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   3533篇
  免费   5篇
综合类   2篇
化学工业   48篇
金属工艺   2篇
机械仪表   2篇
建筑科学   24篇
矿业工程   1篇
能源动力   4篇
轻工业   29篇
无线电   14篇
一般工业技术   25篇
冶金工业   3366篇
自动化技术   21篇
  2021年   1篇
  2020年   3篇
  2019年   1篇
  2018年   2篇
  2017年   7篇
  2016年   2篇
  2015年   3篇
  2014年   6篇
  2013年   24篇
  2012年   3篇
  2011年   7篇
  2010年   7篇
  2009年   6篇
  2008年   4篇
  2007年   3篇
  2006年   8篇
  2005年   8篇
  2004年   8篇
  2003年   11篇
  2002年   4篇
  2001年   4篇
  2000年   5篇
  1999年   106篇
  1998年   998篇
  1997年   602篇
  1996年   377篇
  1995年   241篇
  1994年   177篇
  1993年   212篇
  1992年   32篇
  1991年   67篇
  1990年   52篇
  1989年   67篇
  1988年   59篇
  1987年   44篇
  1986年   34篇
  1985年   34篇
  1983年   7篇
  1982年   21篇
  1981年   16篇
  1980年   19篇
  1979年   3篇
  1978年   7篇
  1977年   94篇
  1976年   130篇
  1975年   5篇
  1974年   2篇
  1968年   1篇
  1967年   1篇
  1955年   1篇
排序方式: 共有3538条查询结果,搜索用时 20 毫秒
61.
BACKGROUND: A retrospective study of 54 children was undertaken to define the clinical presentation and secretory patterns of adrenal tumors and to evaluate the outcome of surgical resection and medical therapy. PROCEDURES: Different factors were studied in univariate and multivariate analysis by using the Cox proportional hazard model. RESULTS: Median age at diagnosis was 4 years. Boys and girls were affected equally. The disease was revealed by virilization (61%) or by a palpable mass (39%) with a 0.1-5.5 year delay from initial symptoms. At initial examination, we found that 76% of children were virilized. Ninety-four percent of the tested tumors secreted androgens, which were associated with glucocorticoids in 36%. Adrenal tumors in children were smaller than in adults. Half of them measured less than 10 cm. There were recurrences in 40% of children. The survival rate at 5 years was 49%, 70% if resection was microscopically complete and 7% if not (P < 0.001). CONCLUSIONS: In children, rare adrenal tumors have different diagnostic and prognostic characteristics than in adults; however, recurrences remain frequent. The efficacy of chemotherapy, mainly o,p'-DDD (Mitotane), remains to be evaluated in comparative trials.  相似文献   
62.
63.
We describe here a new type of mitochondrial mutation (dum24; for dark uniparental minus inheritance) of the unicellular photosynthetic alga Chlamydomonas reinhardtii. The mutant fails to grow under heterotrophic conditions and displays reduced growth under both photoautotrophic and mixotrophic conditions. In reciprocal crosses between mutant and wild-type cells, the meiotic progeny only inherit the phenotype of the mating-type minus parent, indicating that the dum24 mutation exclusively affects the mitochondrial genome. Digestion with various restriction enzymes followed by DNA gel blot hybridizations with specific probes demonstrated that dum24 cells contain four types of altered mitochondrial genomes: deleted monomers lacking cob, nd4, and the 3' end of the nd5 gene; deleted monomers deprived of cob, nd4, nd5, and the 5' end of the cox1 coding sequence; and two types of dimers produced by end-to-end fusions between monomers similarly or differently deleted. Due to these mitochondrial DNA alterations, complex I activity, the cytochrome pathway of respiration, and presumably, the three phosphorylation sites associated with these enzyme activities are lacking in the mutant. The low respiratory rate of the dum24 cells results from the activities of rotenone-resistant NADH dehydrogenase, complex II, and alternative oxidase, with none of these enzymes being coupled to ATP production. To our knowledge, this type of mitochondrial mutation has never been described for photosynthetic organisms or more generally for obligate aerobes.  相似文献   
64.
C Obek  P Louis  F Civantos  MS Soloway 《Canadian Metallurgical Quarterly》1999,161(2):494-8; discussion 498-9
PURPOSE: Digital rectal examination is integral to staging prostate cancer. Ultrasound guided biopsy establishes the diagnosis, and it may provide useful information regarding disease grade and extent. Treatment decisions are largely based on information gained from digital rectal examination and biopsy but this information is only useful if it correlates with the radical prostatectomy specimen and prognosis. We correlated digital rectal examination and transrectal ultrasound guided biopsy results with a detailed analysis of the radical prostatectomy specimen. MATERIALS AND METHODS: The accuracy of an abnormal digital rectal examination for predicting the location and extent of cancer was assessed in 89 patients thought to have clinical stage T2 disease. We evaluated 155 patients with clinical stages T1c and T2 disease to correlate the location of positive biopsies with the tumor site in the prostate. Radical prostatectomy specimens were completely sectioned at 2 mm. intervals, and tumor extent and location were recorded. RESULTS: In 85 patients a unilateral lesion was suspicious on digital rectal examination, that is stage cT2. The final pathological review revealed cancer on the suspicious side in 82 cases (96%) with tumor confined to the same lobe in only 23 (27%), bilateral disease in 59 (69%) and tumor confined to the contralateral lobe in 3 (4%). In 4 patients with a palpable bilateral abnormality a bilateral lesion was confirmed on final pathological evaluation. Digital rectal examination demonstrated a 36 and 31% incidence of extracapsular tumor extension and positive surgical margins, respectively, on the clinically benign side. In 100 patients only unilateral biopsy was positive. The final pathological evaluation revealed cancer in the biopsy positive side in 95 cases (95%) with tumor confined to the ipsilateral lobe in only 26 (26%), bilateral disease in 69 (69%) and tumor confined to the contralateral lobe in 5 (5%). In 46 of the 55 patients (84%) with bilateral positive biopsies tumor involved both sides but the pathologist did not identify cancer in both lobes in 9 (16%). While 100 patients had a unilateral negative biopsy, analysis of the prostatectomy specimen revealed carcinoma in the benign lobe in 74 (74%). Moreover, extracapsular tumor extension and a positive surgical margin were observed on the biopsy negative side in 31% of the patients. The degree to which digital rectal examination and biopsy results confirmed the final pathological evaluation was assessed using the kappa statistic, which revealed only slight agreement with each factor. The correlation of digital rectal examination and biopsy results with the location of extracapsular extension and positive margins was evaluated by the Spearman coefficient of correlation, which indicated poor agreement. When patients with unilateral versus bilateral positive biopsy were compared with respect to prognostic parameters, the difference was statistically significant for initial serum prostate specific antigen, the percentage of surface involved by tumor, biopsy and final Gleason scores, and the incidence of extracapsular extension of tumor. CONCLUSIONS: Digital rectal examination and the interpretation of prostate biopsy are not accurate clinical tools for defining the location and extent of prostatic carcinoma. Bilateral positive biopsy may be useful as an adjunct to the current clinical staging system.  相似文献   
65.
