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   7423篇
  免费   34篇
  国内免费   1篇
电工技术   12篇
综合类   1篇
化学工业   248篇
金属工艺   9篇
机械仪表   30篇
建筑科学   40篇
矿业工程   1篇
能源动力   22篇
轻工业   114篇
水利工程   4篇
石油天然气   4篇
无线电   85篇
一般工业技术   149篇
冶金工业   6599篇
原子能技术   9篇
自动化技术   131篇
  2023年   11篇
  2022年   8篇
  2021年   21篇
  2020年   15篇
  2019年   23篇
  2018年   31篇
  2017年   17篇
  2016年   17篇
  2015年   15篇
  2014年   26篇
  2013年   57篇
  2012年   27篇
  2011年   42篇
  2010年   35篇
  2009年   39篇
  2008年   40篇
  2007年   48篇
  2006年   32篇
  2005年   32篇
  2004年   27篇
  2003年   48篇
  2002年   21篇
  2001年   17篇
  2000年   24篇
  1999年   232篇
  1998年   2061篇
  1997年   1143篇
  1996年   814篇
  1995年   428篇
  1994年   360篇
  1993年   395篇
  1992年   52篇
  1991年   102篇
  1990年   60篇
  1989年   79篇
  1988年   81篇
  1987年   74篇
  1986年   76篇
  1985年   70篇
  1984年   10篇
  1983年   18篇
  1982年   32篇
  1981年   51篇
  1980年   69篇
  1979年   12篇
  1978年   21篇
  1977年   152篇
  1976年   336篇
  1975年   14篇
  1973年   11篇
排序方式: 共有7458条查询结果,搜索用时 15 毫秒
21.
Near-haploidy is a rare cytogenetic finding in childhood acute lymphoblastic leukaemia (ALL) and is associated with a poor prognosis. A second hyperdiploid line, occurring presumably by endoreduplication of the near-haploid stemline, is often observed. We present a case of common ALL in relapse characterized morphologically by a dual population of small and large lymphoblasts. Cytogenetic analysis supplemented with fluorescence in-situ hybridization (FISH) studies localized near-haploidy and hyperdiploidy to the small and large blast population respectively. DNA ploidy determination confirmed two abnormal clones with near-haploidy as the predominant one. A novel t(9;12)(q11;q13) was present in the near-haploid clone and was duplicated in the hyperdiploid clone. This finding identified cells bearing near-haploidy to be the clonogenic population following malignant transformation and confirmed endoreduplication as the mechanism for the presence of associated hyperdiploidy.  相似文献   
22.
The aim of this study was to document plasma retinol status and nocturnal vision in ten eutrophic adolescents with cystic fibrosis (CF) receiving daily retinol supplementation. Plasma retinol, alpha and beta carotenes and retinol binding protein were measured in ten clinically stable CF patients (mean age: 14.3 years; Shwachman score: 80-100). Nocturnal vision evaluation was performed with a Beyne optometer. Plasma retinol (mean 0.42 +/- 0.16 mg/l), alpha carotene and beta carotene levels were below the lower limit of normal in all but one patient. Five out of ten patients with normal standard opthalmological examination presented a poor (n = 3 patients) or a pathological (n = 2) dark adaptation test. These two patients showed a dramatic increase in nocturnal vision after 1 year of adapted retinol supplementation. CONCLUSION: Low vitamin A levels occur frequently in clinically stable, eutrophic and retinol supplemented CF adolescents. Since vitamin A deficiency is associated with poor nocturnal vision and since this pattern can be reversed by adapted retinol supplementation, we recommend monitoring plasma vitamin A levels in CF patients and evaluation of dark adaptation in retinol deficient patients.  相似文献   
23.
The purpose of this study was to examine the factors which affect the level of fatigue among patients participating in clinical trials in which this symptom had been assessed with the EORTC QLQ-C30. Data were assembled from 2390 patients in ten clinical trials in which the QLQ-C30 had been used to assess baseline and on-study quality of life. The relationship between the level of fatigue reported by the patients on the fatigue scale of this questionnaire and patient and disease characteristics was assessed in univariate and multivariate cross-sectional analyses. In addition, changes in fatigue scores were compared in a longitudinal analysis among patients on two arms of an anti-emetic trial whose emesis control was markedly different. Baseline fatigue levels differed substantially among patients taking part in the different trials. Factors associated with greater fatigue severity on univariate analysis included: female gender, presence of metastatic disease, and poorer performance status. In addition, on multivariate analyses the oldest patients were found to have less fatigue, as were patients with breast cancer, while patients with ovarian and lung cancer experienced greater fatigue. Patients on the arm of the anti-emetic trial in which emesis was better controlled showed significantly less increase in fatigue after receiving chemotherapy. The fatigue scale of the QLQ-C30 appears to provide a useful approach to assessing this important symptom. The relationships found between fatigue and patient and disease characteristics need further exploration as does the degree to which the QLQ-C30 fully captures this dimension of quality of life.  相似文献   
24.
