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   49257篇
  免费   2185篇
  国内免费   62篇
电工技术   347篇
综合类   39篇
化学工业   10707篇
金属工艺   936篇
机械仪表   966篇
建筑科学   1479篇
矿业工程   100篇
能源动力   1302篇
轻工业   8345篇
水利工程   443篇
石油天然气   216篇
武器工业   7篇
无线电   1908篇
一般工业技术   7567篇
冶金工业   10331篇
原子能技术   275篇
自动化技术   6536篇
  2024年   93篇
  2023年   396篇
  2022年   842篇
  2021年   1318篇
  2020年   934篇
  2019年   1118篇
  2018年   1636篇
  2017年   1631篇
  2016年   1740篇
  2015年   1366篇
  2014年   1743篇
  2013年   3457篇
  2012年   2721篇
  2011年   2929篇
  2010年   2292篇
  2009年   2189篇
  2008年   2014篇
  2007年   1785篇
  2006年   1387篇
  2005年   1226篇
  2004年   1125篇
  2003年   999篇
  2002年   939篇
  2001年   669篇
  2000年   666篇
  1999年   720篇
  1998年   3212篇
  1997年   2168篇
  1996年   1441篇
  1995年   846篇
  1994年   692篇
  1993年   766篇
  1992年   280篇
  1991年   289篇
  1990年   205篇
  1989年   230篇
  1988年   222篇
  1987年   204篇
  1986年   166篇
  1985年   217篇
  1984年   168篇
  1983年   134篇
  1982年   163篇
  1981年   180篇
  1980年   190篇
  1979年   98篇
  1978年   99篇
  1977年   353篇
  1976年   745篇
  1973年   85篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
Little is known about the long-term effects of Borrelia burgdorferi (Bb) infection in untreated patients with peripheral facial palsy. We investigated 12 patients with elevated serum Bb antibody levels, with a median follow-up time of 11 years, during which 3 of the 12 still exhibited intrathecal antibody production of antibodies against Bb flagellar antigen, and 2 of the 3 had normal serum Bb antibodies. Four of the 12 had elevated serum antibody titres at the late follow-up examination. Arthralgia, reported by 7 patients, was the single most common complaint. Four patients showed extensive oculomotor disturbances, which were not correlated to antibody titres or intrathecal antibody synthesis. In 1 of the patients with intrathecal Bb antibody production, most symptoms were eradicated by antibiotic treatment 6 years after the initial infection. We conclude that even several years after a Bb infection, intrathecal Bb antibody production can still occur in serum Bb IgG antibody negative patients with a history of facial palsy.  相似文献   
123.
BACKGROUND: We studied availability to mammography among Spanish women aged 40 to 70 years, variation in use of the mammography by autonomous community, and the situation and importance of breast cancer screening programs among other factors, in the access to mammography. SUBJECTS AND METHODS: A cross-sectional population survey was conducted in 1994 in a sample of 3,218 women. A questionnaire was used to collect data on the variable access (receipt of at least one mammogram in the last 2 years) as well as different access-related variables. Information on breast cancer screening programs was collected by contacting the responsible institutions. We considered that a program had total coverage if it included all the municipalities in the province and partial if it did not include all municipalities. RESULTS: Twenty-eight percent of women had performed a mammogram. This proportion varied among autonomous communities (AACC) from 11.5 to 73.8%. Breast cancer screening programs existed in 8 AACC. The multivariant analysis revealed an association between access to mammography and the existence of a screening program, especially when the later had total coverage (OR = 7.64; 95% CI = 5.24-11.10). An association was also found between access to mammography and physician-related factors, place of residence and attitudes of women toward mammography. CONCLUSIONS: Less than one third of women aged 40-70 have performed a mammography in the last 2 years, and this proportion varies among AACC. Gynecologist visits and the existence of breast cancer screening programs are fundamental factors in the access to mammography in Spain.  相似文献   
124.
Proton induced nuclear reactions on enriched 111Cd and 112Cd have been studied up to 30 MeV in the context of routine production of the medically used isotope 111In with low and medium energy cyclotrons. The excitation functions of 111Cd(p,n)111m,gIn and 112Cd(p,2n)111m,gIn as production reactions and 111Cd(p,2n)110mIn, 111Cd(p,2n)110In, 111Cd(p,3n)109ml,m2,In, 112Cd(p,3n)110mIn, 112Cd(p,3n)110gIn as competing processes have been measured using the activation method involving the stacked-foil technique. The deduced thick target yields are compared with those obtained experimentally.  相似文献   
125.
The authors describe the cases of two women suffering from AL amyloidosis with affection of the heart muscle in particular. In one of the patients there were also massive amyloid deposits in the lungs which is an exceptional finding. The observations confirm the adverse prognosis of amyloidosis, the rapid progression of the disease after the onset of symptoms of cardiac failure. The first cardial manifestation in both patients were palpitations. The first patient, but not the second one, had a myeloma confirmed on necropsy.  相似文献   
126.
