首页 | 官方网站   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   6575篇
  免费   41篇
  国内免费   3篇
工业技术   6619篇
  2021年   18篇
  2019年   14篇
  2018年   24篇
  2017年   16篇
  2016年   26篇
  2015年   21篇
  2014年   26篇
  2013年   107篇
  2012年   45篇
  2011年   83篇
  2010年   39篇
  2009年   44篇
  2008年   69篇
  2007年   57篇
  2006年   60篇
  2005年   52篇
  2004年   38篇
  2003年   52篇
  2002年   46篇
  2001年   31篇
  2000年   39篇
  1999年   205篇
  1998年   1526篇
  1997年   849篇
  1996年   605篇
  1995年   314篇
  1994年   299篇
  1993年   312篇
  1992年   72篇
  1991年   97篇
  1990年   103篇
  1989年   122篇
  1988年   100篇
  1987年   88篇
  1986年   89篇
  1985年   82篇
  1984年   32篇
  1983年   29篇
  1982年   33篇
  1981年   44篇
  1980年   71篇
  1979年   20篇
  1978年   30篇
  1977年   150篇
  1976年   303篇
  1975年   21篇
  1974年   11篇
  1973年   21篇
  1971年   14篇
  1970年   10篇
排序方式: 共有6619条查询结果,搜索用时 15 毫秒
61.
To determine whether a 3-week hepatitis B (HB) vaccination could achieve protective immunity, 89 healthy non-immunized young adults received three doses of 20 micrograms each of HBs antigen (GenHevac B, Pasteur) and were randomly assigned to schedule A (n = 44): two doses at day 0, one dose at day 21; or schedule B (n = 45): one dose at days 0, 10 and 21. Seroprotection rates (anti-HBs > or = 10 mIU ml-1) for groups A and B respectively were: 23 and 40% at day 21; and 77 and 91% at day 82 (not significant). Anti-HBs geometric mean titres were higher in group B than in group A (p < 0.05) at days 21 (6.4 versus 3.8) and 82 (77.6 versus 33.5). One year after primary vaccination, the seroprotection rate remained as high as 90% in the vaccinees of group B; after boosting all vaccinees had protective levels of anti-HBs antibodies. Thus 3-week HB vaccination with GenHevac B allowed early and durable protective immunity.  相似文献   
62.
Each amino acid is represented by a vector of numerical measurements for the attributes of volume, area, hydrophilicity, polarity, hydrogen bonding, shape, and charge. Inter-residue distances are then calculated according to common metrics, and we introduce a new clustering objective function derived from information-theoretic considerations. The arguments of the function are the inter-object distances of the things to be clustered: in this case the amino acids. By means of approximating the solution of an integer programming problem, then, the residues are partitioned into clusters. The clusters obtained are compared with groups obtained in substitution/mutation studies and found to be similar. Thus, probably the strongest and most objective evidence to date is supplied for believing that physico-chemical properties account for the viability of substitutions and that the important similarities/differences are explained by a relatively small and simple set of properties.  相似文献   
63.
The objective of this study is to describe usual medical management and costs associated with recurrent respiratory infections in subjects with chronic obstructive bronchitis in France. A prospective survey was performed in Autumn 1994 on a national sample of private practice pulmonologists (N = 71). Two hundred forty-four patients, presenting at least one infection of the lower respiratory tract, were included. Bronchitis was the most frequent acute exacerbation observed (94%). Pneumonia concerned 9% of the patients. Biological tests, X-rays and pulmonary function tests were prescribed for, respectively, 59, 65 and 45% of the patients. Following the visit, 15 patients were hospitalized (6%). The direct medical cost per acute exacerbation was estimated 3,289 francs (1994 value) of which 60% were hospital-related. An average 10.4 day sick-leave was prescribed to 21% of patients in employment. For those patients, this sick-leave was associated to an extra-cost of 1,264-1,876 francs for Social Security and of 0-2,553 francs out of pocket per episode varying according to their Benefit Regimen.  相似文献   
64.
The purpose of this study was to determine the etiologic factors of denture stomatitis. Fifteen subjects with clinical evidence of localized simple denture stomatitis, fifteen subjects without clinical signs of denture stomatitis, and forty-five subjects with clinical evidence of generalized simple denture stomatitis were investigated clinically and mycologically. Subjects were evaluated according to age, sex, duration of denture usage, smoking habits, frequency of denture brushing, overnight denture wearing, pH level of saliva and degree of candidal colonization and candidal formation. Salivary samples and swabs were taken from the palate and the mucosal surfaces of the dentures investigated mycologically in order to identify the yeast colonies. Smears were taken from the palate and investigated in order to identify candidal formation. No statistically significant relationship was found between denture stomatitis and age, sex, duration of denture usage, frequency of denture brushing, overnight denture wearing or pH level of saliva. There was however, a statistically significant relationship between denture stomatitis and denture hygiene, smoking habits, candidal colonization and candidal formation.  相似文献   
65.
