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   4322篇
  免费   53篇
  国内免费   11篇
电工技术   44篇
综合类   2篇
化学工业   402篇
金属工艺   82篇
机械仪表   168篇
建筑科学   56篇
矿业工程   3篇
能源动力   112篇
轻工业   83篇
水利工程   26篇
石油天然气   8篇
无线电   511篇
一般工业技术   485篇
冶金工业   1860篇
原子能技术   19篇
自动化技术   525篇
  2023年   26篇
  2022年   57篇
  2021年   57篇
  2020年   50篇
  2019年   53篇
  2018年   66篇
  2017年   57篇
  2016年   81篇
  2015年   54篇
  2014年   65篇
  2013年   171篇
  2012年   88篇
  2011年   129篇
  2010年   106篇
  2009年   99篇
  2008年   102篇
  2007年   96篇
  2006年   90篇
  2005年   80篇
  2004年   59篇
  2003年   60篇
  2002年   66篇
  2001年   55篇
  2000年   63篇
  1999年   107篇
  1998年   632篇
  1997年   344篇
  1996年   255篇
  1995年   139篇
  1994年   125篇
  1993年   155篇
  1992年   60篇
  1991年   54篇
  1990年   54篇
  1989年   52篇
  1988年   48篇
  1987年   40篇
  1986年   40篇
  1985年   46篇
  1984年   23篇
  1983年   34篇
  1982年   36篇
  1981年   37篇
  1980年   36篇
  1979年   34篇
  1978年   17篇
  1977年   53篇
  1976年   74篇
  1975年   14篇
  1973年   11篇
排序方式: 共有4386条查询结果,搜索用时 0 毫秒
171.
172.
The course of two neonates and one 4-month-old infant with laboratory and clinical evidence of central hypothyroidism is described. All three presented with failure to thrive and improved after L-T4 therapy. Early recognition and treatment of newborns and infants with central hypothyroidism is important to maximize the potential for growth and development. Two of the three infants have been documented to have transient central hypothyroidism of hypothalamic origin, not previously reported.  相似文献   
173.
In 12 depressed inpatients referred for bilateral electroconvulsive therapy (ECT), each patient was titrated at the first treatment session by using an ascending method-of-limits procedure with a step-wise increase in pulse frequency (frequency titration) or train duration (duration titration). At the second treatment session, seizure threshold was redetermined by using the method (frequency or duration titration) not used at the first treatment. Frequency or duration was maintained at the lowest level when the other parameter was titrated. Seizure threshold was significantly lower with duration titration (mean, 90 mC; SD, 27.3) than frequency titration (mean, 114 mC; SD, 35.6; p = 0.03). On average, patients in the duration-titration group required 1.2 (SD, 0.6) subconvulsive stimulations before a seizure was elicited, and patients in the frequency-titration group required 1.7 (SD, 0.9) subconvulsive stimulations before a seizure was elicited, a nonsignificant difference. These findings suggest that to elicit a seizure during ECT, increasing train duration may be slightly more efficient than increasing frequency. Basic and other clinical research findings indicate that increasing pulse width may be an inefficient way to elicit a seizure. Therefore the following sequence in the determination of seizure threshold is worth considering when using dose-titration or related techniques: the train duration should be increased first before increasing pulse frequency, and the decision to increase pulse width should be reserved for patients who do not seize at the maximal duration and frequency settings. Further empiric research is needed to establish the utility of this approach.  相似文献   
174.
Our previous studies in iron-loaded rat heart cells showed that in vitro iron loading results in peroxidative injury, manifested in a marked decrease in rate and amplitude of heart cell contractility and rhythmicity, which is correctable by treatment with deferoxamine (DF). In the present studies we explored the role of mitochondrial damage in myocardial iron toxicity. Iron loading by 24-hour incubation with 0.36 mmol/L ferric ammonium citrate resulted in a decrease in the activity of nicotinamide adenine dinucleotide (NADH)-cytochrome c oxidoreductase (complex I+III) to 35.3%+/-11.2% of the value in untreated controls; of succinate-cytochrome c oxidoreductase (complex II+III) to 57.4%+/-3.1%; and of succinate dehydrogenase to 63.5%+/-12.6% (p < 0.001 in all cases). The decrease in activity of other mitochondrial enzymes, including NADH-ferricyanide reductase, succinate ubiquinone oxidoreductase (complex II), cytochrome c oxidase (complex IV), and ubiquinol cytochrome c oxidoreductase (complex III), was less impressive and ranged from 71.5%+/-15.8% to 91.5%+/-14.6% of controls. That the observed loss of respiratory enzyme activity was a specific effect of iron toxicity was clearly demonstrated by the complete restoration of enzyme activities by in vitro iron chelation therapy. Sequential treatment with iron and doxorubicin caused a loss of complex I+III and complex II+III activity that was greater than that seen with either agent alone but was only partially correctable by DF treatment. Alterations in cellular adenosine triphosphate measurements paralleled very closely the changes observed in respiratory complex activity. These findings demonstrate for the first time the impairment of cardiac mitochondrial respiratory enzyme activity caused by iron loading at conditions formerly shown to produce severe abnormalities in contractility and rhythmicity.  相似文献   
175.
