首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4173篇
  免费   221篇
  国内免费   68篇
系统科学   199篇
丛书文集   142篇
教育与普及   13篇
理论与方法论   19篇
现状及发展   59篇
综合类   4030篇
  2024年   4篇
  2023年   15篇
  2022年   21篇
  2021年   27篇
  2020年   34篇
  2019年   18篇
  2018年   20篇
  2017年   37篇
  2016年   31篇
  2015年   42篇
  2014年   747篇
  2013年   535篇
  2012年   441篇
  2011年   451篇
  2010年   465篇
  2009年   472篇
  2008年   401篇
  2007年   91篇
  2006年   75篇
  2005年   73篇
  2004年   50篇
  2003年   50篇
  2002年   64篇
  2001年   48篇
  2000年   45篇
  1999年   34篇
  1998年   24篇
  1997年   19篇
  1996年   23篇
  1995年   11篇
  1994年   12篇
  1993年   19篇
  1992年   13篇
  1991年   19篇
  1990年   9篇
  1989年   10篇
  1988年   8篇
  1987年   3篇
  1986年   1篇
排序方式: 共有4462条查询结果,搜索用时 15 毫秒
261.
运用层次分析法来确定每个评价因子的权重,采用调查问卷的方式获得公众对河南农业旅游示范点的评价意见,然后利用经韦伯-费希纳定律改进后的综合评价指数模型对各示范点做出综合评价.  相似文献   
262.
在含有临界指数的p-Laplace方程,由于嵌入Hp^1.p(Ω)→L^p*b(Ω,|x|^-α)不紧,如果直接运用标准变分方法去讨论其临界点的存在是比较困难的.为此,采用集中紧原理,证明了方程的非平凡解的存在性.  相似文献   
263.
Objective: To examine the values of endoscopic ultrasonography (EUS) on diagnosis and treatment of esophageal hamartoma. Methods: We compared and analyzed various kinds of imaging examinations such as barium esophagram, contrast-enhanced computed tomography (CT) and conventional gastroscopy in retrospectively reviewing the clinical data of an esophageal hamartoma patient seen in our clinic in the First Affiliated Hospital, School of Medicine, Zhejiang University, Hangzhou, China. Having received various imaging examinations, this patient was finally diagnosed with esophageal hamartoma and underwent gastroscopic resection of hamartoma with the diagnostic information obtained from EUS. The patient had been regularly followed up for 13 months after treatment. Results: Barium esophagram, CT and conventional gastroscopy detected the lesion, but were unable to distinguish it from common esophagopolypus and other submucosal lesions, and unable to determine etiopathogenisis. EUS detected the hamartoma and identified its internal structure, echo, exact size, depth of invasion, origin and the relationship between adjacent tissues and organs, differentiating the lesion from other submucosal tumors and clearly defining the diagnosis. EUS-guided fine needle aspiration (FNA) also helped to identify the etiological diagnosis. Conclusion: EUS was superior to other imaging means in diagnosis and treatment of hamartoma.  相似文献   
264.
在不同的温度和时间下煅烧Cu片得到CO2前驱体,再由Cu2O前驱体电化学还原得到铜电极(Oxide reduced copper, OR-Cu).通过X射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)等测试手段对材料进行了表征,通过循环伏安(CV)和气质联用仪(GC-MS)等表征手段探讨了材料对CO2的电化学催化性能以及对电合成DMC产率的影响.结果表明,OR-Cu电极的晶界以及Cu2O初始厚度是影响材料电催化性能的主要因素.700 ℃下煅烧2 h的Cu2O前驱体,再经电化学还原所得到的电极具有明显的晶界及合适的Cu2O初始厚度,具有最高的电催化活性,以其作为电极合成DMC的产率在室温下可达到87%.  相似文献   
265.
超细碳酸钙生产中的晶型诱导   总被引:4,自引:0,他引:4  
文章探讨了在石灰悬浮液浓度3-5%,反应温度20-50℃条件下不同添加剂对超细碳酸钙晶型的诱导影响,并开发出新的多角型和Z丝型超细碳酸钙。  相似文献   
266.