An extruding wire knife was used to give adult male CFHB rats a minimally traumatic unilateral mechanical lesion of the medial forebrain bundle. In addition, some rats received bilateral intrastriatal injections of one of three fluorescent retrograde tracers either eight days before or eight days after the lesion. Injections made after the lesion revealed that about half of the animals had complete lesions of the nigrostriatal tract, while the other half were incompletely lesioned, the mean proportion of non-axotomized neurons being 23%. Over the 10 weeks following the lesions, the number of tyrosine hydroxylase-immunoreactive cells in the lesioned substantia nigra fell linearly, reaching a mean of 29% of that of the control substantia nigra. In the animals which were completely lesioned, neuronal survival at 10 weeks varied between 6 and 12%. That the disappearance of tyrosine hydroxylase-immunoreactive neurons was due to cell death rather than the loss of tyrosine hydroxylase itself was confirmed by labelling the cells with Fluoro Gold before axotomy; the tracer was seen in survival neurons, microglia and in a few involuted neurons which continued to be tyrosine hydroxylase-immunoreactive. This percentage of neurons surviving axotomy corresponds to the proportion of substantia nigra neurons which project to the contralateral striatum, and these neurons were in the region of the substantia nigra from which the contralateral projection originated. It is concluded that following mechanical transection of the nigrostriatal tract, all truly axotomized substantia nigra neurons die over a period of about 10 weeks.  相似文献   
127.
BACKGROUND: The efficacy of squamous cell carcinoma antigen (SCC-Ag) in laryngeal cancer to predict those patients who will relapse after primary treatment (surgery or radiotherapy) and its utility to detect relapses early and thereby increase salvage rates and cure were assessed. METHODS: Sixty healthy donors and 168 patients with laryngeal cancer were included in this prospective trial. Squamous cell carcinoma antigen was measured at diagnosis in all patients, 24 hours and 1 week after surgery in 113 patients and every 10 Gy of administered dose and 2 weeks after treatment in 49 patients primarily referred to radiotherapy. The marker was determined every 3-6 months during follow-up. All patients who relapsed had SCC-Ag studies before and after salvage treatment. RESULTS: The selected cut-off value was 1.5 ngr/ml (mean value in control group, 0.65 + 2 standard deviation [0.38]). Seventy-eight percent of patients with cancer had elevated SCC-Ag values at diagnosis. Squamous cell carcinoma antigen was statistically related to TNM categories (T, P < 0.04; N, P < 0.05; Stage, P < 0.01). Seventy-five percent of those patients with previously elevated pretreatment values normalized after treatment. Incomplete surgical resection (P < 0.0001) or persistence of the disease after radiotherapy (P < 0.01) were related to high posttreatment values. Squamous cell carcinoma antigen was elevated in 88% of the patients who relapsed. In 55% of the recurrences, SCC-Ag was elevated 3 months before pathologic confirmation of relapse. Salvage by surgery or radiotherapy was effective in 70% of the patients. Squamous cell carcinoma antigen posttreatment values were the most important factor in predicting disease free survival (DFS) (P < 0.0001) and overall survival (P < 0.03). CONCLUSIONS: Squamous cell carcinoma antigen is an excellent marker of residual disease after primary treatment that can lead to the addition of other therapeutic procedures (surgery and postoperative radiotherapy). The absence of posttreatment SCC-Ag is the best predictor of DFS, its presence detects recurrence in early stages, permitting salvage of an increased proportion of patients primarily referred for palliative treatment.  相似文献   
128.
129.
Different intravascular ultrasound (IVUS) systems vary in their image presentation. The purpose of this study was to compare four IVUS systems in vitro to determine the accuracy of tissue characterization of atherosclerotic plaque compared with histology. Ninety-eight plaque segments from 23 formalin-fixed human iliac arteries were imaged in saline at room temperature with four different IVUS systems. To assess the accuracy of IVUS in describing plaque, three types of analysis were performed: (1) the ability to identify the presence and extent of lumen or plaque boundary; (2) sensitivity, specificity, and interobserver variability of IVUS in qualitatively identifying plaque components compared with histology; and (3) quantification of calcification. The synthetic aperture device had a lower sensitivity in identifying lumen and plaque boundaries (87%, 38% respectively) compared with other machines (96%-100%, 95%-100%). All three mechanically rotating systems had fair to good sensitivities for identifying calcification (57%-73%) or lipid filled areas (50%-83%). The sensitivity of discriminating fibrous tissue from fatty areas was low (39%-52%). The synthetic aperture system had a significantly lower sensitivity for identifying all three tissue types (4%-21%). There was significant interobserver variability (kappa value = 0.47-0.68) as well as machine to machine variability (kappa value = 0.52) for tissue characterization. Calcified areas were underestimated by System 1 (p < .05) and System 4 (p < .01) because of weaker echo reflections or poor image quality. There are significant differences in image representation among these four IVUS systems in the diagnosis of tissue components of complex atherosclerotic plaque. These variabilities should be considered when interpreting studies performed with different machines.  相似文献   
130.
It is very difficult to have a clear and homogeneous idea of the embryo metabolism. In fact it may vary from one species to another and also according to the embryonic stage: i.e. before and after genomic activation. Basic compounds such as glucose may be toxic, but obviously, it is more the problem of the quantity introduced in the culture media and an unsuitable balance between the metabolites which may impair the embryonic development. At low concentration glucose is actively metabolised by embryos. High levels of amino acids are deleterious (due to release of ammonia), but they are necessary at low concentrations. Addition of serum or other biological fluids is generally useless. Further knowledge on embryo metabolism is necessary to avoid culture medium related delay or developmental blocks. Sequential media are at least partly the answer.  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