PURPOSE: We determined the impact of preexisting co-morbidities on survival of men with clinical stages T1b and T2NXM0 prostate cancer treated with surgery or radiation therapy. MATERIALS AND METHODS: A weighted co-morbidity score was determined for 276 consecutive men treated with surgery (138) or radiation therapy (138) at a Veterans Affairs medical center and was correlated with actuarial freedom from death due to co-morbid disease. RESULTS: After a median potential followup of 7.0 years 91 patients (33%) died of co-morbid disease and 20 (7%) died of cancer related causes. There were highly significant correlations between actuarial survival and weighted co-morbidity (p < 0.000001), and the 10-year actuarial survivals in men with no or severe co-morbidities were 66 and 9%, respectively. Associations between patient age and co-morbidity score were highly significant (p < 0.0001). The age adjusted risk of co-morbid death was 5.7 times greater in men with severe compared to no co-morbidities. There were also significant correlations between actuarial survival and weighted co-morbidity among patients treated with surgery (p = 0.02) and radiation therapy (p = 0.0002). Patient age and severity of co-morbidities were significantly greater among men treated with radiation therapy compared to surgery, and age adjusted risk of co-morbid death among men with a co-morbidity score of 1 was 3.8 times greater among men treated with radiation therapy (p = 0.025). CONCLUSIONS: Cancer related deaths are unusual within 5 to 10 years after surgery or radiation therapy in men with stages T1b and 2 prostate cancer. The risk of death during this interval is directly related to the severity of co-morbid conditions, which should be factored in an individual when assessing the advisability of therapeutic intervention. Since patient co-morbidities impact all cause survival, quantitative assessment of co-morbidities using validated instruments offers a method to control partially for the variabilities of health status among men receiving different treatments for localized prostate cancer.  相似文献   
66.
Clinical characteristics associated with bacillary angiomatosis and bacillary peliosis (BAP) in patients with human immunodeficiency virus (HIV) infection were evaluated in a case-control study; 42 case-patients and 84 controls were matched by clinical care institution. Case-patients presented with fever (temperature, > 37.8 degrees C; 93%), a median CD4 lymphocyte count of 21/mm3, cutaneous or subcutaneous vascular lesions (55%), lymphadenopathy (21%), and/or abdominal symptoms (24%). Many case-patients experienced long delays between medical evaluation and diagnosis of BAP (median, 4 weeks; range, 1 day to 24 months). Case-patients were more likely than controls to have fever, lymphadenopathy, hepatomegaly, splenomegaly, a low CD4 lymphocyte count, anemia, or an elevated serum level of alkaline phosphatase (AP) (P < .001). In multivariate analysis, a CD4 lymphocyte count of < 200/mm3 (matched odds ratio [OR], 9.9; P < .09), anemia reflected by a hematocrit value of < 0.36 (OR, 19.7; P < .04), and an elevated AP level of > or = 2.6 mukat/L (OR, 23.9; P < .05) remained associated with disease after therapy with zidovudine was controlled for. BAP should be considered an AIDS-defining opportunistic infection and should be included in the differential diagnosis for febrile, HIV-infected patients with cutaneous or osteolytic lesions, lymphadenopathy, abdominal symptoms, anemia, or an elevated serum level of AP.  相似文献   
67.
OBJECTIVE: To describe a simple injection apparatus and method for performing intracytoplasmic sperm injection in a clinical IVF program. DESIGN: A prospective clinical trial of intracytoplasmic sperm injection. SETTING: A private office-based fertility program. PATIENTS: Five couples undergoing IVF-ET with intracytoplasmic sperm injection as a treatment for male factor infertility. INTERVENTIONS: Intracytoplasmic sperm injection was performed at room temperature (23.5 to 24.5 degrees C) in a simple zwitterion-buffered medium. MAIN OUTCOME MEASURES: Fertilization rates, cleavage rates, clinical pregnancy rates, implantation rates. RESULTS: Intracytoplasmic sperm injection was performed on 44 fresh oocytes from five patients. Twenty-three oocytes fertilized (52.3%) and 22 zygotes cleaved (95.7%). Three of five patients became pregnant (60%), resulting in the live birth of one normal male infant, one continuing singleton pregnancy, and one continuing twin gestation (46XX, 46XY). The implantation rate was 23.5%. CONCLUSION: Intracytoplasmic sperm injection can be performed successfully in a simple medium at room temperature using commercially available microtools.  相似文献   
68.
The peripherin gene has three potential ATG translation initiation sites at positions 38, 56, and 290. The second ATG has been proposed to be the initiation codon used for translation of the protein, but there is no experimental evidence for this conjecture. We have isolated a full-length peripherin cDNA (designated as p61-11) from a rat brain cDNA library. Upon sequencing, we found that this cDNA contains a point mutation at the second potential translation initiation codon, which changes this ATG to ACG. When expressed in SW13 cl.2 vim- cells, a cell line without any detectable cytoplasmic intermediate filaments, the protein product of p61-11 cannot form a filamentous network and the major product is 45 kDa in size, which is most likely initiated from the third ATG. The protein product from the first ATG (57 kDa in size) of p61-11 is also detected albeit in smaller amounts. We introduced a frame-shift mutation upstream of the third ATG in p61-11 to create p61-11FS and showed that the third ATG is able to initiate translation efficiently even in the presence of the first ATG, and the 45 kDa protein leads to a diffuse nonfilamentous staining pattern in vim- cells confirming that the first ATG may not be the preferred translation initiation codon, since it cannot suppress a downstream ATG. We increased the translation efficiency from the first ATG of p61-11 by mutating the three nucleotides preceding this first ATG and thereby placing it in a better Kozak consensus sequence for translation initiation. The resulting 57 kDa protein is able to form a filamentous network in vim- cells. We corrected the mutation in the original p61-11 by polymerase chain reaction and generated two peripherin constructs: perM1M2 (which contains all three translation initiation codons) and per delta 1M2 (the first ATG is deleted, but the other two are present). When transfected, their protein products, about 57 kDa in size, form filamentous networks in the absence of other cytoplasmic intermediate filaments. Since there is no 45 kDa protein detected for these latter two constructs, it is reasonable to conclude that in the presence of the second ATG, little or no translation is initiated from the third ATG. Taken together, these results strongly suggest that the second ATG is the preferred translation initiation codon for the peripherin gene.  相似文献   
69.
70.
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-23

京公网安备 11010802026262号