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   6137篇
  免费   440篇
  国内免费   2篇
  2023年   28篇
  2022年   39篇
  2021年   157篇
  2020年   82篇
  2019年   115篇
  2018年   140篇
  2017年   115篇
  2016年   199篇
  2015年   327篇
  2014年   357篇
  2013年   410篇
  2012年   583篇
  2011年   543篇
  2010年   327篇
  2009年   302篇
  2008年   367篇
  2007年   338篇
  2006年   288篇
  2005年   292篇
  2004年   287篇
  2003年   246篇
  2002年   223篇
  2001年   60篇
  2000年   34篇
  1999年   47篇
  1998年   61篇
  1997年   41篇
  1996年   51篇
  1995年   30篇
  1994年   45篇
  1993年   30篇
  1992年   33篇
  1991年   33篇
  1990年   25篇
  1989年   34篇
  1988年   16篇
  1987年   25篇
  1986年   21篇
  1985年   18篇
  1984年   27篇
  1983年   13篇
  1982年   15篇
  1981年   18篇
  1980年   18篇
  1979年   13篇
  1978年   11篇
  1977年   15篇
  1976年   9篇
  1975年   10篇
  1973年   10篇
排序方式: 共有6579条查询结果,搜索用时 62 毫秒
131.
Pigs used in research are often housed in barren environments. The effects of ice as a simple enrichment tool for newly weaned pigs were investigated. Four replicates of 120 pigs were separated into 3 groups. One group was given free access to blocks of ice (ice group), another group had access to Classic Kong toys (Kong group), and a 3rd group did not receive any enrichment (control group). The behavior of each group was observed every 5 min from 08:00 hr to 12:00 hr during 4 consecutive days. Pigs were motivated to explore the ice blocks (4.85% ± 1.34) over the Classic Kong toys (2.03% ± 0.59). No differences in other behaviors were found between treatments. Ice is an effective and easy-to-replace enrichment device. Blocks of ice can be used as enrichment devices for pigs housed in research facilities.  相似文献   
132.
ABSTRACT

The results of various types of manual and free pollination are reported in Helleborus bocconei and Helleborus foetidus, two sympatric species of the understorey of submediterranean woods that flower in January–March and are pollinated by bumble-bees of the genus Bombus. Both species were found to be partially self-compatible even if at different rates; geitonogamy seems to be more frequent in H. foetidus and spontaneous self-pollination does not usually occur for morphological reasons, and the species are not apomictic. Efficiency in seed production after free pollination was very high indicating that in spite of harsh climatic condition, there were no limitations imposed by pollen or pollinators.  相似文献   
133.
The present study was undertaken to identify whether inflammation or oxidative stress is the primary abnormality in the kidney in spontaneously hypertensive rats (SHR). Renal inflammation and oxidative stress were evaluated in 2- and 3-week-old prehypertensive SHR and age-matched genetically normotensive control Wistar-Kyoto (WKY) rats. Blood pressure was similar in WKY and SHR rats at 2 and 3 weeks, of age. Renal inflammation (macrophage and nuclear factor-κB) was elevated in SHR at 3 weeks, but not at 2 weeks, of age compared with age-matched WKY rats. Renal oxidative stress (nitrotyrosine, 8-hydroxy-2′-deoxyguanosine and p47phox) was also clearly elevated in 3-week-old SHR compared with age-matched WKY rats. Additionally, NADPH oxidase subunit p47phox was found elevated in 2-week-old SHR compared to age-matched WKY rats. Moreover, antioxidant (N-acetyl-l-cysteine and Tempol) treatment reduced renal inflammation in prehypertensive SHR. We therefore conclude that the oxidative stress appears before inflammation as a primary abnormality in the kidney in prehypertensive SHR.  相似文献   
134.
A comparative performance evaluation of DNA extraction methods from anti-diabetic botanical supplements using various commercial kits was conducted, to determine which produces the best quality DNA suitable for PCR amplification, sequencing and species identification. All plant materials involved were of suboptimal quality showing various levels of degradation and therefore representing real conditions for testing herbal supplements. Eight different DNA extraction methods were used to isolate genomic DNA from 13 medicinal plant products. Two methods for evaluation, DNA concentration measurements that included absorbance ratios as well as PCR amplifiability, were used to determine quantity and quality of extracted DNA. We found that neither DNA concentrations nor commonly used UV absorbance ratio measurements at A 260/A 280 between 1.7 and 1.9 are suitable for globally predicting PCR success in these plant samples, and that PCR amplifiablity itself was the best indicator of extracted product quality. However, our results suggest that A 260/A 280 ratios below about 1.3 and above 2.3 indicated a DNA quality too poor to amplify. Therefore, A 260/A 280 measurements are not useful to identify samples that likely will amplify but can be used to exclude samples that likely will not amplify reducing the cost for unnecessarily subjecting samples to PCR. The two Nucleospin® plant II kit extraction methods produced the most pure and amplifiable genomic DNA extracts. Our results suggest that there are clear, discernable differences between extraction methods for low quality plant samples in terms of producing contamination-free, high-quality genomic DNA to be used for further analysis.  相似文献   
135.
The quick detection of domestic and peridomestic triatomines in their environments becomes difficult without the use of dislodgement substances that flush them out from their shelters. At present, tetramethrin 0.2% is being widely used in control programs. Although it is an efficient dislodging agent, its toxicity might affect the health of captured triatomines, of other insects and, to a lesser extent, of other animals, including humans. Here, we tested if semiochemicals released by disturbed adults of Triatoma infestans and/or Rhodnius prolixus can make larvae of the same species exit from their refuges. In a walking olfactometer we found that: 1) larvae of T. infestans were repelled by the odors released by disturbed adults of their own species and of R. prolixus, 2) larvae of R. prolixus did not change their behavior in the presence of odors released by adults of both species, and 3) activity levels were not modulated by these odors in any of both species. Besides, in pseudo‐natural conditions we found an increased flushing‐out activity of larvae of T. infestans when their shelters were sprayed with isobutyric acid or 3‐pentanol, and of larvae of R. prolixus when sprayed with 3‐methyl1butanol. We succeeded in this work to dislodge larvae of triatomines from artificial shelters using natural volatile compounds, allowing the capture of live bugs for further investigations (e.g., xenodiagnosis or genetic studies) and favoring ecological aspects (e.g., minimizing environmental insecticide‐contamination and non‐targeted mortality).  相似文献   
136.
137.

