全文获取类型
收费全文 | 179850篇 |
免费 | 2273篇 |
国内免费 | 708篇 |
专业分类
电工技术 | 3752篇 |
综合类 | 138篇 |
化学工业 | 28410篇 |
金属工艺 | 7811篇 |
机械仪表 | 5148篇 |
建筑科学 | 4676篇 |
矿业工程 | 1011篇 |
能源动力 | 4692篇 |
轻工业 | 16454篇 |
水利工程 | 1715篇 |
石油天然气 | 3135篇 |
武器工业 | 2篇 |
无线电 | 20070篇 |
一般工业技术 | 34170篇 |
冶金工业 | 33149篇 |
原子能技术 | 4291篇 |
自动化技术 | 14207篇 |
出版年
2021年 | 1317篇 |
2019年 | 1275篇 |
2018年 | 2107篇 |
2017年 | 2083篇 |
2016年 | 2230篇 |
2015年 | 1671篇 |
2014年 | 2834篇 |
2013年 | 7973篇 |
2012年 | 4683篇 |
2011年 | 6484篇 |
2010年 | 5176篇 |
2009年 | 6037篇 |
2008年 | 6035篇 |
2007年 | 6003篇 |
2006年 | 5073篇 |
2005年 | 4824篇 |
2004年 | 4576篇 |
2003年 | 4234篇 |
2002年 | 4145篇 |
2001年 | 4174篇 |
2000年 | 3930篇 |
1999年 | 4101篇 |
1998年 | 10405篇 |
1997年 | 7402篇 |
1996年 | 5682篇 |
1995年 | 4310篇 |
1994年 | 3645篇 |
1993年 | 3628篇 |
1992年 | 2648篇 |
1991年 | 2561篇 |
1990年 | 2444篇 |
1989年 | 2466篇 |
1988年 | 2397篇 |
1987年 | 2159篇 |
1986年 | 2089篇 |
1985年 | 2398篇 |
1984年 | 2209篇 |
1983年 | 2035篇 |
1982年 | 1906篇 |
1981年 | 1994篇 |
1980年 | 1832篇 |
1979年 | 1843篇 |
1978年 | 1794篇 |
1977年 | 2135篇 |
1976年 | 2710篇 |
1975年 | 1585篇 |
1974年 | 1578篇 |
1973年 | 1702篇 |
1972年 | 1370篇 |
1971年 | 1278篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Rate of passage from the rumen was estimated from samples from the rumen, duodenum, ileum, and rectum after four crossbred heifers were dosed with cobalt ethylenediaminetetraacetic acid and ytterbium-labeled alfalfa or corn grain as digesta markers. Diets consisted of chopped alfalfa hay or an 80% concentrate mixture fed twice daily at 2.0% of body weight (dry matter per day) in two trials. Rates of particulate passage were calculated by two mathematical models, and influence of site of sampling on rate of ruminal passage was appraised. Rate of fluid passage calculated from ruminal samples was more rapid than rates from samples at the duodenum, ileum, and rectum for both the alfalfa diet (10.6 versus 6.3, 5.6, and 5.6% X h-1) and the 80% concentrate diet (10.3 versus 6.8, 7.2, and 6.1% X h-1). With the alfalfa diet, rates of fluid passage estimated from ruminal and rectal samples were correlated .98 despite the marked disparity of means. Rates of passage of particulates estimated from samples from the duodenum, ileum, and rectum were not correlated. Estimates of rate of digesta passage from the rumen differed with postruminal site of sampling. 相似文献
992.
F. R. DEL VALLE H. VILLANUEVA J. REYES-GOVEA M. ESCOBEDO M. J. MUNOZ J. PONCE M. VEGA M. HERNANDEZ G. VELAZQUEZ 《Journal of food science》1985,50(2):551-552
A nitrogen balance study, comparing a low-cost soy-oats infant formula with and without supplemental methionine, was carried out with infants of ages 3 - 26 months at a single equivalent level of nitrogen intake (approximately 850 mg/kg/day). Nitrogen retention was found to be unaffected by methionine supplementation. Com- parison of data obtained in this study with previous research re- vealed that a critical value for intake of total L-sulfur amino acids apparently exists in infants 3 - 26 months of age, below which methionine supplementation will increase nitrogen retention, but above which it will not. This value appears to coincide with that of 49 mg/kg/day recommended for infants by the FNB. 相似文献
993.
