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   3763篇
  免费   175篇
  国内免费   8篇
电工技术   59篇
综合类   7篇
化学工业   912篇
金属工艺   67篇
机械仪表   70篇
建筑科学   104篇
矿业工程   21篇
能源动力   117篇
轻工业   301篇
水利工程   28篇
石油天然气   21篇
无线电   377篇
一般工业技术   702篇
冶金工业   418篇
原子能技术   36篇
自动化技术   706篇
  2023年   43篇
  2022年   122篇
  2021年   140篇
  2020年   83篇
  2019年   100篇
  2018年   111篇
  2017年   103篇
  2016年   134篇
  2015年   90篇
  2014年   150篇
  2013年   249篇
  2012年   201篇
  2011年   250篇
  2010年   176篇
  2009年   155篇
  2008年   157篇
  2007年   155篇
  2006年   143篇
  2005年   116篇
  2004年   109篇
  2003年   99篇
  2002年   94篇
  2001年   54篇
  2000年   52篇
  1999年   66篇
  1998年   76篇
  1997年   68篇
  1996年   56篇
  1995年   47篇
  1994年   30篇
  1993年   37篇
  1992年   27篇
  1991年   19篇
  1990年   23篇
  1989年   22篇
  1988年   33篇
  1987年   24篇
  1986年   18篇
  1985年   29篇
  1984年   17篇
  1983年   22篇
  1982年   24篇
  1981年   12篇
  1980年   20篇
  1979年   22篇
  1978年   19篇
  1977年   18篇
  1976年   24篇
  1975年   18篇
  1973年   12篇
排序方式: 共有3946条查询结果,搜索用时 15 毫秒
81.
We present specific new expressions for thermal stresses as Green's functions for a plane boundary value problem of steady-state thermoelasticity for a semi-layer. We also obtain new integration formulas of Green's type, which determine the thermal stresses in the form of integrals of the products of the given distributed internal heat source, boundary temperature, and heat flux and derived kernels. Elementary functions results obtained are formulated in a theorem, which is proved using the harmonic integral representations method to derive thermal stresses Green's functions, which are written in terms of Green's functions for Poisson's equation. A new solution to particular two-dimensional boundary value problem for a semi-layer under a boundary constant temperature gradient is obtained in explicit form. Graphical presentations for thermal stresses Green's functions created by a unit heat source (line load in out-of-plane direction) and by a temperature gradient are also included.  相似文献   
82.
The study examines the dynamics of carbon emissions baselines of electricity generation in Indian states and Chinese provinces in the backdrop of ongoing electricity sector reforms in these countries. Two Indian states—Gujarat and Andhra Pradesh, and three Chinese provinces–Guangdong, Liaoning and Hubei have been chosen for detailed analysis to bring out regional variations that are not captured in aggregate country studies. The study finds that fuel mix is the main driver behind the trends exhibited by the carbon baselines in these five cases. The cases confirm that opportunities exist in the Indian and Chinese electricity sectors to lower carbon intensity mainly in the substitution of other fuels for coal and, to a lesser extent, adoption of more efficient and advanced coal-fired generation technology. Overall, the findings suggest that the electricity sectors in India and China are becoming friendlier to the global environment. Disaggregated analysis, detailed and careful industry analysis is essential to establishing a power sector carbon emissions baseline as a reference for CDM crediting. However, considering all the difficulties associated with the baseline issue, our case studies demonstrate that there is merit in examining alternate approaches that rely on more aggregated baselines.  相似文献   
83.
Ribonucleic acid (RNA) metabolism of the normal and copper-treated ( Cu-T200 IUD) human endometrium was investigated. The relative concentration of total, messenger, ribosomal and transfer RNA was measured in normal and Cu-treated endometrium using the technique of affinity chromatography in polysepharose. The transition from the proli ferative to the secretory endometrium in normal women was accompanied by significant increases (p less than .05) in total RNA, messenger RNA and in ribosomal RNA. The relative proportions of bound and free messenger RNA were also modified by endometrial maturation changing from 70% bound messenger RNA in the proliferative to 83% in the secretory phase. Cu-T200 Cu release appeared to particularly affect RNA metabolism in the secretory phase. During the proliferative phase only the concentration of transfer RNA and the proportion of bound to free messenger RNA were modified by the Cu-T200. The Cu-T200 induced significant decreases (p less than .01 and p less than .05) in all RNA parameters, with the exception of the RNA/deoxyribonucleic acid ratio.  相似文献   
84.
Nowadays, consumers demand red wines with deep colour, soft tannins and fruit scents, but these wines can only be obtained from grapes with complete phenolic maturity. Diverse methods have been proposed for measuring phenolic maturity. However, all these methods only provide the average value and do not consider any possible heterogeneity. Throughout ripening, grapes were separated according to their density, which revealed the existence of a large heterogeneity. Grapes at harvest were also separated by density in three groups. The higher the density of the grapes the higher ethanol content, pH, colour intensity, total phenolic index and anthocyanin and proanthocyanidin concentrations, and the lower the titratable acidity and bitterness of the wines. When the grapes were denser the wines were also better balanced in flavour and mouthfeel sensation. These results suggest that grape heterogeneity may influence the final wine composition and quality and therefore it should be considered at harvest.  相似文献   
85.
