全文获取类型
收费全文 | 14604篇 |
免费 | 375篇 |
国内免费 | 37篇 |
专业分类
电工技术 | 206篇 |
综合类 | 21篇 |
化学工业 | 2224篇 |
金属工艺 | 315篇 |
机械仪表 | 330篇 |
建筑科学 | 527篇 |
矿业工程 | 28篇 |
能源动力 | 305篇 |
轻工业 | 1767篇 |
水利工程 | 112篇 |
石油天然气 | 37篇 |
无线电 | 1677篇 |
一般工业技术 | 2168篇 |
冶金工业 | 3292篇 |
原子能技术 | 179篇 |
自动化技术 | 1828篇 |
出版年
2023年 | 68篇 |
2022年 | 153篇 |
2021年 | 274篇 |
2020年 | 170篇 |
2019年 | 244篇 |
2018年 | 273篇 |
2017年 | 264篇 |
2016年 | 333篇 |
2015年 | 194篇 |
2014年 | 346篇 |
2013年 | 697篇 |
2012年 | 542篇 |
2011年 | 665篇 |
2010年 | 537篇 |
2009年 | 513篇 |
2008年 | 575篇 |
2007年 | 546篇 |
2006年 | 441篇 |
2005年 | 448篇 |
2004年 | 346篇 |
2003年 | 343篇 |
2002年 | 320篇 |
2001年 | 320篇 |
2000年 | 308篇 |
1999年 | 344篇 |
1998年 | 1074篇 |
1997年 | 669篇 |
1996年 | 492篇 |
1995年 | 366篇 |
1994年 | 297篇 |
1993年 | 309篇 |
1992年 | 176篇 |
1991年 | 152篇 |
1990年 | 157篇 |
1989年 | 151篇 |
1988年 | 145篇 |
1987年 | 116篇 |
1986年 | 124篇 |
1985年 | 160篇 |
1984年 | 112篇 |
1983年 | 101篇 |
1982年 | 85篇 |
1981年 | 101篇 |
1980年 | 93篇 |
1979年 | 98篇 |
1978年 | 53篇 |
1977年 | 117篇 |
1976年 | 162篇 |
1975年 | 55篇 |
1973年 | 61篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
141.
The thermal stability of polymers is a main issue when used as friction elements under dry sliding. Cast polyamide grades processed with either natrium or magnesium catalysors are slid on a small-scale and a large-scale test configuration to reveal the effect of softening or degradation on the sliding stability and to investigate possibilities for extrapolation of friction and wear rates between both testing scales. The combination of softening and afterwards transition into the glassy state is detrimental for the sliding stability of natrium catalysed polyamides, characterised by heavy noise during sliding. A transfer film formed under continuous softening also provides high friction. Melting during initial sliding is necessary for stabilisation in both friction and wear, and eventual softening of a molten film near the end of the test then not deteriorates the sliding stability. Softening of magnesium catalysed polyamides is favourable for the formation of a coherent transfer film resulting in more stable sliding than natrium catalysed polyamides. The differences in softening mechanisms of both polyamide grades is correlated to structural changes investigated by thermal analysis and Raman spectroscopy: the γ crystalline structure prevails in magnesium catalysed samples and the α crystalline structure is predominant in natrium catalysed samples. For internal oil lubricated polyamides, a time dependent degradation of the polyamide bulk deteriorates the supply of internal oil lubricant to the sliding interface, resulting in high friction and wear under overload conditions. As the degradation mechanisms during sliding are strongly correlated to the test set-up, extrapolation is only possible for friction in a limited application range, while wear rates cannot be extrapolated. 相似文献
142.
Nguyen Van Nguu 《Nutrient Cycling in Agroecosystems》1987,14(2):135-142
The shortening of fallow period in several areas in tropical Africa has reduced soil fertility and exposed soils to erosion and run-off. Fertilizer application and crop conservation practices are needeed to sustain high crop yield and to conserve the natural resource base for upland crop production in the continent. Field trials were carried out to evaluate the effect of fertilizer application and soil and crop residues management practices on yield of maize (Zea mays L.) planted on a Plinthudult soil at Bertoua, Eastern Cameroon. Maize yields increased significantly with nitrogen and phosphorus fertilizer application. Under the rainfall pattern prevailing in the area, the amount of nitrogen required for maximum yield was higher in the second season. On the other hand, the amount of phosphorus required for maximum yield appeared to decrease with time. The burning of crop residues and weeds prior to planting together with no-till practive gave higher yield of maize than other soil and crop residues management practices. 相似文献
143.
W. D. Pohle R. L. Gregory B. Van Giessen 《Journal of the American Oil Chemists' Society》1964,41(10):649-650
The peroxide value and thiobarbituric acid value were compared with the flavor score for a series of different types of fats,
with and without added monoglyceride and with and without different stabilizers. The data indicated that the flavor score
cannot be estimated for any given fat from either the peroxide value or the thiobarbituric acid value. Either can be used
to follow the development of off flavors in a given product or formulation but the relative level may vary from product to
product.
Presented at the AOCS Meeting in Minneapolis, 1963. 相似文献
144.