Prevention of the green-grey discoloration of liquid whole eggs cooked at high temperatures was studied. Disodium ethylenediaminetetraacetic acid (NaZEDTA) was used to prevent the discoloration in whole egg adjusted to pH 8.5. Liquid egg samples in flexible packages were subjected to retort conditions (121°C, for 20 min) and evaluated for off-color development. Discoloration was more severe at higher temperatures and longer cooking times; however, the use of 0.015% NazEDTA was effective in preventing green-grey color development. 相似文献
994.
995.
Adaptive Radar Detection of Distributed Targets in Homogeneous and Partially Homogeneous Noise Plus Subspace Interference 总被引:5,自引:0,他引:5
Bandiera F. De Maio A. Greco A.S. Ricci G. 《Signal Processing, IEEE Transactions on》2007,55(4):1223-1237
This paper addresses adaptive radar detection of distributed targets in noise plus interference assumed to belong to a known or unknown subspace of the observables. At the design stage we resort to either the GLRT or the so-called two-step GLRT-based design procedure and assume that a set of noise-only data is available (the so-called secondary data). Detection algorithms have been derived modeling noise vectors, corresponding to different range cells, as independent, zero-mean, complex normal ones, sharing either the same covariance matrix (homogeneous environment) or the same covariance matrix up to possibly different (mean) power levels between primary data, i.e., range cells under test, and secondary ones (partially homogeneous environment). The performance assessment has been conducted by Monte Carlo simulation, also in comparison to previously proposed detection algorithms, and confirms the effectiveness of the newly proposed ones 相似文献
996.
997.
S. K. SASTRY F. D. McCAFFERTY E. G. MURAKAMI G. D. KUHN 《Journal of food science》1985,50(5):1501-1502
Dry red kidney beans were canned using two different pretreatments: soaking for 12 hr at 20°C, and vacuum hydration for 5 min followed by soaking for 2 hr at temperatures from 45-59.1°C. Samples were then packed, processed to commercial sterility, and tested for percentage of split beans after processing. Vacuum hydration pretreatments greatly decreased the incidence and severity of splitting in the canned product and accelerated water uptake while retaining the same moisture content after soaking as the conventional soak treatment. Vacuum-hydrated beans gained less moisture during retorting than conventionally treated samples. 相似文献
998.
Aiming at producing high temperature structural ceramics, ultra-fine SiC powders were synthesized by the gas phase reaction
of silicon tetrachloride with methane in a d.c. thermal plasma. The influence of parameters as the SiCl4 feeding rate, C/Si and H2/C molar ratios and internal pressure on the powder properties were investigated. The SiC powders were characterized by chemical
analysis, Fourier transform infrared spectroscopy, X-ray diffraction and scanning electronic microscopy. The experimental
set-up allows the production of β-SiC powders at a rate of 200 g h−1 with particle size around 0.1 μm. The main impurities in the as-produced powder handled at ambient atmosphere are: oxygen
(1.8–2.5%) and free carbon (3–4%). Interesting relationships were found between the SiCl4 feeding rate and the H2/C molar ratio and between the C/Si molar ratio and the internal pressure. The internal pressure plays a major role in controlling
the particle size. 相似文献
999.
Two experiments were conducted to compare methods of determining oxygen consumption by portal-drained viscera and to relate this consumption to total oxygen consumption by lactating dairy cows. Oxygen consumption was the product of portal blood flow (determined by dilution of dye infused into a mesenteric vein) and portal-arterial concentration differences of oxygen. In Experiment 1, portal-arterial concentrations of oxygen were determined in samples from three cows using equations based on blood pH, hemoglobin, partial pressure of carbon dioxide, and partial pressure of oxygen (method 1) or by direct oxygen determination with an oximeter (method 2). Overall, there were no differences in oxygen concentrations or oxygen consumption by portal-drained viscera between methods. In Experiment 2, oxygen consumption by portal-drained viscera (method 2) and total oxygen consumption were measured in two lactating cows. Oxygen consumption by portal-drained viscera accounted for 18% of total oxygen consumption. 相似文献
1000.
The structure function of a binary coherent system is approximated by using only a few of its minimal path sets and minimal cut sets. These two incomplete structure functions are represented as disjoint sums. The average of each is a lower and upper bound, respectively, for the system reliability. The usefulness of these bounds is demonstrated on example networks 相似文献