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   4155篇
  免费   102篇
  国内免费   2篇
电工技术   38篇
综合类   3篇
化学工业   787篇
金属工艺   98篇
机械仪表   89篇
建筑科学   147篇
矿业工程   10篇
能源动力   56篇
轻工业   441篇
水利工程   33篇
石油天然气   5篇
无线电   347篇
一般工业技术   477篇
冶金工业   1292篇
原子能技术   20篇
自动化技术   416篇
  2022年   93篇
  2021年   100篇
  2020年   45篇
  2019年   69篇
  2018年   54篇
  2017年   61篇
  2016年   72篇
  2015年   60篇
  2014年   71篇
  2013年   165篇
  2012年   110篇
  2011年   134篇
  2010年   115篇
  2009年   116篇
  2008年   132篇
  2007年   139篇
  2006年   125篇
  2005年   108篇
  2004年   89篇
  2003年   97篇
  2002年   108篇
  2001年   80篇
  2000年   75篇
  1999年   100篇
  1998年   288篇
  1997年   179篇
  1996年   137篇
  1995年   96篇
  1994年   97篇
  1993年   83篇
  1992年   54篇
  1991年   36篇
  1990年   71篇
  1989年   71篇
  1988年   62篇
  1987年   53篇
  1986年   48篇
  1985年   50篇
  1984年   51篇
  1983年   43篇
  1982年   40篇
  1981年   37篇
  1980年   34篇
  1979年   29篇
  1978年   24篇
  1977年   45篇
  1976年   72篇
  1974年   29篇
  1973年   24篇
  1971年   23篇
排序方式: 共有4259条查询结果,搜索用时 15 毫秒
991.
An irony of territorial behavior is that successful territory defense requires males to be aggressive, but highly aggressive males often inadvertently repel females, potential mates that the territory was created to attract. The authors provide empirical evidence that learning to anticipate the arrival of a female not only can overcome this well-recognized cost of territorial behavior but also can provide a significant paternity advantage. Following a short training period in which male blue gourami fish (Trichogaster trichopterus) either received classically conditioned pairings of a signal and a receptive female or received unpaired presentations of the signal and a female, male subjects were given an opportunity to mate. Signaling of female accessibility not only enabled classically conditioned males to attenuate their initial aggressive response to arriving females but, more important, classically conditioned males were able to spawn with females sooner, clasp females more often, and produce more young than males that did not have the benefit of a signal. Results provide the first demonstration of direct reproductive benefits resulting from classical conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
992.
Prevention programs for eating disorders attempt to simultaneously prevent new cases from arising (primary prevention) and encourage students who already have symptoms to seek early treatment (secondary prevention), even though ideal strategies for these 2 types of prevention may be incompatible with each other. In the present study, an eating disorder prevention program was evaluated in a sample of female college freshmen. In the intervention, classmates who had recovered from eating disorders described their experiences and provided information about eating disorders. At follow-up, intervention participants had slightly more symptoms of eating disorders than did controls. The program may have been ineffective in preventing eating disorders because by reducing the stigma of these disorders (to encourage students with problems to seek help), the program may have inadvertently normalized them. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
993.
We randomly assigned 65 women who had been sexually abused by a father, stepfather, or other close relative to 1 of 3 treatment conditions: a 10-week interpersonal transaction (IT) group, a 10-week process group, or a wait list condition. Subjects were evaluated at pretreatment, posttreatment, and (if assigned to a group) a 6-month follow-up on measures of social adjustment, depression, fearfulness, and general distress. Results suggested that both the IT and process group formats were more effective than the wait list condition in reducing depression and in alleviating distress; changes were maintained at follow-up. Subjects in the process group format exhibited improvement in social adjustment, whereas subjects in the wait list condition actually deteriorated. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
994.
This study investigated and compared ideas about parenting in Argentine, Belgian, French, Israeli, Italian, Japanese, and U.S. mothers of 20-month-olds. Mothers evaluated their competence, satisfaction, investment, and role balance in parenting and rated attributions of successes and failures in 7 parenting tasks to their own ability, effort, or mood, to difficulty of the task, or to child behavior. Few cross-cultural similarities emerged; rather, systematic culture effects for both self-evaluations and attributions were common, such as varying degrees of competence and satisfaction in parenting, and these effects are interpreted in terms of specific cultural proclivities and emphases. Child gender was not an influential factor. Parents' self-evaluations and attributions help to explain how and why parents parent and provide further insight into the broader cultural contexts of children's development.  相似文献   
995.
