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   3894篇
  免费   363篇
  国内免费   290篇
  4547篇
  2024年   15篇
  2023年   95篇
  2022年   120篇
  2021年   146篇
  2020年   155篇
  2019年   161篇
  2018年   159篇
  2017年   132篇
  2016年   178篇
  2015年   143篇
  2014年   192篇
  2013年   256篇
  2012年   144篇
  2011年   237篇
  2010年   127篇
  2009年   205篇
  2008年   181篇
  2007年   176篇
  2006年   146篇
  2005年   125篇
  2004年   134篇
  2003年   103篇
  2002年   99篇
  2001年   87篇
  2000年   62篇
  1999年   54篇
  1998年   56篇
  1997年   53篇
  1996年   49篇
  1995年   58篇
  1994年   49篇
  1993年   48篇
  1992年   29篇
  1991年   38篇
  1990年   26篇
  1989年   25篇
  1988年   19篇
  1985年   26篇
  1984年   40篇
  1983年   34篇
  1982年   37篇
  1981年   24篇
  1980年   23篇
  1979年   32篇
  1978年   38篇
  1977年   23篇
  1976年   22篇
  1975年   42篇
  1974年   46篇
  1973年   32篇
排序方式: 共有4547条查询结果,搜索用时 15 毫秒
111.
Polymer films of sago starch/polyvinyl alcohol (PVA) were prepared by casting and cured under ultra violet (UV) radiation. Different blends were made varying the concentration of sago starch and PVA. Tensile strength (TS) and elongation at break (Eb) of the prepared films were studied. Films made up of sago starch and PVA with a ratio of 1:2 showed the highest TS and Eb. The physico-mechanical properties of prepared films were improved by grafting with acrylic monomers with the aid of UV radiation. A series of formulations was prepared with two monomers 2-ethyl 2-hydroxymethyl 1,3 methacrylate (EHMPTMA) and 2-ethylhexylacrylate (EHA) and a photoinitiator. Monomer concentration, soaking time and radiation dose were optimized in terms of grafting and mechanical properties. The highest TS was at 50% EHMPTMA and 48% EHA and 2% photo initiator at 5 min soaking time and recorded value was 6.58 MPa. The prepared films were further characterized with NMR spectroscopy and scanning electron microscope (SEM).  相似文献   
112.
Certain drugs or treatments that are known to affect bone quality or integrity might have side effects on the extracellular matrix of articular cartilage. We investigated the effects of vitamin D and calcium deficiency, estrogen deficiency, and hypercortisolism alone or in combination with bisphosphonates or sodium fluoride in an animal model, viz., the Göttingen miniature pig (n=29). The articular cartilage from knee joints was analyzed for its content of glycosaminoglycans (GAGs, as macromolecules responsible for the elasticity of articular cartilage) by a spectrometric method with dimethylene blue chloride. In cryo- or paraffin sections, alkaline phosphatase (AP, as an enzyme indicating mineralization or reorganization of articular cartilage matrix) was localized by enzyme histochemistry, and positive cells were counted, whereas differently sulfated GAGs were stained histochemically. A significant decrease in GAG content was measured in ovariectomized and long-term glucocorticoid-treated animals compared with untreated animals. In the glucocorticoid/sodium fluoride group, GAGs were significantly diminished, and significantly fewer AP-positive chondrocytes were counted compared with the control. GAG content was slightly higher, and significantly more AP-positive chondrocytes were counted in short-term glucocorticoid-treated animals then in the control group. GAGs, as part of proteoglycans, are responsible for the water-storage capacity that gives articular cartilage its unique property of elasticity. Thus, ovariectomy and long-term glucocorticoid therapy, especially when combined with sodium fluoride, have detrimental effects on this tissue.This work was in part supported by Deutsche Forschungsgemeinschaft (DFG) project no. Schr 430/5–1, 5–2 and G 1289/1–1, 1–2  相似文献   
113.