Male strain A/J mice were exposed for 6 h a day, 5 days a week to environmental tobacco smoke (ETS) generated from Kentucky 1R4F reference cigarettes. Chamber concentrations were 87 mg/m3 of total suspended particulate matter (TSP), 246 p.p.m. of CO and 16 mg/m3 of nicotine. After 5 months, 33% of the ETS exposed and 11% of the control animals had one or several lung tumors; the difference was statistically not significant. A second group of animals exposed for 5 months to ETS was allowed to recover for another 4 months in filtered air. When they were killed, 85% of the ETS animals had lung tumors (average number per lung: 1.4 +/- 0.2), whereas in the control group 38% had lung tumors (average number of lung tumors in all animals 0.5 +/- 0.2). The differences in tumor incidence and multiplicity were statistically significant. More than 80% of all tumors were adenomas, the rest adenocarcinomas. When animals were pretreated with a carcinogen, lung tumor multiplicity was lower in the ETS exposed animals after 5 months compared with controls injected with a carcinogen and kept in air. However, after an additional 4 month recovery period in air, lung tumor multiplicities were the same in ETS plus carcinogen exposed mice as in carcinogen-treated air-exposed controls. Histopathologic and morphometric analysis of the lung tissue failed to reveal any differences between ETS exposed and control animals. However, immediately after ETS exposure, immunohistochemistry revealed increased staining for CYP1A1 in airway epithelia and lung parenchyma; following recovery in air, the staining disappeared again. Analysis of cell kinetics showed an initial burst of increased DNA synthesis in the epithelial cells of the airways and a smaller early positive response in the parenchyma. Feeding of butylated hydroxytoluene during ETS exposure did not modulate lung tumor development. It was concluded that ETS is a pulmonary carcinogen in strain A/J mice.  相似文献   
25.
Gonadotropin-releasing hormone (GnRH) molecular variants in the brain and pituitary gland of pejerrey, Odontesthes bonariensis (Atheriniformes), were characterized by gradient reverse phase high performance liquid chromatography (RP-HPLC). Eluted fractions were tested in radioimmunoassays with different antisera. The results show that the brain extract contains three forms of GnRH: one is immunologically and chromatographically similar to cIIGnRH (chicken II), and another is similar to sGnRH (salmon). A third GnRH appears to be chromatographic and immunologically different from the nine other known forms of the vertebrate hormone. This is the only variant present in the pituitary gland.  相似文献   
26.
A cryptic multidrug resistance (MDR) system in Stenotrophomonas maltophilia, the expression of which is selectable by tetracycline, is described. Tetracycline resistance was the consequence of active efflux of the antibiotic, and it was associated with resistance to quinolones and chloramphenicol, but not to aminoglycosides or beta-lactam antibiotics. MDR is linked to the expression of an outer membrane protein (OMP54) both in a model system and in multidrug-resistant clinical isolates.  相似文献   
27.
28.
The spectrum of photoaging in human skin shows a recognizable progression from subtle to profound, reflecting the various anatomic and structural changes that appear with aging skin. Qualification and quantification of these changes on both the macroscopic and microscopic levels enable the physician to select and evaluate appropriate therapeutic approaches to the problem of chronic solar damage. Comparative studies remain to be done that will further refine our abilities to match the right solutions to the recognized problems.  相似文献   
29.
The main ionotropic receptors (GABAA, NMDA and AMPA) display a sequential participation in neuronal excitation in the neonatal hippocampus. GABA, the principal inhibitory transmitter in the adult CNS, acts as an excitatory transmitter in early postnatal stage. Glutamatergic synaptic transmission is first purely NMDA-receptor based and lacks functional AMPA receptors. Therefore, initially glutamatergic synapses are 'silent' at resting membrane potential, NMDA channels being blocked by Mg2+. However, when GABA and glutamatergic synapses are coactivated during the physiological patterns of activity, GABAA receptors can facilitate the activation of NMDA receptors, playing the role conferred to AMPA receptors later on in development. Determining the mechanisms underlying the development of this 'ménage à trois' will shed light not only on the wide range of trophic roles of glutamate and GABA in the developing brain, but also on the significance of the transition from neonatal to adult forms of plasticity.  相似文献   
30.
Although both B7 and its counter-receptor CD28 are expressed in the thymus, the role of B7 in thymic selection is not clear. We investigated the role of B7 in intrathymic deletion of antigen-specific T cells using a TCR transgenic model specific for antigen ovalbumin (OVA) and H-2Ad. Intraperitoneal injection of OVA induced apoptosis of thymocytes and drastic reduction of thymocyte numbers. This was significantly inhibited by co-injection of CTLA-4-Ig which blocks B7 co-stimulation. Deletion of T cells in the thymus following i.p. injection of OVA was associated with T cell pre-activation as demonstrated by T cell proliferation and cytokine production. Injection of CTLA-4-Ig blocked all these activation events and rescued thymocytes from activation-induced cell death. These results demonstrate that B7 is required for the activation-induced cell death of MHC class II-restricted thymocytes in vivo.  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