全文获取类型
收费全文 | 684623篇 |
免费 | 7838篇 |
国内免费 | 1466篇 |
专业分类
电工技术 | 12563篇 |
综合类 | 573篇 |
化学工业 | 107240篇 |
金属工艺 | 28515篇 |
机械仪表 | 21816篇 |
建筑科学 | 15239篇 |
矿业工程 | 5344篇 |
能源动力 | 17497篇 |
轻工业 | 53908篇 |
水利工程 | 8554篇 |
石油天然气 | 17927篇 |
武器工业 | 52篇 |
无线电 | 73047篇 |
一般工业技术 | 141050篇 |
冶金工业 | 117902篇 |
原子能技术 | 17342篇 |
自动化技术 | 55358篇 |
出版年
2021年 | 6518篇 |
2019年 | 6200篇 |
2018年 | 10711篇 |
2017年 | 10903篇 |
2016年 | 11486篇 |
2015年 | 7163篇 |
2014年 | 12096篇 |
2013年 | 31173篇 |
2012年 | 18674篇 |
2011年 | 25036篇 |
2010年 | 19968篇 |
2009年 | 22427篇 |
2008年 | 22742篇 |
2007年 | 22379篇 |
2006年 | 19442篇 |
2005年 | 17673篇 |
2004年 | 16762篇 |
2003年 | 16370篇 |
2002年 | 15767篇 |
2001年 | 15458篇 |
2000年 | 14808篇 |
1999年 | 14613篇 |
1998年 | 34808篇 |
1997年 | 24983篇 |
1996年 | 19436篇 |
1995年 | 14816篇 |
1994年 | 13323篇 |
1993年 | 13160篇 |
1992年 | 10144篇 |
1991年 | 9851篇 |
1990年 | 9633篇 |
1989年 | 9363篇 |
1988年 | 9109篇 |
1987年 | 8146篇 |
1986年 | 7997篇 |
1985年 | 9067篇 |
1984年 | 8218篇 |
1983年 | 7903篇 |
1982年 | 7122篇 |
1981年 | 7165篇 |
1980年 | 6959篇 |
1979年 | 7081篇 |
1978年 | 6960篇 |
1977年 | 7646篇 |
1976年 | 9424篇 |
1975年 | 6255篇 |
1974年 | 5988篇 |
1973年 | 6123篇 |
1972年 | 5308篇 |
1971年 | 4950篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
101.
102.
103.
M. A. Lotonov 《Measurement Techniques》1996,39(11):1083-1091
It is proved that biased estimates of parameters are always more effective than unbiased estimates in the case of asymmetric
distribution laws of random quantities. Expressions are obtained for the root mean square deviation of a biased estimate of
a parameter from its value for the cases of normal, Poisson, Rayleigh, and gamma distribution laws.
Translated from Izmeritel'naya Tekhnika No. 11, pp. 8–13, November, 1996. 相似文献
104.
This article describes the phenomenon of morphological instability in solid-solid phase transformations during thin-film interdiffusion,
specifically related to the initial stages of precipitation when phase growth occurs along the interface between thin films.
The experimental observations that revealed this effect will be presented, and a working hypothesis will be discussed. Experimental
observations suggest that the ledge mechanism of growth is present in this system but does not inhibit the formation of interfacial
instabilities. It is proposed that morphological stability for solid-phase growth occurring during thin-film interdiffusion
can be treated by the inclusion of a solute-source term into the two-dimensional perturbation approach generally used to study
unstable growth morphologies. Experimental observations that provide qualitative and semiquantitative support for the solute-source
model are also presented. 相似文献
105.
BACKGROUND: The incidental discovery of an adrenal mass poses the problem of distinguishing between the frequent benign masses and the infrequent malignant ones that require surgery. Univocal guidelines to approach this problem are unavailable. OBJECTIVE: To perform a survey of the clinical management of incidentally discovered adrenal masses (ie, adrenal incidentalomas). DESIGN: A multicentric retrospective analysis of hospital medical records of adrenal incidentalomas diagnosed during a 5-year period; the medical records were scrutinized for demographic data and clinical details by means of a specifically tailored questionnaire. SETTING: The major surgical and medical centers of Piedmont, a northern Italian region with approximately 4 million inhabitants. The recruitment pattern of these centers was unselected. PATIENTS: The definition of adrenal incidentaloma was limited to patients with a physical examination and a clinical history unindicative of adrenal disease. Exclusion criteria also included hypertension of suspected endocrine origin and a history of neoplasms known to metastasize frequently in the adrenal glands. Two hundred twenty-four medical records were collected, and 210 were analyzed (14 excluded a posteriori). RESULTS: Most patients were in their 50s and 60s, and women were predominantly affected. The frequency of adrenocortical cancer was 13% among patients operated on. The tumor diameter was highly correlated with the risk of cancer; a cutoff at 5 cm had a sensitivity of 93% with a specificity of 64% in discriminating between benign and malignant cortical lesions. CONCLUSIONS: The occurrence of adrenocortical carcinoma among adrenal incidentalomas is not rare. The evaluation of the mass size is a simple and effective method for selecting patients at risk for cancer. The indication for surgery of masses larger than 5 cm, or of masses of any diameter that have suspicious imaging characteristics, limits unnecessary operations and costs. 相似文献
106.
