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   44727篇
  免费   2074篇
  国内免费   42篇
电工技术   333篇
综合类   31篇
化学工业   8336篇
金属工艺   782篇
机械仪表   850篇
建筑科学   1447篇
矿业工程   78篇
能源动力   948篇
轻工业   5604篇
水利工程   395篇
石油天然气   180篇
武器工业   4篇
无线电   2759篇
一般工业技术   7797篇
冶金工业   10884篇
原子能技术   253篇
自动化技术   6162篇
  2023年   360篇
  2022年   379篇
  2021年   940篇
  2020年   767篇
  2019年   720篇
  2018年   1554篇
  2017年   1578篇
  2016年   1642篇
  2015年   1160篇
  2014年   1474篇
  2013年   2916篇
  2012年   2519篇
  2011年   2378篇
  2010年   1870篇
  2009年   1700篇
  2008年   1954篇
  2007年   1724篇
  2006年   1349篇
  2005年   1097篇
  2004年   1049篇
  2003年   920篇
  2002年   835篇
  2001年   554篇
  2000年   549篇
  1999年   680篇
  1998年   3491篇
  1997年   2120篇
  1996年   1393篇
  1995年   850篇
  1994年   729篇
  1993年   710篇
  1992年   260篇
  1991年   261篇
  1990年   264篇
  1989年   252篇
  1988年   232篇
  1987年   208篇
  1986年   203篇
  1985年   288篇
  1984年   237篇
  1983年   182篇
  1982年   200篇
  1981年   240篇
  1980年   194篇
  1979年   170篇
  1978年   132篇
  1977年   231篇
  1976年   454篇
  1975年   123篇
  1973年   105篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
41.
42.
43.
BACKGROUND: Recently, investigators have reported the use of endoscopic myotomy in the treatment of esophageal achalasia. As with the open operation, considerable disagreement exists regarding the appropriate length of the myotomy and the need for a concomitant antireflux procedure. METHODS: Patients presenting with symptomatic achalasia between 1993 and 1997 were included in this prospective study. Preoperative studies included barium upper gastrointestinal study, endoscopy, and esophageal manometry. Laparoscopic myotomy was completed in all 20 patients; 18 had concomitant Toupet fundoplication. RESULTS: Operative times ranged from 95 to 345 minutes (mean 216). Blood loss ranged from 50 to 300 cc (mean 100 cc). There were 7 minor complications (5 mucosal injuries repaired laparoscopically, 1 bile leak and 1 splenic capsular tear). Nine patients began a liquid diet on the first day postoperatively; 19 were tolerating liquids by postoperative day 3. Hospital stay ranged from 2 to 20 days (mean 5). Eighteen patients had complete relief of dysphagia, with less than one reflux episode per month. One individual continues to have mild persistent solid food dysphagia. Another patient initially did well but subsequently developed mild recurrent dysphagia and reflux. One patient required laparoscopic take-down of the wrap because of recurrent dysphagia and now has no problems swallowing, but does complain of mild reflux. Two other patients also have mild reflux, 1 of whom did not undergo fundoplication. CONCLUSIONS: Laparoscopic Heller myotomy can be performed safely with excellent results in patients with achalasia. Adding a partial fundoplication appears to help control postoperative symptoms of reflux. This procedure should be considered the procedure of choice in patients with symptomatic esophageal achalasia.  相似文献   
44.
45.
46.
The plasma membrane (PM) H(+)-ATPase has been proposed to play important transport and regulatory roles in plant physiology, including its participation in auxin-induced acidification in coleoptile segments. This enzyme is encoded by a family of genes differing in tissue distribution, regulation, and expression level. A major expressed isoform of the maize PM H(+)-ATPase (MHA2) has been characterized. RNA gel blot analysis indicated that MHA2 is expressed in all maize organs, with highest levels being in the roots. In situ hybridization of sections from maize seedlings indicated enriched expression of MHA2 in stomatal guard cells, phloem cells, and root epidermal cells. MHA2 mRNA was induced threefold when nonvascular parts of the coleoptile segments were treated with auxin. This induction correlates with auxin-triggered proton extrusion by the same part of the segments. The PM H(+)-ATPase in the vascular bundies does not contribute significantly to auxin-induced acidification, is not regulated by auxin, and masks the auxin effect in extracts of whole coleoptile segments. We conclude that auxin-induced acidification in coleoptile segments most often occurs in the nonvascular tissue and is mediated, at least in part, by increased levels of MHA2.  相似文献   
47.
