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   8552篇
  免费   562篇
  国内免费   27篇
电工技术   132篇
综合类   6篇
化学工业   1766篇
金属工艺   322篇
机械仪表   585篇
建筑科学   168篇
矿业工程   1篇
能源动力   370篇
轻工业   627篇
水利工程   30篇
石油天然气   11篇
无线电   1742篇
一般工业技术   1825篇
冶金工业   622篇
原子能技术   149篇
自动化技术   785篇
  2023年   103篇
  2022年   149篇
  2021年   274篇
  2020年   211篇
  2019年   181篇
  2018年   240篇
  2017年   267篇
  2016年   312篇
  2015年   223篇
  2014年   336篇
  2013年   541篇
  2012年   550篇
  2011年   668篇
  2010年   481篇
  2009年   500篇
  2008年   472篇
  2007年   369篇
  2006年   331篇
  2005年   303篇
  2004年   276篇
  2003年   236篇
  2002年   251篇
  2001年   211篇
  2000年   203篇
  1999年   197篇
  1998年   307篇
  1997年   182篇
  1996年   140篇
  1995年   94篇
  1994年   99篇
  1993年   79篇
  1992年   43篇
  1991年   55篇
  1990年   38篇
  1989年   32篇
  1988年   28篇
  1987年   29篇
  1986年   17篇
  1985年   13篇
  1984年   12篇
  1983年   14篇
  1982年   10篇
  1981年   14篇
  1980年   8篇
  1979年   6篇
  1977年   10篇
  1976年   10篇
  1975年   3篇
  1974年   5篇
  1971年   2篇
排序方式: 共有9141条查询结果,搜索用时 15 毫秒
961.
Although it has been known that pomegranate (Punica granatum L.) contains several steroid hormones, concrete experimental proofs about that have not been published until now. In order to identify and evaluate the contents of steroid hormones including estrone in pomegranate, we analyzed pomegranate seed, fruit juice and commercial preparations. We developed a reproducible and sensitive method for separation and identification of steroid hormones in pomegranate samples using both high performance liquid chromatography (HPLC)–PDA and gas chromatography (GC)–MS. In case of HPLC, an isocratic elution method using 35% aqueous acetonitrile solution at 1.0 ml/min with photodiode-array (PDA) detection at 225 nm and 254 nm was found to optimally separate and identify the steroid hormones from the pomegranate samples with a run time of less than 30 min. The pomegranate samples were comparatively analyzed to the HPLC results by GC/FID or GC/MS detection on a HP-1 (30 m length, 0.32 mm I.D.) with helium as carrier gas under the oven temperature control as follows: start 220 °C for 5 min, raising 5 °C per min, final 280 °C for 10 min. The HPLC and GC methods were successfully applied to the identification of steroid hormones in pomegranate samples. Our results suggested that there were no steroid estrogens including estrone, estradiol and testosterone in pomegranate seed, fruit juice and preparations. Consequently, we assumed that the previously reported analysts of pomegranate were misunderstood their analytical results according to either the estrogen-like effects or similarity of peak retention time and Rf values in experiments.  相似文献   
962.
In order to dispose of high-level radioactive waste (HLW) safely in geological formations, it is necessary to assess the feasibility, safety, appropriateness, and stability of the disposal concept at an underground research laboratory (URL) constructed in the same geological formation as the host rock. In this study, minimum requirements and the conceptual design for an efficient construction of a small scale URL were derived based on a literature review. To confirm the validity of the conceptual design for construction at KAERI, a geological survey including a seismic refraction survey, electronic resistivity survey, borehole drilling, and in situ and laboratory tests were carried out. The mechanical stability of the URL was investigated with a consideration of the surface topography, tunnel geometry, tunnel slope, sequential excavation, in situ stress ratio, erosion effect, and rock property variation along the tunnel using the three-dimensional code, FLAC3D. From the study, it was possible to conclude that the small scale URL will be effectively constructed in a granite mass at KAERI and will satisfy the minimum requirements.  相似文献   
963.
The impact of stress induced by biaxial mechanical bending on multiple-gate FET (MuGFET) performance is studied. For relatively low levels of bending-induced surface strain (/spl sim/ 0.1%), significant enhancements in the driving current can be achieved and maintained with gate-length scaling. This makes package strain a potentially attractive approach to enhancing MuGFET-based CMOS performance at low cost. For bending-induced strain, the enhancements in electron mobility and [110] hole mobility are well predicted by the piezoresistance model using the coefficients for bulk-Si, but the impact of stress on (100) hole mobility is more complex.  相似文献   
964.
