全文获取类型
收费全文 | 2615篇 |
免费 | 28篇 |
国内免费 | 1篇 |
专业分类
电工技术 | 27篇 |
化学工业 | 227篇 |
金属工艺 | 22篇 |
机械仪表 | 29篇 |
建筑科学 | 61篇 |
矿业工程 | 2篇 |
能源动力 | 24篇 |
轻工业 | 185篇 |
水利工程 | 13篇 |
石油天然气 | 23篇 |
无线电 | 98篇 |
一般工业技术 | 250篇 |
冶金工业 | 1492篇 |
原子能技术 | 27篇 |
自动化技术 | 164篇 |
出版年
2022年 | 13篇 |
2021年 | 10篇 |
2020年 | 8篇 |
2019年 | 14篇 |
2018年 | 17篇 |
2017年 | 15篇 |
2016年 | 19篇 |
2015年 | 20篇 |
2014年 | 16篇 |
2013年 | 58篇 |
2012年 | 43篇 |
2011年 | 70篇 |
2010年 | 35篇 |
2009年 | 50篇 |
2008年 | 68篇 |
2007年 | 54篇 |
2006年 | 46篇 |
2005年 | 30篇 |
2004年 | 32篇 |
2003年 | 51篇 |
2002年 | 26篇 |
2001年 | 35篇 |
2000年 | 30篇 |
1999年 | 56篇 |
1998年 | 470篇 |
1997年 | 264篇 |
1996年 | 165篇 |
1995年 | 102篇 |
1994年 | 103篇 |
1993年 | 98篇 |
1992年 | 23篇 |
1991年 | 20篇 |
1990年 | 40篇 |
1989年 | 28篇 |
1988年 | 35篇 |
1987年 | 16篇 |
1986年 | 20篇 |
1985年 | 24篇 |
1984年 | 22篇 |
1983年 | 21篇 |
1982年 | 17篇 |
1981年 | 23篇 |
1980年 | 25篇 |
1978年 | 22篇 |
1977年 | 53篇 |
1976年 | 113篇 |
1975年 | 12篇 |
1973年 | 10篇 |
1972年 | 9篇 |
1969年 | 7篇 |
排序方式: 共有2644条查询结果,搜索用时 15 毫秒
1.
Vladimir Rozenshtein Alexander Berg Haim Levanon Uwe Krueger Dietmar Stehlik Yuri Kandrashkin Art Van Der Est 《Israel journal of chemistry》2003,43(3-4):373-381
Light-induced spin-polarized transient EPR spectra are reported for several water-soluble copper porphyrins. The spectra are assigned to the doublet ground state, with emissive spin polarization resulting from photoexcitation and subsequent electronic relaxation. In contrast to other systems for which polarization of a doublet ground state has been observed, the exchange interactions in the copper porphyrins are strong and the geometry is fixed. It is proposed that intersystem crossing from the photoexcited trip-doublet to the trip-quartet state can lead to net polarization of the spin system and that this polarization is maintained during electronic decay, possibly via charge-transfer and exciplex states. The intensity of the observed spin polarization is essentially independent of the molecular orientation in the external field, but is strongly dependent on the nature of the charged peripheral groups. Possible reasons for this behavior are discussed. 相似文献
2.
D Galasko T Saitoh Y Xia LJ Thal R Katzman LR Hill L Hansen 《Canadian Metallurgical Quarterly》1994,44(10):1950-1951
We determined apolipoprotein E (ApoE) genotypes in 122 autopsied demented patients. The frequency of the ApoE epsilon 4 allele was 39.6% in Alzheimer's disease (AD), 29.0% in the Lewy body variant of AD (LBV), and 6.25% in diffuse Lewy body disease. For AD and LBV patients, the epsilon 4 frequency was significantly higher than that reported in nondemented controls (10 to 15%). Therefore, LBV and AD share ApoE epsilon 4 as a genetic risk factor, providing further evidence that these conditions overlap. 相似文献
3.
4.
HG Tiselius C Berg AM Fornander MA Nilsson 《Canadian Metallurgical Quarterly》1993,7(1):381-9; discussion 389-90
Urinary citrate appears to be an important factor in the crystallization process of calcium oxalate and calcium phosphate. The urinary excretion of citrate was found to be significantly lower in patients with calcium oxalate stone disease as compared with normal subjects, and about 30 per cent of the calcium stone formers can be considered as hypocitraturic. The lowest excretion of citrate was recorded in urine collected during the night. Citrate has significant effects on supersaturation with respect to both calcium oxalate and calcium phosphate, it also inhibits the growth of these crystals. In addition, citrate appears to be capable of inhibiting the aggregation of crystals composed of calcium oxalate, brushite, and hydroxyapatite. The heterogenous growth of calcium oxalate on calcium phosphate is also counteracted by citrate. As a consequence of the crucial role of citrate in these processes, stone prevention with alkaline citrate has become an attractive form of treatment in patients with recurrent stone formation. Single evening dose administration of sodium potassium citrate resulted in an of sodium potassium citrate resulted in an increased excretion of citrate, reduced levels of the calcium/citrate ratio as well as supersaturation with respect to calcium oxalate and a decreased rate of stone formation. However, conflicting results of stone preventive treatment with alkaline citrate have been reported by different groups, and long-term follow-up of patients treated in a randomized way is necessary to definitely assess the efficacy of alkaline citrate. 相似文献
5.
