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   8113篇
  免费   656篇
  国内免费   1153篇
  9922篇
  2024年   39篇
  2023年   180篇
  2022年   199篇
  2021年   270篇
  2020年   336篇
  2019年   371篇
  2018年   344篇
  2017年   337篇
  2016年   331篇
  2015年   332篇
  2014年   464篇
  2013年   722篇
  2012年   345篇
  2011年   385篇
  2010年   333篇
  2009年   414篇
  2008年   408篇
  2007年   404篇
  2006年   426篇
  2005年   326篇
  2004年   297篇
  2003年   282篇
  2002年   248篇
  2001年   229篇
  2000年   168篇
  1999年   174篇
  1998年   130篇
  1997年   116篇
  1996年   125篇
  1995年   117篇
  1994年   95篇
  1993年   114篇
  1992年   78篇
  1991年   119篇
  1990年   60篇
  1989年   59篇
  1988年   48篇
  1987年   55篇
  1986年   49篇
  1985年   44篇
  1984年   61篇
  1983年   42篇
  1982年   47篇
  1981年   39篇
  1980年   34篇
  1979年   25篇
  1978年   26篇
  1977年   17篇
  1976年   18篇
  1975年   11篇
排序方式: 共有9922条查询结果,搜索用时 0 毫秒
31.
ABSTRACT Capturing songbirds at their nests can be challenging and time consuming. Although traps designed for capturing songbirds at their nests have been described in the literature, few are effective for capturing species with open‐cup nests. We describe a cylindrical trap designed to capture songbirds at nests up to 2 m above ground in grasses, forbs, shrubs, and small saplings. The nest trap is constructed using a rigid hoop, two pieces of mist net, three stakes, and twist ties. We used this trap to capture female Dickcissels (Spiza americana) and female Indigo Buntings (Passerina cyanea) at their nests, with success rates of 85% (N= 196) and 60–73% (N= 16), respectively. Trapping success was comparable to that using other passerine nest trap designs. Nest abandonment after trapping attempts was rare and similar to that reported in previous studies. Our nest trap is lightweight, easy to make, versatile enough to use in a variety of grassland and shrub habitats, and easily carried and deployed in the field.  相似文献   
32.
S. Haneuse  J. Chen 《Biometrics》2011,67(1):309-318
Summary A recently funded study of the impact of oral contraceptive use on the risk of bone fracture employed the randomized recruitment scheme of Weinberg and Wacholder (1990, Biometrics 46, 963–975). One potential complication in the bone fracture study is the potential for differential response rates between cases and controls; participation rates in previous, related studies have been around 70%. Although data from randomized recruitment schemes may be analyzed within the two‐phase study framework, ignoring potential differential participation may lead to biased estimates of association. To overcome this, we build on the two‐phase framework and propose an extension by introducing an additional stage of data collection aimed specifically at addressing potential differential participation. Four estimators that correct for both sampling and participation bias are proposed; two are general purpose and two are for the special case where covariates underlying the participation mechanism are discrete. Because the fracture study is ongoing, we illustrate the methods using infant mortality data from North Carolina.  相似文献   
33.
Objective: To investigate the construct validity of the Short Form‐36 (SF‐36) Health Survey questionnaire in obese patients. Research Methods and Procedures: Our series consisted of 1735 obese patients (age, 44.7 ± 11.0 years; 1346 women) consecutively enrolled in the QUOVADIS study, an observational multicenter study of obese treatment‐seeking outpatients. The construct validity of the SF‐36 was assessed by main component analysis. Age‐, gender‐, and education‐adjusted general linear models were used to investigate the relationship between BMI and SF‐36 domains or factors identified by main component analysis. Results: BMI was significantly associated with poor health‐related quality of life in all eight SF‐36 domains, and the strongest association was observed with physical activity. Main components analysis generated a six‐factor solution explaining 59% of the observed variance. BMI was strongly associated with factors based on the loading of items regarding the physical activity domain and factors based on role‐physical and role‐emotional items or general health and bodily pain items. In contrast, mental health‐, vitality‐, and social functioning‐based factors were not related to BMI. Discussion: In obese treatment‐seeking outpatients, the clustering of SF‐36 items in main components is not significantly different from the domain‐based approach generally used, thus confirming the robustness of such a generic questionnaire in this specific condition. However, the peculiar clustering of some SF‐36 items and their relationship with BMI suggest that the health‐related quality of life profile of subjects belonging to that population may be better described with alternative aggregations of the SF‐36 items or with disease‐tailored questionnaires.  相似文献   
34.