A triplet pregnancy in a 23-year-old woman was terminated at 15 weeks of gestation because of her severe hypertension, lung edema, and secondary hyperthyroidism. The pregnancy consisted of a hydatidiform mole with a 46,XY karyotype and two fetuses each with 46,XX and a 46,XY karyotype. To determine the zygosity and genetic origin of the mole and fetuses, PCR- and computer-assisted genotyping were performed at 27 CA-repeat marker loci that were distributed evenly over the genome. As a result, genotypes of the three pregnancy products were distinct from each other, indicating that the triplets were trizygotic. The mole lacked any maternal alleles but inherited both of the paternal alleles and/or one paternal allele in duplicate. This, along with the XY sex chromosome constitution, indicated that the mole resulted from dispermic androgenesis. The mother developed a persistent trophoblastic tumor thereafter.  相似文献   
66.
67.
Dried blood spot (DBS) specimens were assessed as an alternative to plasma for human immunodeficiency virus type 1 (HIV-1) serotyping by V3 loop peptide enzyme immunoassay. Nested PCR capable of distinguishing HIV-1 subtypes B and E was used as the reference standard. Ninety-two percent of DBS samples were typeable as either HIV-1 subtype B or E. Serotype results with DBS and plasma were identical for 254 of 257 specimens. A simple DBS collection method provides a convenient alternative for conducting HIV-1 serotype surveillance while retaining sensitivity and specificity.  相似文献   
68.
In this study the desizing, scouring, bleaching and slack mercerising of cotton fabric by a one–step process at different sodium hydroxide concentrations, impregnation temperatures and curing times were examined. The results indicate that when the fabrics are mercerised at lower temperature, the strength retention and colour strength are enhanced. When the sodium hydroxide concentration is increased, these properties also increase. When cotton fabrics are mercerised for 3 min at 40C and cured for 30 s at 120C, their physical properties are similar to those obtained using a conventional two–step approach.  相似文献   
69.
INTRODUCTION: Global amnesia (AG) is a dysfunction of memory, in which there is a sudden incapacity to remember recent events and assimilate new information. The aetiology is currently under discussion. The main hypotheses under consideration are an ischemic mechanism and an epileptic mechanism. OBJECTIVE: To study the cerebral blood flow in patients with temporary global amnesia (AGT) or permanent global amnesia (AGP) using functional neuroimaging techniques. MATERIAL AND METHODS: Four patients were studied. Two had severe alterations of fixation memory which had been present for months. The other two had a sudden loss of recent memory, although space-person orientation and immediate memory remained normal. In all cases the cerebral blood flow (FSC), was assessed by single photon emission tomography (SPECT), using Tc-HMPAO as the radio tracer. RESULTS: We describe 4 patients, 2 men and 2 women aged between 42 and 64.2 with AGT and the others with AGP. On SPECT there was temporal hypoperfusion in all 4 patients. In 2 cases there was associated parietal hypoperfusion and in 1 case there was frontal hypoperfusion. CONCLUSIONS: These findings suggest that AGT and AGP are of vascular origin and that SPECT is a useful diagnostic technique in patients with AG.  相似文献   
70.
The western region of Saudi Arabia is an area with a high prevalence of urolithiasis. This study was designed to find the effect of climatic changes on the occurrence of urinary stone colic as well as the effect of Ramadan fasting and pilgrimage festival. The emergency room (E.R.) records at King Abdulaziz University Hospital in Jeddah were studied for 3 consecutive years. Males diagnosed as urinary colic during this period were recorded on monthly basis with correction for 30 days a month. Data were recorded before, during and after the fasting month as well as before and after the pilgrimage festival. The results showed a steady increase in urinary stone colic in the hot season with a maximum rate in the months of June, July and August. The mean number of males with stone colic in these months was 45.33, 44.19 and 45.16 respectively. The lowest number was in March (28.06) with a rate of 4.11 per 1000 patients. A strong correlation was found between urinary stone colic and both temperature and atmospheric pressure with a P value of < 0.0001. No significant correlation was observed with relative humidity and similarly no significant change in relation to Ramadan fasting or the pilgrimage festival. These results suggest that there is a clear stone season in this area corresponding to the hot summer months. No significant increase in urinary stone colic was observed in relationship to the fasting month of Ramadan or the pilgrimage festival.  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