A unique feature of p21 that distinguishes it from the other cyclin-dependent kinase (CDK) inhibitors is its ability to associate with the proliferating cell nuclear antigen (PCNA), an auxiliary factor for DNA polymerases delta and epsilon. While it is now well established that inhibition of cyclin/CDK complexes by p21 can result in G1 cell cycle arrest, the consequences of p21/PCNA interaction on cell cycle progression have not yet been determined. Here, we show, using a tetracycline-regulated system, that expression of wild-type p21 in p53-deficient DLD1 human colon cancer cells inhibits DNA synthesis and causes G1 and G2 cell cycle arrest. Similar effects are observed in cells expressing p21CDK-, a mutant impaired in the interaction with CDKs, but not in cells expressing p21PCNA-, a mutant deficient for the interaction with PCNA. Analysis of cells treated with a p21-derived PCNA-binding peptide provides additional evidence that the growth inhibitory effects of p21 and p21CDK result from their ability to bind to PCNA. Our results suggest that p21 might inhibit cell cycle progression by two independent mechanisms, inhibition of cyclin/CDK complexes, and inhibition of PCNA function resulting in both G1 and G2 arrest.  相似文献   
176.
The fusion (F) protein of bovine respiratory syncytial virus (BRSV) was expressed by using a baculovirus vector. Antigenicity was tested by immunofluorescence analysis with F-specific monoclonal and polyclonal antibodies. Antibodies to recombinant F protein raised in a rabbit neutralized BRSV and human respiratory syncytial virus infectivity when tested in a plaque reduction assay. The recombinant F protein was evaluated as a source of antigen in an enzyme-linked immunosorbent assay (ELISA), and this ELISA was compared with the virus neutralization (VN) test for detecting BRSV antibodies in 10 consecutive serum samples from four calves vaccinated with a live modified BRSV vaccine and from two nonvaccinated control calves. The ELISA with the baculovirus-expressed F protein as an antigen compared favorably with the VN test and is a rapid, sensitive, and specific method for detecting serum antibodies to BRSV.  相似文献   
177.
178.
OBJECTIVE: This study aimed to examine the characteristics of intraretinal changes associated with macular holes and epiretinal membranes by scanning retinal thickness analysis. STUDY DESIGN: The study design was a nonconsecutive case series. PATIENTS: Fifty-six eyes of patients who had either a suspected or clinically diagnosed macular hole or epiretinal membrane were recruited. INTERVENTIONS: A commercial prototype of the scanning retinal thickness analyzer (RTA) was used. It projected a laser slit beam onto the retina and scanned it, in 200 or 400 msec, across a 2- x 2-mm area, yielding multiple optical cross sections that were recorded digitally. RESULTS: Epiretinal membranes were detected, and sites of attachment could be identified. Full-thickness holes corresponded to intraretinal cavities in which the inner retinal surface was broken, usually at the center. The majority of eyes with full-thickness macular holes showed increased retinal thickness surrounding the hole. The so-called "cuff of subretinal fluid," however, often was not present by retinal thickness analysis, despite clinical diagnosis to the contrary, even though retinal thickness analysis is capable of detecting such fluid. In 20 (42%) of 47 eyes diagnosed or suspected of having macular holes, scanning retinal thickness analysis showed findings different from those reported by retinal specialists. CONCLUSIONS: Examination of macular holes with the scanning RTA provides useful information in the diagnosis of macular holes in addition to that obtained through conventional techniques. The findings support the idea that many macular holes develop in association with intraretinal cystic changes. The precise chronology of the events remains to be determined.  相似文献   
179.
We currently recommend excision of adrenal incidentalomas > or = 4 cm in size and all hormonally active tumors. The optimal management and follow-up of smaller nonfunctioning tumors are controversial. The aim of this study was to determine the clinical outcome of a well defined population of patients with incidentalomas followed without operative intervention. The study group comprised 231 patients, identified from the records of abdominal or thoracic computed tomographic (CT) scans performed between 1985 and 1989. The primary outcome variable analyzed was survival. Follow-up was obtained by office records, telephone contact, or letter. There were 101 male and 130 female patients with a mean age at diagnosis of 64 years (range 5-86 years). Most adrenal tumors were unilateral (right 113; left 98); 20 were bilateral. Mean tumor size was 2 cm (range 1-6 cm). In nine (4%) patients the tumor was > or = 4 cm. Follow-up [mean 7 years; range 1 month (patient died) to 11.7 years] was complete in 224 (97%) patients. Ninety-one (39%) patients had one or more additional CT scans performed during the follow-up period, with only four patients demonstrating a > 1 cm increase in the size of the adrenal mass. Surgical excision of these four lesions identified benign pathology. Eighty-one (35%) patients died of conditions unrelated to adrenal pathology. No patient developed subsequent adrenal hyperfunction or adrenal malignancy. Within the context of our guidelines, conservative management of adrenal incidentalomas considered benign or nonfunctioning at diagnosis is appropriate. Additional information provided by repeat CT scanning appears to confer limited benefit. This study does not support laparoscopic removal of small, nonfunctional adrenal tumors, as has been suggested.  相似文献   
180.
This paper calculates the mean duration of the postpartum amenorrhoea (PPA) and examines its demographic, and socioeconomic correlates in rural north India, using data collected through 'retrospective' (last but one child) as well as 'current status' (last child) reporting of the duration of PPA. The mean duration of PPA was higher in the current status than in the retrospective data; the difference being statistically significant. However, for the same mothers who gave PPA information in both the data sets, the difference in mean duration of PPA was not statistically significant. The correlates were identical in both the data sets. The current status data were more complete in terms of the coverage, and perhaps less distorted by reporting errors caused by recall lapse. A positive relationship of the mean duration of PPA was found with longer breast-feeding, higher parity and age of mother at the birth of the child, and the survival status of the child. An inverse relationship was found with higher education of a woman, higher education of her husband and higher socioeconomic status of her household, these variables possibly acting as proxies for women's better nutritional status.  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