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   7725篇
  免费   18篇
  国内免费   1篇
电工技术   13篇
化学工业   133篇
金属工艺   5篇
机械仪表   30篇
建筑科学   18篇
矿业工程   5篇
能源动力   15篇
轻工业   93篇
水利工程   4篇
石油天然气   3篇
无线电   107篇
一般工业技术   129篇
冶金工业   7063篇
原子能技术   5篇
自动化技术   121篇
  2021年   9篇
  2020年   6篇
  2019年   8篇
  2018年   11篇
  2016年   8篇
  2015年   11篇
  2014年   17篇
  2013年   28篇
  2012年   16篇
  2011年   29篇
  2010年   14篇
  2009年   21篇
  2008年   29篇
  2007年   33篇
  2006年   29篇
  2005年   32篇
  2004年   22篇
  2003年   31篇
  2002年   36篇
  2001年   22篇
  2000年   32篇
  1999年   232篇
  1998年   2394篇
  1997年   1325篇
  1996年   852篇
  1995年   447篇
  1994年   363篇
  1993年   445篇
  1992年   55篇
  1991年   62篇
  1990年   74篇
  1989年   67篇
  1988年   77篇
  1987年   72篇
  1986年   68篇
  1985年   50篇
  1984年   7篇
  1983年   27篇
  1982年   33篇
  1981年   37篇
  1980年   69篇
  1979年   7篇
  1978年   17篇
  1977年   148篇
  1976年   294篇
  1975年   12篇
  1973年   10篇
  1971年   6篇
  1969年   4篇
  1966年   6篇
排序方式: 共有7744条查询结果,搜索用时 31 毫秒
51.
We examined the resting metabolic rate (RMR) and sympathetic nervous system activity of young male vegetarians (n = 17) and nonvegetarians (n = 40). Subjects were characterized for RMR by indirect calorimetry, norepinephrine kinetics from infusions of tritiated norepinephrine, energy and macronutrient intake from a 3-day food diary, and body composition by underwater weighing. Vegetarians reported a greater relative intake of carbohydrates (62% +/- 5% v 51% +/- 6%, P < .01) and a lower relative intake of fat (25% +/- 5% v 33% +/- 6%, P < .01) than nonvegetarians, whereas no differences were observed in daily energy intake, body composition, or maximal aerobic capacity (VO2max) between groups. Vegetarians exhibited an 11% higher absolute RMR (1.29 +/- 0.15 v 1.16 +/- 0.13 kcal/min, P < .01), a higher plasma concentration of norepinephrine (216 +/- 33 v 165 +/- 18 pg/mL, P < .01), and a greater norepinephrine appearance rate (0.50 +/- 0.08 v 0.36 +/- 0.09 micrograms/min, P < .01) than nonvegetarians. After statistically controlling for differences in relative amounts of carbohydrate and fat in the diet and for norepinephrine concentrations, no significant differences in adjusted RMR between vegetarians and nonvegetarians were noted. These results suggest that the higher RMR observed in young male vegetarians is partially mediated by differences in dietary macronutrient composition and increased sympathetic nervous system activity.  相似文献   
52.
Disorders of memory are a frequent cause of consultation and are observed in dementias, in some depressive syndromes and in normal ageing. The specialized memory consultation is based on two successive examinations: one by a neurologist, the other by a psychologist, using standardized batteries of tests. In 100 consecutive subjects, 3 main groups of about 20 to 30 individuals each could be identified: dementia syndromes, psychiatric disorders and age-related disorders of memory. The remaining subjects had various diseases. In subjects with memory complaints a psychometric evaluation performed by a team of specialists seems to be the only means of refining the diagnosis enough for a personalized management. In addition, the specialized consultation team acquires the knowledge that will help it, in the future, to lay down the bases of medical prevention of pathological cerebral ageing.  相似文献   
53.