In this paper we describe the synthesis of a series of alpha-substituted analogues of the potent and selective group II metabotropic glutamate receptor (mGluR) agonist (1S,1'S,2'S)-carboxycyclopropylglycine (2, L-CCG 1). Incorporation of a substitutent on the amino acid carbon converted the agonist 2 into an antagonist. All of the compounds were prepared and tested as a series of four isomers, i.e., two racemic diastereomers. On the basis of the improvement in affinity realized for the alpha-phenylethyl analogue 3, in this paper we explored the effects of substitution on the aromatic ring as a strategy to increase the affinity to these compounds for group II mGluRs. Affinity for group II mGluRs was measured using [3H]glutamic acid (Glu) binding in rat forebrain membranes. Antagonist activity was confirmed for these compounds by measuring their ability to antagonize (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-induced inhibition of forskolin stimulated cyclic-AMP in RGT cells transfected with human mGluR2 and mGluR3. Meta substitution on the aromatic ring of 3 with a variety of substituents, both electron donating (e.g., methyl, hydroxy, amino, methoxy, phenyl, phenoxy) and electron withdrawing (e.g., fluorine, chlorine, bromine, carboxy, trifluoromethyl) gave from 1.5- to 4.5-fold increases in affinity. Substitution with p-fluorine, as in 97 (IC50 = 0.022 +/- 0.002), was the exception. Here, a greater increase in affinity was realized than for either the ortho- or meta-substituted analogues; 97 was the most potent compound resulting from monosubstitution of the aromatic. At best, only modest increases in affinity were realized for certain compounds bearing either two chlorines or two fluorines, and two methoxy groups gave no improvement in affinity (all examined in a variety of substitution patterns). Three amino acids, 4, 5, and 104, were resolved into their four constituent isomers, and affinity and functional activity for group II mGluRs was found to reside solely in the S,S,S-isomers of each, consistent with 1. With an IC50 = 2.9 +/- 0.6 nM, the resolved xanthylmethyl compound 168 was the most potent compound from this SAR. Amino acid 168 demonstrated high plasma levels following intraperitoneal (i.p.) administration and readily penetrated into the brain. This compound, however, had only limited (approximately 5%) oral bioavailability. Systemic administration of 168 protected mice from limbic seizures produced by the mGluR agonist 3,5-dihydroxyphenylglycine, with an ED50 = 31 mg/kg (i.p., 60 min preinjection). Thus, 168 represents a valuable tool to study the role of group II mGluRs in disease.  相似文献   
996.
Compared 15 heterosexual, 14 homosexual, and 10 bisexual male pedophiles (mean age 39 yrs) to 14 nonviolent nonsex offenders (mean age 25 yrs), using the Halstead-Reitan Neuropsychological Test Battery, the Luria-Nebraska Neuropsychological Battery, the Wechsler Adult Intelligence Scale (WAIS), and computerized tomography scans. Ss were classified into their groups based on criminal history, a standard sex history assessment, and a phallometric test of erotic preference. Pedophiles tended to have lower IQs than controls and showed significantly more impairment on all measures. Left temporo-parietal pathology was noted more often for pedophiles. Findings suggest that neuropsychological examination can provide useful and potentially discriminating information in pedophilia and should be considered an important supplement in clinical assessments of the disorder. (French abstract) (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
997.
Proposes that social status is a function of both individual and group characteristics and argues that 2 factors are necessary to predict peer popularity—prosocial interaction and person–group similarity. Prosocial behavior, viewed as a prerequisite for high social status, was predicted to be consistently correlated with status across peer groups. The relation between status and other social behaviors such as aggression was predicted to be mediated by the degree of similarity between the individual and the peer group. Two studies were conducted with 217 males (mean age 10 yrs) at a summer program for children with behavioral and social problems. Data from both acceptance and rejection measures of status support the proposed model. This evidence helps to integrate discrepant findings in the sociometric literature and demonstrates the utility of social psychological theories of interpersonal attraction in the study of peer status. (45 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
998.