The article describes the experience of implementing patient-focused care from a physician's perspective. Pitfalls that guarantee failure are presented, many of which are avoidable with early participation by all parties involved: administrators, nursing staff, physicians, and patients. Contamination of the process with downsizing needs, lack of support for staff in dealing with necessary change, loss of administrative commitment, and withholding of appropriated funds are among the key errors to be avoided. 相似文献
107.
H Asahara K Fujisawa T Kobata T Hasunuma T Maeda M Asanuma N Ogawa H Inoue T Sumida K Nishioka 《Canadian Metallurgical Quarterly》1997,40(5):912-918
Green fluorescent protein (GFP) is increasingly being used in plant biology from the cellular level to whole plant level. At the cellular level, GFP is being used as an in vivo reporter to assess frequency of transient and stable transformation. GFP has also proven to be an invaluable tool in monitoring trafficking and subcellular localization of protein. At the organ level and up, many exciting applications are rapidly emerging. The development of brighter GFP mutants with more robust folding properties has enabled better macroscopic visualization of GFP in whole leaves and plants. One interesting example has been the use of GFP to monitor virus movement in and among whole plants. GFP is also emerging as a powerful tool to monitor transgene movement and transgenic plants in the field. In a proof-of-concept study, tobacco was transformed with a modified version of the GFP gene controlled by a constitutive (35S) promoter. GFP expression in progeny plants ranged from 0% to 0.5%, and approximately 0.1% GFP was the minimal amount needed for unambiguous macroscopic detection. GFP is the first truly in vivo reporter system useful in whole plants, and we project its usefulness will increase even further as better forms of GFP genes become available. 相似文献
108.
O Doherty M Booth N Waran C Salthouse D Cuddeford 《Canadian Metallurgical Quarterly》1997,141(23):589-592
The energy expenditure and heart rate of five Shetland ponies were measured during transport and compared with the values recorded while they were at rest and walking. Secondary aims of the study were to investigate whether there was any correlation between heart rate and energy expenditure and whether different orientations affected the values to different degrees. The measurements were recorded at one-minute intervals while the ponies were at rest, walking and being transported in four different orientations, on journeys lasting 30 minutes. There were significant increases (P < 0.001) in both heart rate and energy expenditure during transport which were similar to those associated with walking, and there was a strong correlation between the two measurements. There was no conclusive evidence that orientation affected the measurements, but it appeared that rear facing transport might have had the least effect. 相似文献
109.
ML Martínez-Frías E Bermejo Sánchez E Rodríguez Pinilla A Villa 《Canadian Metallurgical Quarterly》1997,46(6):593-596
INTRODUCTION: Even though there are few epidemiological studies evaluating the birth weights of different groups of malformed babies with chromosomal abnormalities, it is widely known that infants with trisomy 18, and to a lesser degree those with trisomy 13 and other chromosomal alterations, have low birth weights. PATIENTS AND METHODS: In this study we present the analysis of the birth weights and gestational ages of a large sample of babies (23,155 malformed and a similar number of nonmalformed babies), separating the different groups of chromosomal anomalies and comparing the weight in the clinical groups of malformed infants. RESULTS: Most of the groups with chromosomal abnormalities present lower birth weights in comparison to the other groups. CONCLUSIONS: The results of our study support the conclusion that the relationship between chromosomal alterations and low birth weight is sufficiently important that such a low birth weight in children with minor or major anomalies should be considered as one more indication to perform chromosomal analysis. 相似文献
110.
J DiRuggiero N Santangelo Z Nackerdien J Ravel FT Robb 《Canadian Metallurgical Quarterly》1997,179(14):4643-4645
We investigated the capacity of the hyperthermophile Pyrococcus furiosus for DNA repair by measuring survival at high levels of 60Co gamma-irradiation. The P. furiosus 2-Mb chromosome was fragmented into pieces ranging from 500 kb to shorter than 30 kb at a dose of 2,500 Gy and was fully restored upon incubation at 95 degrees C. We suggest that recombination repair could be an extremely active repair mechanism in P. furiosus and that it might be an important determinant of survival of hyperthermophiles at high temperatures. 相似文献