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   4572篇
  免费   420篇
  国内免费   10篇
电工技术   47篇
综合类   6篇
化学工业   1357篇
金属工艺   71篇
机械仪表   104篇
建筑科学   200篇
矿业工程   14篇
能源动力   115篇
轻工业   517篇
水利工程   33篇
石油天然气   12篇
无线电   490篇
一般工业技术   796篇
冶金工业   572篇
原子能技术   47篇
自动化技术   621篇
  2022年   43篇
  2021年   49篇
  2020年   63篇
  2019年   103篇
  2018年   121篇
  2017年   130篇
  2016年   161篇
  2015年   140篇
  2014年   201篇
  2013年   311篇
  2012年   220篇
  2011年   248篇
  2010年   223篇
  2009年   216篇
  2008年   232篇
  2007年   177篇
  2006年   187篇
  2005年   152篇
  2004年   145篇
  2003年   133篇
  2002年   126篇
  2001年   95篇
  2000年   106篇
  1999年   73篇
  1998年   91篇
  1997年   73篇
  1996年   83篇
  1995年   63篇
  1994年   75篇
  1993年   61篇
  1992年   52篇
  1991年   35篇
  1990年   55篇
  1989年   50篇
  1988年   31篇
  1987年   36篇
  1986年   33篇
  1985年   44篇
  1984年   48篇
  1983年   41篇
  1982年   39篇
  1981年   37篇
  1980年   34篇
  1979年   45篇
  1978年   17篇
  1977年   42篇
  1976年   30篇
  1975年   23篇
  1974年   23篇
  1973年   24篇
排序方式: 共有5002条查询结果,搜索用时 15 毫秒
81.
In a general population sample from the Detroit site of the Collaborative Genetic Study of Nicotine Dependence (COGEND), we tested Black-White differences in nicotine dependence, measured by "how soon after wake-up the smokers smoked their first cigarette (time to first cigarette TTFC)", and its relationship with number of cigarettes per day (CPD). Analysis was conducted on respondents who have smoked > or =100 cigarettes in lifetime and were current smokers (n = 1,442; 1,087 Whites and 355 Blacks). In univariate analysis, we found no significant race differences on time to first cigarette (chi2 = 2.9, p value = 0.41), but significant race differences on CPD (chi2 = 154.3, p<.01), both categorized by the Fagerstr?m Test of Nicotine Dependence (FTND) cutoffs. We estimated the probability of TTFC < or =30 min given CPD using probit models. The interactions between race and CPD indicated significant differences in dependence at various levels of CPD. The same probability of nicotine dependence was associated with smaller increments in CPD for Blacks than for Whites. The data support the hypothesis that the relationship between CPD and nicotine dependence as reflected in relapse varies by race, and that Black smokers are dependent at lower levels of CPD than Whites.  相似文献   
82.
83.
We conducted a systematic review of what is known about the relationship between post-traumatic stress disorder (PTSD) and smoking to guide research on underlying mechanisms and to facilitate the development of evidence-based tobacco treatments for this population of smokers. We searched Medline, PsychINFO, and the Cochrane Central Register of Controlled Trials and identified 45 studies for review that presented primary data on PTSD and smoking. Smoking rates were high among clinical samples with PTSD (40%-86%) as well as nonclinical populations with PTSD (34%-61%). Most studies showed a positive relationship between PTSD and smoking and nicotine dependence, with odds ratios ranging between 2.04 and 4.52. Findings also suggest that PTSD, rather than trauma exposure itself, is more influential for increasing risk of smoking. A small but growing literature has examined psychological factors related to smoking initiation and maintenance and the overlapping neurobiology of PTSD and nicotine dependence. Observational studies indicate that smokers with PTSD have lower quit rates than do smokers without PTSD. Yet a few tobacco cessation treatment trials in smokers with PTSD have achieved quit rates comparable with controlled trials of smokers without mental disorders. In conclusion, the evidence points to a causal relationship between PTSD and smoking that may be bidirectional. Specific PTSD symptoms may contribute to smoking and disrupt cessation attempts. Intervention studies that test behavioral and pharmacological interventions designed specifically for use in patients with PTSD are needed to reduce morbidity and mortality in this population.  相似文献   
84.
85.
Seeds from fruits such as Citrullus (C.) lanatus (watermelon) and Limonia (L.) acidissima (wood apple) are not commonly utilized but could be suitable in numerous food formulations. It was shown that the protein content of defatted seed flours was 71.38 and 49.51 % and that these contained considerable amounts of minerals such as Na, Mn, Mg, K, Cu, Fe and Zn. The defatted L. acidissima seed flour was superior to C. lanatus in essential amino acids. The flours obtained from both seeds were also evaluated for functional properties and characterized by X-ray diffraction, differential scanning calorimeter and scanning electron microscope (SEM). Amorphous nature was observed in defatted C. lanatus and L. acidissima flours due to the low percentage of degree of crystallinity. Spherical morphologies were observed through SEM. The exothermic peak was recorded in defatted C. lanatus and L. acidissima flour.  相似文献   
86.