6.
Environmental contaminants such as trace elements may be present in all foods. Foods, raw materials and ingredients for food production are to an increasing extent traded across borders. Hence, there is a need for international legislation on trace elements as contaminants in food. In 1961, the FAO and WHO established the Codex Alimentarius to elaborate international food legislation. Contaminants in food are handled by the Codex Committee for Food Additives and Contaminants (CCFAC). The Codex Alimentarius system for developing legislation concerning trace elements as contaminants in food is based upon the Codex General Standard for Contaminants and Toxins in Food (GSCTF). By October 2001, the principles for setting maximum limits (MLs) for contaminants in food are agreed, and work is in progress on MLs for trace elements such as lead and cadmium in the various food categories. The status for the proposals is presented and discussed. The EU Regulation 466/2001 of 8 March 2001 sets MLs for lead and cadmium in various foods. This regulation will apply from 5 April 2002. The EU regulation is more detailed but similar to the Codex draft standards for lead and cadmium in food. In future, legislators and administrators in the Codex and EU and analytical chemists will discuss how to use more and better analytical data as risk-management tools to protect public health. Trace elements' speciation is an important aspect of this discussion. 相似文献
7.
Poly(vinyl alcohol) is crosslinked in dilute solution (c=0.1 wt%) with glutaraldehyde. The reaction product is characterized by viscometry and gel permeation chromatography (g.p.c.). The intrinsic viscosity decreases with increasing degree of crosslinking and does not depend on temperature. G.p.c. reveals that the reaction product is not homogeneous, but consists of a mixture of particles with different sizes, possibly both intra- and intermolecularly crosslinked molecules. The intramolecularly crosslinked molecules are smaller in size than the initial polymer molecules and their size depends on the degree of crosslinking. They possess a narrow particle size distribution even if the initial polymer sample had a broad molecular weight distribution. 相似文献
8.
M E Rentschler S R Platt K Berg J Dumpert D Oleynikov S M Farritor 《IEEE transactions on information technology in biomedicine》2008,12(1):66-75
Long-term human space exploration will require contingencies for emergency medical procedures including some capability to perform surgery. The ability to perform minimally invasive surgery (MIS) would be an important capability. The use of small incisions reduces surgical risk, but also eliminates the ability of the surgeon to view and touch the surgical environment directly. Robotic surgery, or telerobotic surgery, may provide emergency surgical care in remote or harsh environments such as space flight, or extremely forward environments such as battlefields. However, because current surgical robots are large and require extensive support personnel, their implementation has remained limited in forward environments, and they would be difficult, or impossible, to use in space flight or on battlefields. This paper presents experimental analysis of miniature fixed-base and mobile in vivo robots to support MIS surgery in remote and harsh environments. The objective is to develop wireless imaging and task-assisting robots that can be placed inside the abdominal cavity during surgery. Such robots will provide surgical task assistance and enable an on-site or remote surgeon to view the surgical environment from multiple angles. This approach is applicable to long-duration space flight, battlefield situations, and for traditional medical centers and other remote surgical locations. 相似文献
9.
10.
SA Cook RT Bronson LR Donahue N Ben-Arie MT Davisson 《Canadian Metallurgical Quarterly》1997,8(2):108-112
OBJECTIVE: To discuss the case of an 8-yr-old boy with an aneurysmal bone cyst of the right proximal humerus, including the features imaged on plain film radiography, computed tomography (CT), magnetic resonance imaging (MRI), including spin echo and fast field echo imaging. CLINICAL FEATURES: The patient suffered for 1 yr from intermittent but progressive pain in his right upper arm and shoulder area. There was no history of trauma or known systemic disease. There was decreased range of motion in abduction of the glenohumeral joint and pain on focal pressure along the deltoid muscle. A complete imaging evaluation consisting of plain film radiography, CT and MRI was performed, which revealed the classical imaging features of an aneurysmal bone cyst. An additional cystic lesion was detected by the MRI that was not appreciated on the plain films or CT. INTERVENTION AND OUTCOME: The patient was referred for biopsy to confirm the preliminary diagnosis of aneurysmal bone cyst. No treatment was instituted. CONCLUSION: Evaluation of aneurysmal bone cyst may be completed with CT scanning and more specifically with MRI MRI coronal T2, weighted images are advantageous for visualization of the main cystic lesion and any additional cysts. Fast field echo images show a better contrast between the cyst and bone marrow with extension of the cyst into the epiphysis as evident in this case. Follow-up studies revealed complete healing of the cyst with only residual densities in the humeral metaphyseal area. 相似文献