Enzyme-linked immunosorbent assay and flow cytometric methods to screen hybridoma culture supernatants for antibodies to bovine neutrophils (surface antigen-specific) were optimized. Sensitivity of the 2 methods was compared. A panel of 14 murine monoclonal antibodies (MAB) to surface antigens of bovine polymorphonuclear neutrophilic leukocytes (neutrophils) was produced by hybridoma technology, and their isotypes were determined by whole-cell ELISA. Monoclonal antibody reactivity with neutrophils, eosinophils, and lymphocytes isolated on phosphate-buffered saline solution and on Ficoll-sodium diatrizoate were compared. Biochemical characterization of antigens recognized by MAB was performed by immunoblot analysis. Neutrophil plasma membranes were isolated on sucrose gradients (20, 32, and 50%) and purified for polypeptide characterization. Neutrophil surface proteins were characterized by external labeling with 125I. The flow cytometric method was proven to be more sensitive and rapid than ELISA to screen hybridoma supernatants. This method allowed light-scatter gating of live neutrophil populations for analysis, which eliminated nonspecific binding of antibodies to contaminating cells and dead neutrophils. The optimal conditions for flow cytometric analyses were 5 x 10(5) neutrophils and 1 micrograms of fluorescein-labeled F(ab')2/assay as the second antibody. The optimal conditions for hybridoma screening by ELISA were neutrophil concentration of 2.5 x 10(5)/well, using a 96-well polystyrene microtitration plate as solid support, and 2,2'-azino-di[3-ethyl-benzthiazoline sulfonate (6)] with H2O2 as the chromatogenic substrate. Tissue culture plates as solid support and 3,3', 5,5'-tetramethyl benzidine, with H2O2 as the chromogenic substrate, were equally as sensitive. Panel MAB reacted differently with neutrophils, eosinophils, and lymphocytes. Isolation of these cells from blood on Ficoll-sodium diatrizoate generally did not alter MAB reactivity. Coomassie blue-stained gels of neutrophil plasma membrane proteins contained about 25 polypeptide bands, 13 of which were major bands. Autoradiography revealed about 11 surface proteins, 5 of which were heavily labeled with 125I. Monoclonal antibody S7G8 identified a 65-kd protein and MAB S8G10 identified 65- and 70-kd proteins. On the basis of molecular weight, MAB S7G8 and S8G10 are comparable to human CD15, CD16, and CD64 molecules. The MAB generated in this study are potential candidates to discern bovine neutrophil function and heterogeneity.  相似文献   
54.
55.
INTRODUCTION: Previous work has shown that parents prefer to be present when their children undergo common invasive procedures, although physicians are ambivalent about parental presence. PURPOSE: To determine the effect of a parent-focused intervention on the pain and performance of the procedure, anxiety of parents and clinicians, and parental satisfaction with care. POPULATION: Children younger than 3 years old undergoing venipuncture, intravenous cannulation, or uretheral catheterization. SETTING: Pediatric emergency department of Boston City Hospital. DESIGN: Randomized controlled trial with three groups; parents present and given instructions on how to help their children; parents present, but no instructions given; and parents not present. INTERVENTION: The parents were instructed to touch, talk to, and maintain eye contact during the procedure. RESULTS: A total of 431 parents was randomized to the intervention (N = 153), present (N = 147), and not present (N = 131) groups. The groups were equivalent with respect to measured sociodemographic variables and parents' previous experience in the pediatric emergency department. No differences emerged with respect to pain (3-point scale measured by parent and clinician, and analysis of cry); performance of the procedure (number of attempts, completion of procedure by first clinician, time); clinician anxiety; or parental satisfaction with care. Parents who were present were more likely to rate the pain of the children as extreme/severe (52%) in comparison to clinicians (15%, kappa .07, poor agreement) and were significantly less anxious than parents who were not present. CONCLUSION: Overall, the intervention was not effective in reducing the pain of routine procedures. Parental presence did not negatively affect performance of the procedure or increase clinician anxiety. Parents who were present were less anxious than those who were not present. CLINICAL IMPLICATION: In general, parents have indicated that they want to be present when their children undergo procedures. The results of this study challenge the traditional belief that parental presence negatively affects our ability to successfully complete procedures. We should encourage parents who want to be present to stay during procedures.  相似文献   
56.