BACKGROUND: In most malignant cells, the relatively low level of glucose-6-phosphatase leads to accumulation and trapping of [18F]fluorodeoxyglucose (FDG) intracellularly, allowing the visualization of increased uptake compared with normal cells. OBJECTIVES: To assess the value of FDG positron emission tomography (PET) to differentiate benign from malignant hepatic lesions and to determine in which types of hepatic tumors PET can help evaluate stage, monitor response to therapy, and detect recurrence. DESIGN: Prospective blinded-comparison clinical cohort study. SETTING: Tertiary care university hospital and clinic. PATIENTS: One hundred ten consecutive referred patients with hepatic lesions 1 cm or larger on screening computed tomographic (CT) images who were seen for evaluation and potential resection underwent PET imaging. There were 60 men and 50 women with a mean (+/-SD) age of 59 +/- 14 years. Follow-up was 100%. INTERVENTIONS: A PET scan using static imaging was performed on all patients. The PET scan imaging and biopsy, surgery, or both were performed, providing pathological samples within 2 months of PET imaging. All PET images were correlated with CT scan to localize the lesion. However, PET investigators were unaware of any previous interpretation of the CT scan. MAIN OUTCOME MEASURES: Visual interpretation, lesion-to-normal liver background (L/B) ratio of radioactivity, and standard uptake value (SUV) were correlated with pathological diagnosis. RESULTS: All (100%) liver metastases from adenocarcinoma and sarcoma primaries in 66 patients and all cholangiocarcinomas in 8 patients had increased uptake values, L/B ratios greater than 2, and an SUV greater than 3.5. Hepatocellular carcinoma had increased FDG uptake in 16 of 23 patients and poor uptake in 7 patients. All benign hepatic lesions (n = 23), including adenoma and fibronodular hyperplasia, had poor uptake, an L/B ratio of less than 2, and an SUV less than 3.5, except for 1 of 3 abscesses that had definite uptake. CONCLUSIONS: The PET technique using FDG static imaging was useful to differentiate malignant from benign lesions in the liver. Limitations include false-positive results in a minority of abscesses and false-negative results in a minority of hepatocellular carcinoma. The PET technique was useful in tumor staging and detection of recurrence, as well as monitoring response to therapy for all adenocarcinomas and sarcomas and most hepatocellular carcinomas. Therefore, pretherapy PET imaging is recommended to help assess new hepatic lesions.  相似文献   
999.
OBJECTIVE: A number of dynamic tests of the hypothalamic-pituitary-adrenal axis provide evidence for a mild central adrenal insufficiency in chronic fatigue syndrome (CFS). The 1 microgram adrenocorticotropin (ACTH) test has been proposed to be more sensitive than the standard 250 micrograms ACTH test in the detection of subtle pituitary-adrenal hypofunctioning. We aimed to establish whether the 1 microgram ACTH test would support such a dysregulation in CFS, and also, given the relative novelty of this test in clinical practice and the uncertainty with regard to appropriate cut-off values for normality, to compare our healthy volunteer data with those of previous studies. PATIENTS AND DESIGN: Twenty subjects with CFS, diagnosed according to Centres for Disease Control and Prevention criteria, were compared with 20 healthy volunteer subjects. All participants underwent a 1 microgram ACTH test beginning at 1400 h. Plasma samples for cortisol estimation were drawn at 0, +30 and +40 min. RESULTS: Baseline cortisol values did not differ between CFS patients and healthy subjects. The delta cortisol (maximum increment from baseline) value was significantly lower in the CFS than the volunteer group (P < 0.05). Comparison of the +30 min cortisol values revealed no significant differences. Using an incremental cortisol of > 250 nmol/l as an arbitrary cutoff point, two (10%) of the healthy subjects and nine (45%) of the CFS subjects failed the test on this basis (chi 2 = 4.3, df = 38, P < 0.05). CONCLUSIONS: This study provides further evidence for a subtle pituitary-adrenal insufficiency in subjects with chronic fatigue syndrome compared to healthy volunteers. Disparities between our healthy volunteer data and those of other groups using the 1 microgram ACTH test suggest that the test may not be as reliable as previously indicated.  相似文献   
1000.
NMR spectroscopy, combined with molecular modeling, was used to determine the conformations of isepamicin and butirosin A in the active site of aminoglycoside 6'-N-acetyltransferase-Ii [AAC-(6')-Ii]. The results suggest two enzyme-bound conformers for isepamicin and one for butirosin A. The dihedral angles that describe the glycosidic linkage between the A and B rings for the two conformers of AAC(6')-Ii-bound isepamicin were phi AB = -7.9 +/- 2.0 degrees and psi AB = -46.2 +/- 0.6 degrees for conformer 1 and phi AB = -69.4 +/- 2.0 degrees and psi AB = -57.7 +/- 0.5 degrees for conformer 2. Unrestrained molecular dynamics calculations showed that these distinct conformers are capable of interconversion at 300 K. When superimposed at the 2-deoxystreptamine ring, one enzyme-bound conformer of isepamicin (conformer 1) places the reactive 6' nitrogen in a similar position as that of butirosin A. Conformer 2 of AAC(6')-Ii-bound isepamicin may represent an unproductive binding mode. Unproductive binding modes (to aminoglycoside modifying enzymes) could provide one reason isepamicin remains one of the more effective aminoglycoside antibiotics. The enzyme-bound conformation of butirosin A yielded an orthogonal arrangement of the 2,6-diamino-2,6-dideoxy-D-glucose and D-xylose rings, as opposed to the parallel arrangement which was observed for this aminoglycoside in the active site of an aminoglycoside 3'-O-phosphotransferase [Cox, J. R., and Serpersu, E. H. (1997) Biochemistry 36, 2353-2359]. The complete proton and carbon NMR assignments of the aminoglycoside antibiotic isepamicin at pH 6.8 as well as the pKa values for it's amino groups are also reported.  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