Ivy leaves extracts are authorised in medicinal products for the treatment of acute bronchitis. Different studies and the long experience on the market show safety and efficacy of this drug. A double-blind, randomised study was conducted to assess the efficacy and tolerability of ivy leaves soft extract with an other ivy leaves extract. 590 patients with acute bronchitis participated in this study. They were treated with test or comparator for 7 days (±1). The Bronchitis Severity Score (BSS) decreased gradually and to a similar extent from Day 1 to Day 7 in both treatment groups. Starting from values of 6.2-6.3 ± 1.2, the BSS decreased by approximately 4.7-4.9 points until Day 7, so that patients left the study with a mean BSS of 1.4-1.6. The BSS subscales cough, sputum, rhales/rhonchi, chest pain during coughing, and dyspnoea improved to a similar extent in both treatment groups. Overall, 2.7% of patients (per group and overall) experienced an adverse event, all of which were non-serious. Fewer patients younger than ten years had adverse events than would have been expected from their share of the study population (p = 0.015; Fisher's exact test). As a conclusion, the test product with ivy leaves soft extract proved to be non-inferior to the comparator ivy leaves extract in improving symptoms of acute bronchitis.  相似文献   
35.
Rare earth ions (Eu3+ or Tb3+)‐activated Ca3 Ga2 Si3O12 (CaGaSi) phosphors were synthesized by using a sol–gel method. Photoluminescence spectra of Eu3+:CaGaSi phosphors exhibited five emission bands at 578, 592, 612, 652 and 701 nm, which were assigned to the transitions (5D07F0, 7F1, 7F2, 7F3 and 7F4), respectively, with an excitation wavelength of λexci = 392 nm. Among these, the transition 5D07F2 (612 nm) displayed bright red emission. In the case of Tb3+:CaGaSi phosphors, four emission bands were observed at 488 (5D47F6), 543 (5D47F5), 584 (5D47F4) and 614 nm (5D47F3) from the measurement of PL spectra with λexci = 376 nm. Among these, the transition 5D47F5 at 543 nm displayed bright green emission. The structure and morphology of the phosphors were studied from the measurements of X‐ray diffraction (XRD), scanning electron microscopy (SEM) and energy‐dispersive X‐ray analysis (EDAX) results. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   
36.
37.
38.
Hydrophobic interaction chromatography, an important and effective purification strategy, is generally used for the purification of variety of biomolecules. A basic understanding of the protein interaction behavior is required to effectively separate these biomolecules. A colloidal type extended Derjaguin, Landau, Verwey, and Overbeek calculations were utilized to study the interactions behavior of model proteins to commercially available hydrophobic chromatographic materials that is, Toyopearl Phenyl 650C and Toyopearl Butyl 650C. Physicochemical properties of selected model proteins were achieved by contact angle and zeta potential measurements. The contact angle of chromatographic materials used was achieved through sessile drop method on disrupted beads and capillary penetration method (CPM) on intact beads. The surface properties were further used to calculate the interactions of the proteins to chromatographic supports. The calculated secondary energy minimum of the proteins with the chromatographic materials (from the contact angle values determined through both methods can be correlated with the retention volumes from the real chromatography. The secondary energy minimum values are higher for each protein to the chromatographic materials calculated from the inputs derived through sessile drop method compared to CPM. For instance, immunoglobulin G has secondary energy minimum value of 0.17 kT compared to 0.11 kT, obtained through sessile drop method and CPM, respectively. Average relative values of the energy minimum calculated for all proteins are as 1.51 kT and 1.29 kT for Toyopearl Butyl 650C and Toyopearl Phenyl 650C, respectively, as a conversion factor for estimation of secondary energy minimum for both methods.  相似文献   
39.
The circadian clock regulates a wide range of electrophysiological and developmental processes in plants. Here, we discuss the direct influence of a circadian clock on biologically closed electrochemical circuits in vivo. The biologically closed electrochemical circuits in the leaves of C. miniata (Kaffir lily), Aloe vera and Mimosa pudica, which regulate their physiology, were analyzed using the charge stimulation method. Plants are able to memorize daytime and nighttime. Even at continuous light or darkness, plants recognize nighttime or daytime and change the input resistance. The circadian clock can be maintained endogenously and has electrochemical oscillators, which can activate ion channels in biologically closed electrochemical circuits. The activation of voltage gated channels depends on the applied voltage, electrical charge, and the speed of transmission of electrical energy from the electrostimulator to plants.  相似文献   
40.
目的:研究案例教学法结合多媒体教学法在超声专业教学中的应用效果。方法:选择2015年3月到2017年3月在我院超声科实习的112名医学院学生进行研究,按照随机数字表法将医学生分成观察组及对照组,各56例,对照组采用传统教学法实施教学,观察组则采用案例教学法以及多媒体教学法实施综合教学,在每学期末,统计并对比两组学生理论成绩和实践成绩,以及两组学生对教学方式的满意度。结果:观察组的理论成绩和实践成绩各项目分值均分别明显高于对照组,差异均有统计学意义(均P0.05)。观察组学生对教学方式的满意度为98.21%,明显高于对照组的87.50%,差异有统计学意义(P0.05)。结论:案例教学法与多媒体教学法的综合应用能够有效提升超声专业学生的理论成绩和实践成绩,还可改善其对教学方式的满意度,效果较好,临床超声教学可考虑将此种教学方式进行广泛推广,从而更有利于培养专业人才。  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