全文获取类型
收费全文 | 459161篇 |
免费 | 6405篇 |
国内免费 | 1157篇 |
专业分类
电工技术 | 8432篇 |
综合类 | 3002篇 |
化学工业 | 68693篇 |
金属工艺 | 19802篇 |
机械仪表 | 17303篇 |
建筑科学 | 11477篇 |
矿业工程 | 2120篇 |
能源动力 | 10674篇 |
轻工业 | 38842篇 |
水利工程 | 4536篇 |
石油天然气 | 5407篇 |
武器工业 | 9篇 |
无线电 | 54250篇 |
一般工业技术 | 90454篇 |
冶金工业 | 65011篇 |
原子能技术 | 6723篇 |
自动化技术 | 59988篇 |
出版年
2021年 | 3150篇 |
2020年 | 2214篇 |
2019年 | 2759篇 |
2018年 | 23429篇 |
2017年 | 22962篇 |
2016年 | 16566篇 |
2015年 | 4169篇 |
2014年 | 5645篇 |
2013年 | 16204篇 |
2012年 | 13318篇 |
2011年 | 26234篇 |
2010年 | 22484篇 |
2009年 | 20603篇 |
2008年 | 21501篇 |
2007年 | 23813篇 |
2006年 | 9297篇 |
2005年 | 12705篇 |
2004年 | 10804篇 |
2003年 | 10268篇 |
2002年 | 8691篇 |
2001年 | 7943篇 |
2000年 | 7672篇 |
1999年 | 7823篇 |
1998年 | 18292篇 |
1997年 | 13250篇 |
1996年 | 10443篇 |
1995年 | 8068篇 |
1994年 | 7401篇 |
1993年 | 7046篇 |
1992年 | 5380篇 |
1991年 | 5152篇 |
1990年 | 4971篇 |
1989年 | 4833篇 |
1988年 | 4736篇 |
1987年 | 3930篇 |
1986年 | 4037篇 |
1985年 | 4768篇 |
1984年 | 4304篇 |
1983年 | 4047篇 |
1982年 | 3634篇 |
1981年 | 3790篇 |
1980年 | 3495篇 |
1979年 | 3442篇 |
1978年 | 3247篇 |
1977年 | 3843篇 |
1976年 | 4897篇 |
1975年 | 2790篇 |
1974年 | 2680篇 |
1973年 | 2693篇 |
1972年 | 2222篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
STUDY OBJECTIVE: To assess the efficacy of laser laparoscopic photocoagulation of endometriomas (2-18 cm) in patients with pain, infertility, or a combination of the two. DESIGN: Retrospective review of all patients with endometriomas from June 1, 1983, to December 31, 1993. SETTING: Department of gynecology and obstetrics at a district general hospital and national training center in minimal access surgery. PATIENTS: One hundred sixty-five women with large endometriomas present at the time of laser laparoscopy. INTERVENTIONS: Carbon dioxide laser or potassium-titanyl-phosphate laser laparoscopic surgery. MEASUREMENTS AND MAIN RESULTS: Ninety (74%) of 122 patients reported improvement or resolution of pain; and 30 of 66 achieved a pregnancy, for a cumulative conception rate of 45%. CONCLUSION: Laser laparoscopy is a practical, safe, and effective technique for the management of large ovarian endometriomas. 相似文献
992.
Wakabayashi R. Kawakami H. Sato G. Amano T. Suzuki Y. 《Vehicular Technology, IEEE Transactions on》1998,47(2):392-405
A VHF omnidirectional radio range (VOR) is a navigation aid radio beacon facility, which provides aircraft with azimuth information relative to the VOR station in question as the origin. In Japan, two types of VOR-the conventional type (referred to as a CVOR) and the Doppler type (referred to as a DVOR)-are currently in use. An element known as the Alford loop antenna (ALA), which changes the loading reactance, is used for the VOR because the horizontally polarized wave and nondirectivity in the horizontal plane are preferred. A VOR antenna consists of a carrier antenna and a sideband antenna-an aircraft receives separate signals from these two antennas and compares them to obtain azimuth information. The mutual coupling between the elements forming the carrier and the sideband antenna affects the directivity of the single elements, resulting in errors in azimuth information. With the mutual coupling between the antenna elements being taken into consideration, a quantitative calculation was made by using the moment method-the results of the calculation made it clear that a loading reactance value of -320 Ω is better to make not mutually coupled elements nondirectional while a loading reactance value of -600 Ω is optimum to minimize the azimuth error of a CVOR 相似文献
993.
Application of the extremum stack to neurological MRI 总被引:2,自引:0,他引:2
Simmons A. Arridge S.R. Tofts P.S. Barker G.J. 《IEEE transactions on medical imaging》1998,17(3):371-382
994.