杀虫剂轮用和混用对害虫种群抗性演化的影响   总被引:10,自引:1,他引:10  
莫建初  庄佩君 《昆虫学报》1999,42(4):337-346
杀虫剂轮用和混用是当前害虫抗药性治理中最常采用的两种用药策略。该文用抗性模型和室内试验对轮用和混用延缓害虫抗性演化的效果及轮用最佳间隔期问题作了研究。模型模拟结果表明,在延缓害虫抗性演化方面轮用和混用对抗性演化的影响主要取决于杀虫剂作用强度、抗性基因型个体的适合度大小和杀虫剂混用后的毒理效应类型。两种杀虫剂轮用时,轮用间隔期以1(即两种杀虫剂隔次施用)为佳。以模型昆虫小菜蛾Plutella xylostella在室内试验结果表明,氰戊菊酯单用(I)及与杀虫单轮用(II)和混用(1∶1)(III)连续处理8代后,抗性个体频率为0.01的小菜蛾种群其3龄幼虫对氰戊菊酯的抗性分别上升了75.87(I)、28.67(II)和58.72(III)倍,与模型模拟结果表现出较好的一致性。据此,作者认为抗性模型可用于预测害虫抗性演化和评价抗性治理策略。  相似文献   
114.
Summary The paper is the second of two papers about statistical considerations that researchers should make while doing in vitro plant biology research. The first paper focused on aspects from developing a plan to do research through the collection of data. This paper continues with information about editing data, handling outliers, analyzing quantitative and qualitative data, comparing treatment means, preparing graphs and tables, and presenting results.  相似文献   
115.
Objective: The objective was to examine the effect of offering a reimbursement incentive on the percentage of inquirers who enrolled in a weight control program and on weight loss and program attendance among enrollees. Research Methods and Procedures: We used a sequential control‐intervention design to observe how inquirers of the University of Alabama at Birmingham EatRight Lifestyle Program responded to an enrollment incentive for potential 50% ($150) reimbursement of the total program fee if they attended 10 of 12 classes and lost at least 6% of their current body weight. Inquirers had to be adults with a BMI ≥30 kg/m2, seeking information about a weight control program, and informed of the program cost. Outcomes included proportion of inquirers enrolled, overall number of classes attended, and weight loss. Results: Of the 401 people who inquired during the study periods, 24.5% and 25.0% enrolled in the intervention and control periods, respectively. There was a trend toward higher attendance in the intervention group, compared with the control group; there were no differences in percentage of weight loss. The odds of attending ≥10 classes were 2.4 times as high, and both losing >6% body weight and attending ≥10 classes were three times as high in the intervention subjects compared with controls, although non‐significant. Discussion: The potential of earning a performance‐based reimbursement incentive did not affect enrollment in the EatRight Lifestyle Program. Performance‐based incentives may be an ideal mechanism for extending coverage of weight‐loss interventions by insurers because of limited financial risk and improved adherence.  相似文献   
116.
Both autoclaving and dry-heat treatments were applied to dairy manure-based compost to achieve target populations of indigenous microorganisms. A 3 strain-mixture of Escherichia coli O157:H7 of ca. 2 log CFU/g was inoculated into acclimated autoclaved compost (AAC) and dry heat-treated compost (DHTC) with different moistures, and stored at 8, 22, or 30 °C. Only selected groups of microorganisms grew in AAC during acclimation, whereas the relative ratio of each group of microorganisms was maintained in DHTC after heat treatment. E. coli O157:H7 grew more in AAC than DHTC in the presence of same level of indigenous mesophiles. However, control compost (no heat treatment) did not support E. coli O157:H7 growth. Our results revealed that both the type and population of indigenous microorganisms is critical for suppressing E. coli O157:H7 growth in compost, and dry-heat treatment can result in a compost product which resembles cured compost with different levels of indigenous microorganisms.  相似文献   
117.
To ascertain the effect of exogenously applied hydrogen peroxide (H2O2) on drought stress, we examined whether the spraying of soybean leaves with H2O2 would alleviate the symptoms of drought stress. Pre-treatment by spraying leaves with H2O2 delayed foliar wilting caused by drought stress compared to leaves sprayed with distilled water (DW). Additionally, the relative water content of drought-stressed leaves pre-treated with H2O2 was higher than that of leaves pre-treated with DW. Therefore, we analyzed the effect of H2O2 spraying on photosynthetic parameters and on the biosynthesis of oligosaccharides related to water retention in leaves during drought stress. Under conditions of drought stress, the net photosynthetic rate and stomatal conductance of leaves pre-treated with H2O2 were higher than those of leaves pre-treated with DW. In contrast to DW spraying, H2O2 spraying immediately caused an increase in the mRNA levels of d-myo-inositol 3-phosphate synthase 2 (GmMIPS2) and galactinol synthase (GolS), which encode key enzymes for the biosynthesis of oligosaccharides known to help plants tolerate drought stress. In addition, the levels of myo-inositol and galactinol were higher in H2O2-treated leaves than in DW-treated leaves. These results indicated that H2O2 spraying enabled the soybean plant to avoid drought stress through the maintenance of leaf water content, and that this water retention was caused by the promotion of oligosaccharide biosynthesis rather than by rapid stomatal closure.  相似文献   
118.