N. De Belie J. Monteny A. Beeldens E. Vincke D. Van Gemert W. Verstraete 《Cement and Concrete Research》2004,34(12):2223-2236
New equipment and procedures for chemical and microbiological tests, simulating biogenic sulfuric acid corrosion in sewerage systems, are presented. Subsequent steps of immersion and drying, combined with mechanical abrasion, were applied to simulate events occurring in sewer systems. Both chemical and microbiological tests showed that the aggregate type had the largest effect on degradation. Concrete with limestone aggregates showed a smaller degradation depth than did the concrete with inert aggregates. The limestone aggregates locally created a buffering environment, protecting the cement paste. This was confirmed by microscopic analysis of the eroded surfaces. The production method of concrete pipes influenced durability through its effect on W/C ratio and water absorption values. In the microbiological tests, HSR Portland cement concrete performed slightly better than did the slag cement concrete. A possible explanation can be a more rapid colonisation by microorganisms of the surface of slag cement samples. A new method for degradation prediction was suggested based on the parameters alkalinity and water absorption (as a measure for concrete porosity). 相似文献
145.
In some industrial fluidized bed processes, notably coal combustion, the bed contains a very wide size range (50 – 5000 μm) of equal density particles. In others the particles change their density as the reaction proceeds, giving a bed of particles having similar sizes but densities varying by a factor of up to 2.
Experiments have been done in a bed 0.29 m diam. at velocities up to 5 m/s, using coarse particles up to 6 mm. They show that segregation by density difference can be reduced to negligible proportions by using high velocities, but that segregation by size appears to be an intrinsic feature of coarse particle systems at all velocities when a very wide size range is present. A tentative form of equation is proposed for segregation by size; the equations of Rowe are useful predictors for segregation by density difference. 相似文献
146.
In this article the use of the direct osmosis test as a complementary experiment to the reverse osmosis test is described. The membranes investigated are cellulose acetate membranes reinforced with mineral fillers. In both tests the same influence of the type of fillers on the water permeability coefficients is found. The salt permeability coefficients indicate the presence of pressure sensitive defects in the reinforced membranes. The direct osmosis test is found to be a suitable test to confirm and predict the membrane properties under reverse osmosis conditions. 相似文献
147.
Stan De Spiegelaere Guy Van Gyes Hans De Witte Wendy Niesen Geert Van Hootegem 《Creativity & Innovation Management》2014,23(3):318-330
European policy is focusing on innovation as a way out of the economic crisis. At the same time, job insecurity is rising as Europe is still in crisis. In this paper, we examine whether job insecurity affects the innovative work behaviour of employees by focusing on the relation between job insecurity, job autonomy, work engagement and innovative work behaviour (IWB). Using employee level survey data, we use structural equation modelling to disentangle the relations between these variables. The partially mediated model shows the best fit with the data. This model shows that job insecurity and autonomy are both directly and indirectly, through work engagement, related with IWB. For autonomy these relations are positive, while they are negative (and smaller) for job insecurity. Moreover, a negative covariance is observed between job insecurity and autonomy. 相似文献
148.
The optimal operating policy for a two-bed adiabatic CO-converter is computed using the Discrete Maximum Principle. The problem is then extended to determine also the optimal design of the system. A sensitivity analysis of the optimal solutions is presented, showing the influence of the principal process parameters. Economical, chemical and physical data have been obtained from an existing CO-converter in a standard 1000 short Tons/day NH3 plant. 相似文献
149.
It is shown that even pure quartz, other silicas or aluminas can enhance the aromatization activity of a ZSM-5 zeolite. Incorporation of gallium oxide onto these supports increases further the production of aromatics. The use of supported gallium oxide co-catalyst obtained by co-evaporation of a colloidal silica and a Ga salt has led to extremely high aromatization performance for the hybrid catalyst. 相似文献
150.
An aerosol dynamics model, AERO2, is presented, which describes the formation of H2SO4-H2O aerosol in a smog chamber. The model is used to analyse how the uncertainties on four input parameters are propagated through an aerosol dynamics model. The input parameters are: the rate of the reaction between SO2 and OH (k1), the ratio between the nucleation rate used in AERO2 and that derived from classical nucleation theory (tn), the H2SO4 mass accommodation coefficient () and a measure of the turbulence intensity in the reactor (ke). Uncertainties for these parameters are taken from the literature. One of the results of the analysis is that AERO2 and aerosol dynamics models in general can only predict upper bounds for the total number (Ntot) and total volume (Vtot) concentrations of the particles. The uncertainties on Ntot and Vtot are mainly due to the uncertainties on k1, and tn. An uncertainty factor of 20–100 still remains when the uncertainty on k1, is reduced to ±5%. Aerosol measurements from three smog chamber experiments have therefore been used, in an attempt to reduce the uncertainty on k1 and tn. Values for k1 are obtained in the reduced range 7.8 × 10−13 to 1.0 × 10−12 cm3 s−1, which is within the range found in the literature. For tn, values in the range 104–107 are obtained, which is close to the upper bound of the range in literature. These values for tn are in marked contrast with a recent set of experiments on nucleation in H2SO4-H2O mixtures, which suggests a value for tn of at most 10−5. 相似文献