The motility imparted by the periplasmic flagella (PF) of Serpulina hyodysenteriae is thought to play a pivotal role in the enteropathogenicity of this spirochete. The complex PF are composed of multiple class A and class B polypeptides. Isogenic strains containing specifically disrupted flaAl or flaB1 alleles remain capable of expressing PF, although such mutants display aberrant motility in vitro. To further examine the role that these proteins play in the maintenance of periplasmic flagellar structural integrity, motility, and fitness for intestinal colonization, we constructed a novel strain of S. hyodysenteriae which is deficient in both FlaA1 and FlaB1. To facilitate construction of this strain, a chloramphenicol gene cassette, with general application as a selectable marker in prokaryotes, was developed. The cloned flaAl and flaB1 genes were disrupted by replacement of internal fragments with chloramphenicol and kanamycin gene cassettes, respectively. The inactivated flagellar genes were introduced into S. hyodysenteriae, and allelic exchange at the targeted chromosomal flaA1 and flaB1 loci was verified by PCR analysis. Immunoblots or cell lysates with antiserum raised against purified FlaA or FlaB confirmed the absence of the corresponding sheath and core proteins in this dual flagellar mutant. These mutations selectively abolished the expression of the targeted genes without affecting the synthesis of other immunologically related FlaB proteins. The resulting flaA1 flaB1 mutant exhibited altered motility in vitro. Surprisingly, it was capable of assembling periplasmic flagella that were morphologically normal as evidenced by electron microscopy. The virulence of this strain was assessed in a murine model of swine dysentery by determining the incidence of cecal lesions and the persistence of S. hyodysenteriae in the gut. Mice challenged with the wild-type strain or a passage control strain showed a dose-related response to the challenge organism. The dual flagellar mutant was severely attenuated in murine challenge experiments, suggesting that the FlaA1 and FlaB1 proteins are dispensable for flagellar assembly but critical for normal flagellar function and colonization of mucosal surfaces of the gastrointestinal tract. This strain represents the first spirochete engineered to contain specifically defined mutations in more than one genetic locus.  相似文献   
57.
The coagulation system plays a major role in the formation of the infected endocardial vegetation in bacterial endocarditis. Since monocytes can express tissue factor (TF) on their surfaces, they are thought to be responsible for the extrinsic activation of the coagulation cascade during this disease. The present study used an in vitro model in which fibrin plates, isolated adherent monocytes, and Streptococcus sanguis were used as an analog for endocardial vegetations. Adherence to fibrin by itself was found to stimulate TF expression on the monocytes, but stimulation by S. sanguis significantly increased TF expression, which was found to be maximal at a bacterium-to-monocyte ratio of 9 or more.  相似文献   
58.
The conventional treatment of inflammatory bowel disease should center around the liberal use of one of the many available forms of 5-ASA. Sulfasalazine should be used initially with the newer mesalamine-only containing drugs being reserved for sulfasalazine-intolerant patients or for those patients who require larger doses of medication. The choice of the delivery method should be made with the knowledge of the extent of disease and the potential coverage areas of the individual delivery methods. Systemic and topical glucocorticoids are an invaluable adjunct to 5-ASA therapy, but their use must be directed with the goal of remission induction. The tapering of glucocorticoids should be as prompt as the maintenance of remission allows, with a useful general guideline of decreasing the dose by 1 mg per day. Immunosuppressive therapy, including azathioprine and 6-mercaptopurine, holds promise for refractory cases of inflammatory bowel disease and for their potential steroid sparing properties; antibiotic therapy with metronidazole and ciprofloxacin in the absence of documented infectious disease offers additional routes to control disease. The majority of patients require a combination of drugs to attain remission. Only further study will reveal the ideal regimen for each of the different subsets of inflammatory bowel disease.  相似文献   
59.
The present study proposes a detection technique for delaminations in a laminated beam. The proposed technique optimizes the spatial distribution of harmonic excitation so as to magnify the difference in response between the delaminated and intact beam. The technique is evaluated by numerical simulation of two-layered aluminum beams. Effects of measurement and geometric noise are included in the analysis. A finite element model for a delaminated composite, based on a layer-wise laminated plate theory is used in conjunction with a step function to simulate delaminations  相似文献   
60.
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