mareth and Gannouch)were conducted in order to study the effect of salinity and gamma radiation(350 Gy)inter-action on two populations of this species.It has been shown that the two irradiated populations of Medicago sativa are fairly tolerant to salt at growth phase compared to the non-irradiated.Exposure to gamma irradiation(350 Gy),alone or in combination with salt stress,increased significantly (p<0.00 1)shoot number,stem height and chlorophyll b pigment especially for the Gannouch population,while no change occurred for the Mareth population.The presence of salt to 9 g/1 affected significantly the root biomass and induced a reduc-tion of shoot development of both control and irradiated alfalfa populations.For all treatments,plants of two populations remained able to produce and to allocate dry matter to the different organs.The survey of Na+/K+ratio showed that the growth of the aerial organs of two non-irradiated populations was at least determined by a selectivity in favour of the K+ions r2=0.97 and r2=0.59 for Mareth and Gannouch non-irradiated populations,respectively).However,the rather weak correlation detected for the irradiated populations,par-ticularly for the irradiated Gannouch.seems to be the consequence of the effect of irradiation that improved potassium availability.essential element for growth and development.Results also showed that the two irradiated populations,especially the Mareth,accumu-lated Na+ions in its photosynthetic organs.This accumulation was associated with an improvement of foliar water content at a level of salinity around 5 g/1.Such a mechanism reflects probably an inclusive behaviour of the plants and a good aptitude to use the dominant ions(Na+)for the osmotic adjustment.However,the non-irradiated populations are unable to adjust their internal osmotic potential.Consequently,irradiated plants have probably adapted to the osmotic stress by either closing their stomata or increasing the osmotic pressure of the leaf cells. 相似文献
162.
罗阿理 《内蒙古大学学报(自然科学版)》1996,27(5):722-724
许多驱动机械装置的马达的功放电路,由于受到机械共振频率的限制,不能高速运行.本电路采用陷波器,对于机械共振频率进行限制,保证驱动系统在高速运行时的稳定性. 相似文献
163.
I. R. C. Bick J. B. Bremner A. Razak bin Modh Ali Le Van Thuc 《Cellular and molecular life sciences : CMLS》1980,36(10):1135-1135
Summary A novel series of alkaloids has been obtained from the Tasmanian plantAthrotaxis selaginoides, the first to be isolated from the plant family Taxodiaceae. 相似文献
164.
UV-irradiation of phenylalanine by 253.7 nm light in the presence of hydrogen peroxide formed 5 ninhydrin reactive products and ammonia. Four of them were identified as aspartic acid, serine, alanine and lysine. 相似文献
165.
166.
Ghulam Ali Saadat A. Siddiqi Shahid M. Ramay Shahid Atiq Murtaza Saleem 《矿物冶金与材料学报》2013,20(2):166-171
Cobalt (Co)-doped Bi0.9La0.1FeO3 multiferroics were synthesized by a sol-gel method based on the autocombustion technique. As-synthesized powder was examined using various characterization techniques to explore the effect of Co substitution on the properties of Bi0.9La0.1FeO3. X-ray diffraction reveals that Co-doped Bi0.9La0.1FeO3 preserves the perovskite-type rhombohedral structure of BiFeO3, and the composition without Co preserves the original structure of the phase; however, a second-phase Bi2Fe4O9 has been identified in all other compositions. Surface morphological studies were performed by scanning electron microscopy. Temperature-dependent resistivity of the samples reveals the characteristic insulating behavior of the multiferroic material. The resistivity is found to decrease with the increase both in temperature and Co content. Room temperature frequency-dependent dielectric measurements were also reported. Magnetic measurements show the enhancement in magnetization with the increase in Co content. 相似文献
167.
Hillier LW Fulton RS Fulton LA Graves TA Pepin KH Wagner-McPherson C Layman D Maas J Jaeger S Walker R Wylie K Sekhon M Becker MC O'Laughlin MD Schaller ME Fewell GA Delehaunty KD Miner TL Nash WE Cordes M Du H Sun H Edwards J Bradshaw-Cordum H Ali J Andrews S Isak A Vanbrunt A Nguyen C Du F Lamar B Courtney L Kalicki J Ozersky P Bielicki L Scott K Holmes A Harkins R Harris A Strong CM Hou S Tomlinson C Dauphin-Kohlberg S Kozlowicz-Reilly A Leonard S Rohlfing T Rock SM Tin-Wollam AM Abbott A 《Nature》2003,424(6945):157-164
168.
A. Ali M. A. Guidicci D. Stevenson 《Cellular and molecular life sciences : CMLS》1976,32(12):1503-1504
Summary This synthesis is especially suitable for production of highly-purified bradykinin-potentiating pentapeptide (BPP5a) because of the high yields of the coupling and deprotection reactions, accompanied by minimal side reactions, and the need for only one simple final purification step.The authors wish to thankJ. Pallak for carrying out the aminoacid analyses and bio-assays. 相似文献
169.
Orme CD Davies RG Burgess M Eigenbrod F Pickup N Olson VA Webster AJ Ding TS Rasmussen PC Ridgely RS Stattersfield AJ Bennett PM Blackburn TM Gaston KJ Owens IP 《Nature》2005,436(7053):1016-1019
Biodiversity hotspots have a prominent role in conservation biology, but it remains controversial to what extent different types of hotspot are congruent. Previous studies were unable to provide a general answer because they used a single biodiversity index, were geographically restricted, compared areas of unequal size or did not quantitatively compare hotspot types. Here we use a new global database on the breeding distribution of all known extant bird species to test for congruence across three types of hotspot. We demonstrate that hotspots of species richness, threat and endemism do not show the same geographical distribution. Only 2.5% of hotspot areas are common to all three aspects of diversity, with over 80% of hotspots being idiosyncratic. More generally, there is a surprisingly low overall congruence of biodiversity indices, with any one index explaining less than 24% of variation in the other indices. These results suggest that, even within a single taxonomic class, different mechanisms are responsible for the origin and maintenance of different aspects of diversity. Consequently, the different types of hotspots also vary greatly in their utility as conservation tools. 相似文献
170.
The ability to deposit and tailor reliable semiconducting films (with a particular recent emphasis on ultrathin systems) is indispensable for contemporary solid-state electronics. The search for thin-film semiconductors that provide simultaneously high carrier mobility and convenient solution-based deposition is also an important research direction, with the resulting expectations of new technologies (such as flexible or wearable computers, large-area high-resolution displays and electronic paper) and lower-cost device fabrication. Here we demonstrate a technique for spin coating ultrathin (approximately 50 A), crystalline and continuous metal chalcogenide films, based on the low-temperature decomposition of highly soluble hydrazinium precursors. We fabricate thin-film field-effect transistors (TFTs) based on semiconducting SnS(2-x)Se(x) films, which exhibit n-type transport, large current densities (>10(5) A cm(-2)) and mobilities greater than 10 cm2 V(-1) s(-1)--an order of magnitude higher than previously reported values for spin-coated semiconductors. The spin-coating technique is expected to be applicable to a range of metal chalcogenides, particularly those based on main group metals, as well as for the fabrication of a variety of thin-film-based devices (for example, solar cells, thermoelectrics and memory devices). 相似文献