Consumer awareness, pesticide and fertilizer contaminations and environmental concerns have resulted in significant demand for organically grown farm produce. Consumption of berries has become popular among health-conscious consumers due to the high levels of valuable antioxidants, such as anthocyanins and other phenolic compounds. The present study evaluated the influence that organic farming (OF) and integrated pest management (IPM) practise exert on the total phenolic content in 22 strawberry samples from four varieties. Postharvest performance of OF and IPM strawberries grown in the same area in the centre of Portugal and harvested at the same maturity stage were compared. Chemical profiles (phenolic compounds) were determined with the aid of HPLC-DAD/MS. Total phenolic content was higher for OF strawberry extracts. This study showed that the main differences in bioactive phytochemicals between organically and IPM grown strawberries concerned their anthocyanin levels. Organically grown strawberries were significantly higher in antioxidant activity than were the IPM strawberries, as measured by DPPH and FRAP assays.  相似文献   
86.
Abstract

Digitisation of historical costumes is an actual multidisciplinary area of research that uses science-based methods of reconstruction in virtual reality. The main aim of this study was to create a method for generating numerical replicas of skirts of the late 1850?s and the1860s. We applied two-dimensional and three-dimensional software to parameterise all the elements of the skirts and recreate them layer by layer. Computer modelling allowed us to gain insight into the interrelation between the parameters of cage crinolines, skirt construction and the behaviour of textile materials on the crinoline’s surface. A replica of a historical costume was generated and the similarity between the historical prototype and its replica was proved. The application of computer graphics tools for the reconstruction of the visible and invisible elements of historical costumes can advance their scientific study. The reconstructions can be an effective instrument for teaching, enriching museum collections and producing online presentations.  相似文献   
87.
Biogenic amines (BAs) are low molecular weight organic bases with biological activity, synthesised by the microbial decarboxylation of precursor amino acids such as histidine, tyrosine or arginine. The consumption of food and beverages containing large amounts of BA can have toxicological effects. This paper reports the resistance of aminogenic lactic acid bacteria strains to pasteurisation (63 °C for 30 min). Skimmed milk inoculated with histamine‐ and tyramine‐producing strains of Enterococcus and Lactobacillus was subjected to pasteurisation. The viable number of Enterococcus durans IPLA 655 was reduced by 1 log, while that of Enterococcus faecalis BA64 was reduced by 3 logs. Lactobacillus brevis CECT 3810 did not survive pasteurisation, while the other Lactobacillus strains examined were more resistant; their viable numbers were reduced by about 2 logs. The two most resistant strains, E. durans IPLA 655 and Lactobacillus curvatus VI6, were subjected to higher temperatures. Enterococcus durans was completely inactivated at 78 °C, while some live cells of Lb. curvatus were observed. These results show that some native, metabolically active BA producers could be found in pasteurised milk owing to their resistance to the thermal treatment; their presence could have a negative impact on the final content of BAs in dairy products made from such milk.  相似文献   
88.
The commercial wine spirit used for this study revealed that the aldehyde content mainly comprises acetaldehyde but other aldehydes such as propionaldehyde, 2‐methylbutyraldehyde, isovaleraldehyde, methylglyoxal, benzaldehyde and others are also present in significant amounts. A typical grape must was used to assess the influence of wine spirit in the analytical and sensorial characteristics of fortified wines. Decreasing levels of anthocyanins, as well the increase in the red colour and tanning capacity, were observed, and seem to be positively correlated with the increase of the aldehyde content present in the wine spirits used to fortify the must. Using the CIE L*a*b* system, this aldehyde content present in the spirit used seemed to be correlated with the decrease of the wines' lightness (darkening effect), the displacement of the hue angle to higher values (yellowing effect) and the increase of the chromaticity (colour saturation) of the wines. Copyright © 2005 Society of Chemical Industry  相似文献   
89.
Biogenic amines (BAs) are low molecular weight organic bases with biological activity, mainly formed by the microbial decarboxylation of amino acids. The consumption of food containing large amounts of some BAs can have toxicological consequences. Histamine is the most active BA and the most frequently involved in food-borne intoxications. This article reports the concentrations of histamine found in 80 cheeses made from different types of milk and subjected to different ripening periods. Histamine was detected in 41.25% of the samples, with values ranging from 20 to over 1000 mg per Kg of cheese. The highest histamine concentrations were recorded in long ripening cheeses made from raw milk. The presence and quantification of histamine-producing lactic acid bacteria was determined by RT-qPCR, and a good association was obtained between the Ct values and histamine concentrations determined by HPLC.  相似文献   
90.
The !-amylases of raw and fermented seeds of African oil bean (Pentaclethra macrophylla Benth) were isolated, partially purified (tenfold) and subjected to different thermal and pH conditions. The specific activities of the purified enzyme from raw and fermented seeds were 0.037 ml-1 min-1 and 0.88 ml-1 min-1 respectively. The !-amylase from fermented seeds was more thermostable with optimum activity at 70 °C, compared to the optimum temperature of 60 °C obtained for the raw seed enzyme. The assayed !-amylases were stable over a wide range of pH (3.0-7.0), with optimum activity found at pH 6.0 and pH 5.0 for raw and fermented seeds respectively. The results show that the !-amylase of Pentaclethra macrophylla complements the microbial amylases in the mainly bacterial fermentation of African oil bean seeds to 'ugba', a highly nutritious food condiment. While the !-amylase in the raw seeds was basically the plant enzyme, that isolated from the fermented seeds was a combination of plant and microbial !-amylases.  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