Experimental results for an optical free-space high-speed link using direct modulated mid-infrared (λ = 8.1 μm) quantum cascade lasers are presented. A total of 800 digitally encoded multimedia channels were transmitted. The reliability of the system against weather influence (fog) was experimentally compared to that of a near-infrared (λ = 0.85 μm) link  相似文献   
965.
Bowenoid papulosis (BP) of the genitalia, characterized by the histological findings of a squamous cell carcinoma, follows a largely benign clinical course. The detection of oncogenic human papilloma viruses (HPV) from BP points to an aetiological role of these viral infections. A 47-year-old man with multiple genital skin lesions was seen over a 10-year period with the diagnosis of BP. Recently, he attended again with a recurrent genital tumour that was diagnosed as squamous cell carcinoma. His genital lesions progressed and became polymorphic in appearance, from a wart-like tumour to a reddish invasive plaque. To screen for the presence of different HPV sequences from different skin lesions and to correlate each HPV type with distinct clinical manifestations, polymerase chain reaction and single-strand conformational polymorphism (PCR-SSCP) were performed. PCR-SSCP revealed the presence of several types of HPV from different genital lesions. Sequencing results disclosed that he had a mixed infection of HPV6b, HPV16, HPV18 and HPV33, respectively. Interestingly, the clinical findings were fairly well correlated with the oncogenic potential of HPV found from each lesion.  相似文献   
966.
The behaviors of concentric-double inclusions dispersed in continuous media are investigated theoretically to find some possibilities of improving toughness of composite materials by dispersing double-inc’usions instead of single-inclusions. The general solutions of the Stokes equation, expressed in terms of the spherical harmonics, are used for analyzing the problems that are related to the concentric-double inclusions. From the analysis, it is found that the pressure and stress fields inside and outside the inclusion can be modified by changing the modulus ratios and the thickness of shell layer. Especially, the positions of the minimum pressure points and the maximum stress points turn out to be controllable with some degree of freedom.  相似文献   
967.
968.
BACKGROUND: There have been many studies concerning pathological changes in bronchial mucosa from asthmatics; however, few studies has been carried out to evaluate pathological changes according to the severity of asthma. OBJECTIVE: This study was designed to evaluate the cellular components in bronchoalveolar lavage fluid (BALF) and histological abnormalities in asthmatics according to the severity of asthma. METHODS: Bronchoalveolar lavages, bronchoscopic biopsies and ultrastructural examinations were performed in 13 asthmatics and 11 (BAL) or four (biopsies) non-asthmatic controls. The proportions of epithelial cells and correlations with PC20Meth which reflects bronchial hyperresponsiveness. Light microscopic examination revealed loss of epithelium, inflammatory cell infiltrations and thickening of the basement membrane which also showed significant correlation with PC20Meth. Hypertrophy of airway smooth muscles and hyperplasia of mucous glands were prominent in asthmatics but there was no difference according to the severity of asthma. Ultrastructural examination revealed that basement membrane thickening on light microscopic examination is due to the increased subepithelial collagen deposition with normal thickness of basal lamina. CONCLUSION: These data suggest that loss of epithelial cells, infiltration of inflammatory cells, especially eosinophils, and increased deposition of subepithelial collagen play major roles in determining the severity of asthma and non-specific bronchial hyperresponsiveness.  相似文献   
969.
Ketones react with primary amines in dioxane at 180 °C in the presence of a catalytic amount of a ruthenium catalyst to give the corresponding α-alkylated ketones in moderate to good yields. With alkyl(methyl) ketones, the alkylation takes place exclusively at less-hindered methyl position over α-methylene.  相似文献   
970.
This paper presents the evaluation of LPG (Liquefied petroleum gas) regulators for home use. For the evaluation, several properties of the regulators were experimentally analyzed, such as the operation of safety device, the adjusting and lock-up pressure, the adjusting spring and the diaphragm, with respect to the used time of the regulators. Experimental results showed that the initial operation performances of regulators were degraded with increase of the service time and also showed that the degradation of the performance and material property could become serious after about six-year-use of the regulators.  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