Background

IMRT provides highly conformal dose distributions creating non uniform spatial intensity using different segments in the beam.

Material & Methods and Results

Different retrospective studies have shown a high capability of IMRT to treat tumours close to the base of skull. Prospective studies have shown a decrease in xerostomia compared with conventional 3D conformal treatment (3DCRT). Modulation of intensity is performed by the movement of the multileaf collimator (MLC) that can deliver the radiation in different ways, such as static field segments, dynamic field segments and rotational delivery (arc therapy and tomotherapy). There are slight differences among the different techniques in terms of homogeneity, dose conformity and treatment delivery time.

Conclusions

The best method to deliver IMRT will depend on multiple factors such as deliverability, practicality, user training and plan quality.  相似文献   
138.
139.
The soluble lipase from Pseudomonas fluorescens (PFL) forms bimolecular aggregates in which the hydrophobic active centers of the enzyme monomers are in close contact. This bimolecular aggregate could be immobilized by multipoint covalent linkages on glyoxyl supports at pH 8.5. The monomer of PFL obtained by incubation of the soluble enzyme in the presence of detergent (0.5% TRITON X-100) could not be immobilized under these conditions. The bimolecular aggregate has two amino terminal residues in the same plane. A further incubation of the immobilized derivative under more alkaline conditions (e.g., pH 10.5) allows a further multipoint attachment of lysine (Lys) residues located in the same plane as the amino terminal residues. Monomeric PFL was immobilized at pH 10.5 in the presence of 0.5% TRITON X-100. The properties of both PFL derivatives were compared. In general, the bimolecular derivatives were more active, more selective and more stable both in water and in organic solvents than the monomolecular ones. The bimolecular derivative showed twice the activity and a much higher selectivity (100 versus 20) for the hydrolysis of R,S-2-hydroxy-4-phenylbutyric acid ethyl ester (HPBEt) in aqueous media at pH 5.0 compared to the monomeric derivative. In experiments measuring thermal inactivation at 75 °C, the bimolecular derivative was 5-fold more stable than the monomeric derivative (and 50-fold more stable than a one-point covalently immobilized PFL derivative), and it had a half-life greater than 4 h. In organic solvents (cyclohexane and tert-amyl alcohol), the bimolecular derivative was much more stable and more active than the monomeric derivative in catalyzing the transesterification of olive oil with benzyl alcohol.  相似文献   
140.
The hydrolysis of phenolic compounds using an immobilized and highly active and stable derivative of laccase from Trametes versicolor is presented. The enzyme was immobilized on aldehyde supports. For this, the enzyme was enriched in amino groups by chemical modification of its carboxyl groups. The aminated enzyme was immobilized with a high recovered activity (over 60%). Aldehyde derivatives were more stable than soluble or aminated-soluble enzyme and the reference derivatives after incubation in different inactivating conditions (high temperatures, different pH values or presence of organic cosolvents). The most stable derivative was obtained immobilizing the chemically aminated enzyme at pH 10 on aldehyde supports with a stabilization factor approximately 280 fold after incubation at pH 7 and 55 °C. In addition, it was possible to prepare immobilized derivatives with a maximal enzyme loading of 60 mg g?1 of support. This derivative could be reused for 10 reaction cycles with negligible lost of activity.  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