Bez R. Cantarelli D. Moioli L. Ortolani G. Servalli G. Villa C. Dallabora M. 《Electron Device Letters, IEEE》1998,19(2):37-39
A new method to erase a standard (double-poly, stacked-gate NOR-type) flash cell is proposed. The method, still using the tunneling mechanism to extract electrons from the floating gate, is based on the concept of keeping the electric field constant during the whole erasing operation. The new method has two main advantages with respect to the conventional one: (1) it does not depend on the supply voltage variation and (2) it allows a better reliability in terms of endurance-induced stress. Results have shown that flash device performances are greatly improved in terms of stability and endurance reliability up to one million cycles 相似文献
995.
996.
S Arold P Franken MP Strub F Hoh S Benichou R Benarous C Dumas 《Canadian Metallurgical Quarterly》1997,5(10):1361-1372
BACKGROUND: Human immunodeficiency virus (HIV) Nef protein accelerates virulent progression of acquired immunodeficiency syndrome (AIDS) by its interaction with specific cellular proteins involved in signal transduction and host cell activation. Nef has been shown to bind specifically to a subset of the Src family of kinases. The structures of free Nef and Nef bound to Src homology region 3 (SH3) domain are important for the elucidation of how the affinity and specificity for the Src kinase family SH3 domains are achieved, and also for the development of potential drugs and vaccines against AIDS. RESULTS: We have determined the crystal structures of the conserved core of HIV-1 Nef protein alone and in complex with the wild-type SH3 domain of the p59fyn protein tyrosine kinase (Fyn), at 3.0 A resolution. Comparison of the bound and unbound Nef structures revealed that a proline-rich motif (Pro-x-x-Pro), which is implicated in SH3 binding, is partially disordered in the absence of the binding partner; this motif only fully adopts a left-handed polyproline type II helix conformation upon complex formation with the Fyn SH3 domain. In addition, the structures show how an arginine residue (Arg77) of Nef interacts with Asp 100 of the so-called RT loop within the Fyn SH3 domain, and triggers a hydrogen-bond rearrangement which allows the loop to adapt to complement the Nef surface. The Arg96 residue of the Fyn SH3 domain is specifically accommodated in the same hydrophobic pocket of Nef as the isoleucine residue of a previously described Fyn SH3 (Arg96-->lle) mutant that binds to Nef with higher affinity than the wild type. CONCLUSIONS: The three-dimensional structures support evidence that the Nef-Fyn complex forms in vivo and may have a crucial role in the T cell perturbating action of Nef by altering T cell receptor signaling. The structures of bound and unbound Nef reveal that the multivalency of SH3 binding may be achieved by a ligand induced flexibility in the RT loop. The structures suggest possible targets for the design of inhibitors which specifically block Nef-SH3 interactions. 相似文献
997.
MG Tucci G Ricotti R Giardino S Carraro G Mattei I Cataldi G Lucarini R Solmi L Tosi G Biagini 《Canadian Metallurgical Quarterly》1997,43(8):1213-1220
Establishing guidelines and experimental models preclinical and clinical evaluations of new agents for treatment, and/or prevention of human diseases has become a task of crucial importance. Psoriasis is such one disease holding great interest for dermatology owing to its high rate of incidence and complexity of treatment. However the absence of psoriatic lesions in animals and the inability to induce them, calls for experimental techniques both in vitro and in vivo. The purpose of this study was to evaluate experimentally the effects of tacalcitol on cell proliferation and differentiation process. Thereafter a human pilot study on psoriatic patients has been developed. 相似文献
998.
999.
Sampled-data techniques are used to examine the dynamic characteristics and closed-loop performance of the quasi-square-wave boost converter. Direct duty-ratio control and current-mode control are considered. With direct duty-ratio control, the system poles remain well damped under all load conditions, and there is no zero in the control-to-output transfer function. Good closed-loop performance is therefore achieved. Under current-mode control, the requirement for a stabilizing ramp is seen to depend on load conditions, and the achievable voltage control-loop bandwidth is smaller than that using direct duty-ratio control; however, there is a significant reduction in the magnitude of the DC source to output-voltage frequency response 相似文献
1000.
The MnO activities in (MnO-CrO
x
-CaO-SiO2)-containing melts, which were saturated with the (Mn, Cr)3O4 spinel phase, were determined at 1500 °C under an oxygen partial pressure of 10−8.99 atm. This was done by equilibrating the samples with platinum. The activity of MnO in the melt was then calculated from the
activity coefficient of manganese in the resultant Pt-Cr-Mn alloy. Darken’s quadratic formalism for ternary metallic solutions
was used to calculate the activity coefficient of manganese in the Pt-Cr-Mn system, in which platinum was considered to be
the solvent. It was found that an increase in the concentration of MnO in the melt increases both the MnO activity and the
activity coefficient of MnO. For a constant MnO concentration in the (MnO-CrO
x
-CaO-SiO2)-containing melts, the activity of MnO can be increased by increasing the basicity of the melt. In order to obtain high-manganese
recoveries from (MnO-CrO
x
-CaO-SiO2)-containing melts into an alloy phase, basic slags in which the activity coefficient of MnO is high should therefore be used. 相似文献