PURPOSE: To evaluate the effect of mitomycin C to an accelerated hyperfractionated radiation therapy. The aim was to test a very short schedule with/without mitomycin C (MMC) with conventional fractionation in histologically verified squamous cell carcinoma of the head and neck region. METHODS AND MATERIALS: From October 1990 to December 1996, 188 patients entered the trial. Tumors originated in the oral cavity in 54, oropharynx in 82, larynx in 20, and hypopharynx in 32 cases, respectively. Patients' stages were predominantly T3 and T4 (158/188, 84%) and most patients had lymph node metastases (144/188, 77%) at diagnosis. Only 22 patients were female, 166 were male, the median age of patients was 57 years (range 34 to 76 years). Patients were randomized to one of the following three treatment options: conventional fractionation (CF) consisting of 70 Gy in 35 fractions over 7 weeks (65 patients) or continuous hyperfractionated accelerated radiation therapy (V-CHART; 62 patients) or continuous hyperfractionated accelerated radiation therapy with 20 mg/sqm MMC on day 5 (V-CHART + MMC; 61 patients). By the accelerated regimens, the total dose of 55.3 Gy was delivered within 17 consecutive days, by 33 fractions. On day 1, a single dose of 2.5 Gy was given, from day 2 to 17 a dose of 1.65 Gy was delivered twice: the interfraction interval was 6 hours or more. RESULTS: Mucositis was very intense after accelerated therapy, most patients experiencing a grade III/IV reaction. The mucosal reaction did not differ whether MMC was administered or not. Patients treated by accelerated fractionation experienced a confluent mucosal reaction 12-14 days following start of therapy and recovered (no reaction) within 6 weeks. The skin reaction was not considered different in the three treatment groups. Those patients treated with additional chemotherapy experienced a grade III/IV hematologic toxicity in 12/61 patients. Initial complete response (CR) was recorded in 43% following CF, 58% after V-CHART, and 67% after V-CHART + MMC, respectively (p < 0.05). Actuarial survival (Kaplan-Meier) was significantly improved in the combined treated patients. Local tumor control was 28%, 32%, and 56% following CF, V-CHART, and V-CHART + MMC, respectively (p < 0.05). CONCLUSION: We conclude that our continuous hyperfractionated accelerated radiation therapy regimen is equal to conventional fractionation, suggesting that by shortening the overall treatment time from 7 weeks to 17 days a reduction in dose from 70 Gy to 55.3 Gy is possible, with maintenance of local tumor control rates. The administration of MMC to the accelerated regimen is tolerable and improves the outcome for patients significantly.  相似文献   
48.
49.
The mechanisms that control lipolysis in intra-abdominal fat cells from various primate species, the marmoset (Callithrix jacchus), the baboon (Papio papio), and the macaque (Macaca fascicularis), were compared to those of human intraabdominal fat cells. Selective beta 1- or beta 2-adrenoceptor agonists induced lipolysis in all species. Selective beta 3-agonists (BRL 37344, CL 316243, and SR 58611) acted as partial agonists in marmoset but were inefficient in other primates, including humans. alpha 2-Adrenoceptor number ([3H]RX 8210002 binding) equalized (baboon) or exceeded (other primates) beta 1/beta 2-adrenoceptors ([3H]CGP 12177 binding). Baboon fat cell membranes expressed similar amounts of coupled beta- and alpha 2-adrenoceptors. In all species, norepinephrine- or epinephrine-induced lipolysis did not reach the lipolytic effect of isoproterenol but their effects were enhanced after alpha 2-adrenoceptor blockade. N6-phenylisopropyladenosine (PIA) induced a full antilipolytic effect in baboon, macaque, and human adipocytes through adenosine receptors ([3H]DPCPX binding). Peptide YY (PYY) weakly inhibited lipolysis in baboon. Adrenocorticotropic hormone (ACTH) was inactive whereas parathyroid hormone (PTH) partially stimulated lipolysis in primates. Histamine was partially lipolytic in marmoset only. This study emphasizes the similarities of the mechanisms controlling the lipolysis in nonhuman primate and in human adipocytes and suggests that the baboon and the macaque should provide unique models for the study of the regulation of lipolysis.  相似文献   
50.
1,6-Hexanediamine (HDA) is a high production volume chemical which is used as an intermediate in the synthesis of paints, resins, inks, and textiles and as a corrosion inhibitor in lubricants. Two- and 13-week studies of the toxicity of the dihydrochloride salt of HDA (HDDC) were conducted in male and female Fischer 344/N rats and B6C3F1 mice using whole-body inhalation exposure. Both species were evaluated for histopathologic and reproductive effects, and rats were examined for clinical chemistry and hematologic changes. In the 2-week inhalation studies, animals were exposed to 10-800 mg HDDC/m3, 6 hr per day. All rats, all female mice, and two of five male mice in the high-exposure group died before the end of the study. Surviving mice in this group had a dose-dependent depression in body weight gain. Clinical signs were primarily related to upper respiratory tract irritation and included dyspnea and nasal discharge in both species. Treatment-related histopathologic lesions included inflammation and necrosis of the laryngeal epithelium of both species and the tracheal epithelium of mice, as well as focal inflammation and ulceration of the respiratory and olfactory nasal mucosa. In the 13-week inhalation studies, animals were exposed to HDDC at concentrations of 1.6-160 mg/m3 for 6 hr per day, 5 days per week. In addition to the base study groups, a supplemental group of rats at each exposure level was included to assess the effect of HDDC on reproduction. No treatment-related changes in organ weights or organ-to-body-weight ratios occurred in rats, and no treatment-related clinical signs or gross lesions were seen in either species. Chemical-related microscopic lesions were limited to the upper respiratory tract (larynx and nasal passages) in the two highest exposure groups and were similar in both species. These lesions included minimal to mild focal erosion, ulceration, inflammation, and hyperplasia of the laryngeal epithelium, in addition to degeneration of the olfactory and respiratory nasal epithelium. HDDC caused no significant changes in sperm morphology or vaginal cytology and no significant adverse effects on reproduction in rats or mice. Hematologic and clinical chemistry changes in rats were minor and sporadic and were not accompanied by related histologic findings. HDDC did not increase the frequency of micronucleated erythrocytes in mice.(ABSTRACT TRUNCATED AT 400 WORDS)  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