陕甘宁盆地奥陶系碳酸盐古风化壳天然气储集层中存在丰富的高岭石族矿物,经扫描电镜观察、单矿物样的X射线衍射、红外吸收光谱及差热分析,确认为地开石,而不是高岭石。由于地开石的存在反映酸性的成岩介质,这不利于作为孔隙封堵物的高铁、锰方解石和白云石的沉淀,而有利于碳酸盐矿物的溶解,对埋藏成岩过程中储层孔隙的保存与发育极为有利;同时由于地开石的分子体积小于高岭石,高岭石向地开石的转化会有多余的孔隙空间出现.并形成更多的晶间孔隙,因而地开石通常是好储层的指示矿物。此外.据该地开石形成温度判断,奥陶系碳酸盐储层所经历的最大古地温为140~160℃。  相似文献   
267.
吡虫啉废水中回收碳酸钾的研究   总被引:2,自引:0,他引:2  
根据碳酸钾、碳酸氢钾、氯化钾及水的四元体系在293K时的溶解度及相图,分析研究了吡虫啉废水中各种盐的析出顺序及析出量,确定了合理的碳酸钾回收工艺.产品纯度可达97.5%,碳酸钾回收率达85%.  相似文献   
268.
Calcium carbonate crystallization was performed in hydrogel of hydroxypropylmethyl cellulose (HPMC). Results showed that the polyhydroxy functionalities in HPMC gel network facilitated the nucleation of aragonite which was not found in experiment without HPMC hydrogel and in experiment using methylcellulose (MC) hydrogel as template. On the other hand, due to the unique assembly of the macromolecules in HPMC hydrogel network, which was different from other hydrogels used in some previous reports, particular crystal morphology, corncob-like CaCO_3, was obtained for the first time.  相似文献   
269.
Objective: Leber's hereditary optic neuropathy (LHON) is a maternally inherited degeneration of the optic nerve caused by point mutations of mitochondrial DNA (mtDNA). Many unsolved questions regarding the penetrance and pathophysiological mechanism of LHON demand efficient and reliable mutation testing. This study aims to develop a minor groove binder (MGB) probe assay for rapid detection of mtDNA11778 mutation and heteroplasmy in Chinese LHON patients by real-time polymerase chain reaction (PCR). Methods: Forty-eight patients suspected of having LHON and their maternal relatives underwent a molecular genetic evaluation, with 20 normal individuals as a control group at the same time. A real-time PCR involving two MGB probes was used to detect the mtDNA 1 1778 mutation and heteroplasmy. A linear standard curve was obtained by pUCmLHONG and pUCmLHONA clones. Results: All 48 LHON patients and their maternal relatives were positive for rntDNA11778 mutation in our assay, 27 heteroplasmic and 21 homoplasmic. Eighteen cases did not show an occurrence of the disease, while 9 developed the disease among the 27 heteroplasmic mutation cases. Eleven did not show an occurrence of the disease, while 10 cases developed the disease among 21 homoplasmic mutation cases. There was a significant difference in the incidence between the heteroplasmic and the homoplasmic mutation types. The time needed for running a real-time PCR assay was only 80 min. Conclusion: This real-time PCR assay is a rapid, reliable method for mtDNA mutation detection as well as heteroplasmy quantification. Detecting this ratio is very important for predicting phenotypic expression of unaffected carriers.  相似文献   
270.
A new feather-degrading bacterium was isolated from a local feather waste site and identified as Bacillus subtilis based on morphological, physiochemical, and phylogenetic characteristics. Screening for mutants with elevated keratinolytic activity using N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis resulted in a mutant strain KD-N2 producing keratinolytic activity about 2.5 times that of the wild-type strain. The mutant strain produced inducible keratinase in different substrates of feathers, hair, wool and silk under submerged cultivation. Scanning electron microscopy studies showed the degradation of feathers, hair and silk by the keratinase. The optimal conditions for keratinase production include initial pH of 7.5, inoculum size of 2% (v/v), age of inoculum of 16 h, and cultivation at 23 ℃. The maximum keratinolytic activity of KD-N2 was achieved after 30 h. Essential amino acids like threonine, valine, methionine as well as ammonia were produced when feathers were used as substrates. Strain KD-N2, therefore, shows great promise of finding potential applications in keratin hydrolysis and keratinase production.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号