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   46084篇
  免费   17154篇
  国内免费   11篇
  2023年   85篇
  2022年   271篇
  2021年   769篇
  2020年   2396篇
  2019年   3970篇
  2018年   4126篇
  2017年   4381篇
  2016年   4577篇
  2015年   4839篇
  2014年   4552篇
  2013年   5194篇
  2012年   3348篇
  2011年   3120篇
  2010年   4027篇
  2009年   2631篇
  2008年   1954篇
  2007年   1590篇
  2006年   1493篇
  2005年   1518篇
  2004年   1479篇
  2003年   1381篇
  2002年   1288篇
  2001年   464篇
  2000年   352篇
  1999年   340篇
  1998年   289篇
  1997年   194篇
  1996年   160篇
  1995年   194篇
  1994年   191篇
  1993年   156篇
  1992年   136篇
  1991年   130篇
  1990年   123篇
  1989年   108篇
  1988年   107篇
  1987年   119篇
  1986年   107篇
  1985年   106篇
  1984年   103篇
  1983年   119篇
  1982年   113篇
  1981年   115篇
  1980年   92篇
  1979年   59篇
  1978年   57篇
  1977年   61篇
  1976年   56篇
  1975年   38篇
  1974年   24篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
There are four main ideas in relapse prevention. First, relapse is a gradual process with distinct stages. The goal of treatment is to help individuals recognize the early stages, in which the chances of success are greatest. Second, recovery is a process of personal growth with developmental milestones. Each stage of recovery has its own risks of relapse. Third, the main tools of relapse prevention are cognitive therapy and mind-body relaxation, which are used to develop healthy coping skills. Fourth, most relapses can be explained in terms of a few basic rules. Educating clients in these rules can help them focus on what is important: 1) change your life (recovery involves creating a new life where it is easier to not use); 2) be completely honest; 3) ask for help; 4) practice self-care; and 5) don’t bend the rules.  相似文献   
93.
doi:10.1111/j.1741‐2358.2009.00333.x
Effect of microwave treatment on the shear bond strength of different types of commercial teeth to acrylic resin Objective: The purpose of this study was to verify the effect of microwave treatment on the shear bond strength of commercial types of teeth to acrylic resin, when the glossy ridge laps were unmodified (groups 1 and 5), bur abraded (groups 2 and 6), bur grooved (groups 3 and 7) or etched by monomer (groups 4 and 8). Background: Controversial findings have shown that mechanical or chemical changes in ridge‐lap surface of the tooth increase or decrease the bond strength between tooth and acrylic resin, and the microwave disinfection may cause different changes on this bond strength. Materials and methods: Eighty specimens (n = 10) were made with the acrylic resin bonded to tooth glossy ridge lap, polymerised in water at 74°C for 9 h, and deflasked after flask cooling. Specimens of the groups 5, 6, 7 and 8 were individually immersed in 150 ml of water and submitted to microwave treatment in an oven at 650 W for 3 min. Control specimens (groups 1, 2, 3 and 4) were not microwave treated. Shear bond strength test was performed in an Instron machine with a cross‐speed of 1 mm/min. Collected data were submitted to anova and Tukey’s test (α = 0.05). Results: Microwave treatment decreased the shear bond strength values of the tooth/resin bond. In the microwaved and non‐microwaved procedures, mechanical retention improved the shear bond strength when compared with the control and monomer treatments. Conclusion: Shear bond strength of the tooth/resin bond was influenced by the microwave treatment and different commercial teeth association, and was lower for the Biotone tooth.  相似文献   
94.
95.
A particulate fraction of adult rat brain (sucrose buoyant density 1.24 gm/ml) catalyzed the incorporation of [3H]dTTP into an acid-insoluble product in an endogenously templated reaction sensitive to ribonuclease pretreatment. Upon fractionation, this activity was identified in the cerebellum, pons, frontal lobes and base. The DNA polymerase present in these brain fractions exhibited a strong preference for the synthetic template dT12–18·poly rA rather than dT12–18·poly dA; dT10 was completely inactive. Purification and equilibrium Cs2SO4 gradient centrifugation of the [3H]DNA product-endogenous template complex suggested that RNA was serving as primer for endogenous DNA synthesis.  相似文献   
96.
97.
98.
Many investigators categorize individuals from hybrid zones to facilitate comparisons among genotypic classes (e.g., parental, F1, backcross) for comparative studies in which components of fitness or geographic variation are being analyzed. Frequently, multiple character sets representing genetically independent traits are used to classify these individuals and various methodologies are employed to combine the classifications obtained from the different character sets. We adapted the principles of total evidence and taxonomic congruence (two formalized approaches used by systematists in formulating phylogenetic hypotheses) to address the problem of discriminating hybridizing species and classifying individuals from hybrid zones. As our model, we used two morphological (coloration and morphometric) and two molecular (allozyme and mitochondrial DNA restriction-fragment-length polymorphism) character sets that differentiate two stone crab species (Menippe adina and M. mercenaria). Using principal-components analysis, we determined that combining character sets and eliminating characters or character sets that did not have large eigenvector coefficients for the principal component that best separated the two species yielded the highest level of discrimination between species and allowed us to classify a broad range of morpho-genotypes as hybrids. For the stone crabs, three diagnostic allozyme loci and five diagnostic coloration characters best separated the species. The two character sets were not completely congruent, but they agreed in their classification of 50% of the individuals from the hybrid zone and rarely strongly disagreed in their classifications. Classification discrepancies between the two character sets probably represent variation between traits in interspecific gene flow rather than intraspecific, ecologically mediated variation. Our results support the assertions of previous investigators who espoused the benefits associated with using multiple character sets to classify individuals from hybrid zones and demonstrate that, if character sets are reasonably congruent and numerically balanced, combining diagnostic characters from multiple character sets (a total-evidence approach) can enhance discriminatory power between species and facilitate the assignment of hybrid-zone individuals to genotypic classes. On the contrary, classifying hybrid-zone individuals using character sets separately (a taxonomic-congruence approach) provides the opportunity to compare levels of introgression between species and to assess reasons for discordance among the data sets.  相似文献   
99.
100.
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