The Treatment of Irritable Bowel Syndrome with Hypnotherapy   总被引:5,自引:0,他引:5  
Previous research from the United Kingdom has shown hypnotherapy to be effective in the treatment of irritable bowel syndrome (IBS). The current study provides a systematic replication of this work in the United States. Six matched pairs of IBS patients were randomly assigned to either a gut-directed hypnotherapy (n=6) or to a symptom monitoring wait-list control condition (n=6) in a multiple baseline across subjects design. Those assigned to the control condition were later crossed over to the treatment condition. Subjects were matched on concurrent psychiatric diagnoses, susceptibility to hypnosis, and various demographic features. On a composite measure of primary IBS symptoms, treatment was superior (p=.016) to symptom monitoring. Results from the entire treated sample (n=11; one subject was removed from analysis) indicate that the individual symptoms of abdominal pain, constipation, and flatulence improved significantly. State and trait anxiety scores were also seen to decrease significantly. Results at the 2-month follow-up point indicated good maintenance of treatment gains. No significant correlation was found between initial susceptibility to hypnosis and treatment gain. A positive relationship was found between the incidence of psychiatric diagnosis and overall level of improvement.  相似文献   
119.
Pneumonia, the inflammatory state of lung tissue primarily due to microbial infection, claimed 52,306 lives in the United States in 20071 and resulted in the hospitalization of 1.1 million patients2. With an average length of in-patient hospital stay of five days2, pneumonia and influenza comprise significant financial burden costing the United States $40.2 billion in 20053. Under the current Infectious Disease Society of America/American Thoracic Society guidelines, standard-of-care recommendations include the rapid administration of an appropriate antibiotic regiment, fluid replacement, and ventilation (if necessary). Non-standard therapies include the use of corticosteroids and statins; however, these therapies lack conclusive supporting evidence4. (Figure 1)Osteopathic Manipulative Treatment (OMT) is a cost-effective adjunctive treatment of pneumonia that has been shown to reduce patients’ length of hospital stay, duration of intravenous antibiotics, and incidence of respiratory failure or death when compared to subjects who received conventional care alone5. The use of manual manipulation techniques for pneumonia was first recorded as early as the Spanish influenza pandemic of 1918, when patients treated with standard medical care had an estimated mortality rate of 33%, compared to a 10% mortality rate in patients treated by osteopathic physicians6. When applied to the management of pneumonia, manual manipulation techniques bolster lymphatic flow, respiratory function, and immunological defense by targeting anatomical structures involved in the these systems7,8, 9, 10.The objective of this review video-article is three-fold: a) summarize the findings of randomized controlled studies on the efficacy of OMT in adult patients with diagnosed pneumonia, b) demonstrate established protocols utilized by osteopathic physicians treating pneumonia, c) elucidate the physiological mechanisms behind manual manipulation of the respiratory and lymphatic systems. Specifically, we will discuss and demonstrate four routine techniques that address autonomics, lymph drainage, and rib cage mobility: 1) Rib Raising, 2) Thoracic Pump, 3) Doming of the Thoracic Diaphragm, and 4) Muscle Energy for Rib 1.5,11  相似文献   
120.
The quality of seed treatment by frugivores has an effect on seed removal after dispersal, seed germination and tree recruitment. We provide information on postdispersal seed removal, germination and subsequent recruitment in tropical forest tree species Antiaris toxicaria in Ghana. We tested whether postdispersal seed removal and germination rates were differentially affected by the following seed treatments: seeds that were spat out by monkeys with all fruit pulp removed and spitting seeds with fruit pulp partially removed as observed in some birds and bats. We used seeds of intact ripened fruits as control. Frugivore seed treatment and distance from bole affected seed removal patterns, whereas intact seeds were significantly removed from all seed stations. The germination success was greater for seeds that were spat out by monkeys and poor for seeds with fruit pulp partially removed and intact fruits. More recruits were recorded at the edge of the adult A. toxicaria canopy radius. There was weak relationship (r2 = 0.042) between the number of recruits and distance away from the adult tree. Results suggest that the subsequent recruitment in tropical forest tree species may be enhanced by some frugivore fruit‐handling behaviour where fruit pulp is removed from the seeds without destroying the seeds.  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