Summary Analytical results are given for whey powders prepared on a commercial or semi-commercial scale by three companies. Altogether, five preparations enriched in β-lactoglobulin, four whey protein isolates and a fraction enriched in α-lactalbumin were analyzed for protein composition, including %β-lactoglobulin, α-lactalbumin, bovine serum albumin, casein (glyco) macropeptide and the main triglycerides. Protein composition was determined by high pressure gel permeation and reversed phase liquid chromatography and by capillary zone electrophoresis. The extent of modification of the native β-lactoglobulin structure was also measured through the degree of lactosylation and the fraction of accessible free sulphydryl groups. One significant finding was that the calculated recovery of protein following quantitation of the chromatogram or electropherogram was seldom above 90% and occasionally below 60% of that loaded onto the column or capillary, raising doubts as to the reliability of the analytical results. Extrapolation by linear regression to 100% recovery allowed estimates to be made of the true β-lactoglobulin composition of the samples. The nine samples could be placed into three distinct groups with estimated true β-lactoglobulin weight % of 70.9 ± 1.1, 62.0 ± 3.4 and 39.5 ± 4.9. Physico-chemical properties of the group of samples are reported elsewhere (Holt et al ., 1999).  相似文献   
87.
Lysophosphatidic acid (LPA) modulates prostaglandin (PG) synthesis via LPA receptor 3 (LPAR3) in the murine endometrium. The lack of functional LPAR3 in mice may lead to embryo mortality. In the present study, we examined the role of LPA in the bovine uterus. We confirmed that LPA is locally produced and released from the bovine endometrium. Moreover, there are enzymes involved in LPA synthesis (phospholipase (PL) D(2) and PLA2G1B) in the bovine endometrium during estrous cycle and early pregnancy. Expression of the receptor for LPA (LPAR1) was positively correlated with the expression of PGE(2) synthase (PGES) and negatively correlated with the expression of PGF(2alpha) synthase (aldose reductase with 20 alpha-hydroxysteroid dehydrogenase activity - PGFS) during early pregnancy. In vivo LPA induced P4 and PGE(2) secretion was inhibited by LPAR1 antagonist (Ki16425). The overall results indicate that LPA is locally produced and released from the bovine endometrium. Moreover, LPAR1 gene expression in the endometrium during the estrous cycle and early pregnancy indicates that LPA may play autocrine and/or paracrine roles in the bovine uterus. LPAR1 gene expression is positively correlated with the expression of the enzyme responsible for luteotropic PGE(2) production (PGES) in endometrium. In cow, LPA stimulates P4 and PGE(2) secretion. Thus, LPA in the bovine reproductive tract may indirectly (via endometrium) or directly support corpus luteum action via the increase of P4 synthesis and the increase of PGE(2)/PGF(2)(alpha) ratio. It suggests that LPA may serve as an important factor in the maintenance of early pregnancy in cow.  相似文献   
88.
The aim of this study was to determine the effects of extraction temperature on the colour as well as on the phenolic and parthenolide content of feverfew (Tanacetum parthenium) aqueous extracts, with a view to incorporating the extracts into a beverage. Results showed that extraction temperatures of 20–70 °C yielded dark-coloured extracts with low total phenol and a relatively high total tannin concentration. However, increasing the temperature to 75 °C produced lighter-coloured extracts with a significantly higher concentration of total phenols, lower total tannin fraction and maximum extraction of parthenolide. Higher extraction temperatures (80–100 °C) yielded suitably pale extracts rich in total phenols, but with progressively higher total tannin to non-tannin ratio and lower parthenolide content. The compromise temperature of 80 °C was found to provide extracts rich in parthenolide content, phenolic content and with a desirable colour, suitable for incorporation into a functional beverage.  相似文献   
89.
Tested R. L. Cromwell and P. R. Dokecki's (1968) theory that excessive response by schizophrenics to strong aspects of meaning of words and deficient response to weak aspects of meaning reflect an inability to disattend from strong aspects of meaning. 79 schizophrenics (40 off and 39 on drugs) were given 2 multiple-choice subtests asking for identification of aspects of meaning of words. Items of one subtest contained both a strong and a weak aspect of meaning and items of the other contained 2 weak aspects of meaning. The subtests had been matched on difficulty and reliability for 166 normal Ss. The schizophrenics identified weak aspects of meaning no worse when strong aspects of meaning were present than when they were absent. Findings argue against Cromwell and Dokecki's theory. Schizophrenics' excessive response to strong aspects of meaning on other tasks probably reflects a defect in screening and a resultant heightened expression of a normal bias. (15 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
90.
Administered a structured questionnaire to a cross-sectional sample of 1,034 mothers and a welfare sample of 1,000 mothers of 6-18 yr olds in New York City. One or more psychiatrists rated each of the children on a 5-point scale of total psychiatric impairment. Factor analysis of the questionnaire items of child behavior yielded 18 factors. The 7 factors with the highest correlations with total psychiatric impairment were selected for the inventory, each represented by 5 items. The correlation of the total 35-item score and total psychiatric impairment was .82. The screening inventory significantly and substantially improved over chance in assigning children to impairment statuses in the cross-sectional sample and the sample used for cross-validation, the welfare sample. Correlations of the total score with child's treatment status and a direct clinical examination rating were expectedly low. In its present form, the instrument is recommended only for comparisons of different child and adolescent groups. (25 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)  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