全文获取类型
收费全文 | 48815篇 |
免费 | 3380篇 |
国内免费 | 1018篇 |
专业分类
电工技术 | 1637篇 |
技术理论 | 4篇 |
综合类 | 1561篇 |
化学工业 | 9336篇 |
金属工艺 | 2040篇 |
机械仪表 | 2368篇 |
建筑科学 | 2021篇 |
矿业工程 | 768篇 |
能源动力 | 1737篇 |
轻工业 | 3572篇 |
水利工程 | 495篇 |
石油天然气 | 1033篇 |
武器工业 | 146篇 |
无线电 | 7507篇 |
一般工业技术 | 7935篇 |
冶金工业 | 4234篇 |
原子能技术 | 561篇 |
自动化技术 | 6258篇 |
出版年
2024年 | 122篇 |
2023年 | 570篇 |
2022年 | 1066篇 |
2021年 | 1539篇 |
2020年 | 1201篇 |
2019年 | 1118篇 |
2018年 | 1274篇 |
2017年 | 1380篇 |
2016年 | 1329篇 |
2015年 | 1505篇 |
2014年 | 2018篇 |
2013年 | 3068篇 |
2012年 | 2837篇 |
2011年 | 3395篇 |
2010年 | 2565篇 |
2009年 | 2616篇 |
2008年 | 2583篇 |
2007年 | 2271篇 |
2006年 | 1994篇 |
2005年 | 1659篇 |
2004年 | 1480篇 |
2003年 | 1554篇 |
2002年 | 1719篇 |
2001年 | 1451篇 |
2000年 | 1057篇 |
1999年 | 959篇 |
1998年 | 1495篇 |
1997年 | 1056篇 |
1996年 | 819篇 |
1995年 | 659篇 |
1994年 | 578篇 |
1993年 | 510篇 |
1992年 | 372篇 |
1991年 | 330篇 |
1990年 | 326篇 |
1989年 | 301篇 |
1988年 | 242篇 |
1987年 | 214篇 |
1986年 | 196篇 |
1985年 | 210篇 |
1984年 | 186篇 |
1983年 | 118篇 |
1982年 | 112篇 |
1981年 | 103篇 |
1980年 | 124篇 |
1979年 | 89篇 |
1978年 | 93篇 |
1977年 | 111篇 |
1976年 | 152篇 |
1975年 | 82篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
Ray tracing has been shown to be an excellent technique for rendering realistic images. However, it is important to reduce the lengthy computation time resulting from the brute-force nature of the original ray-tracing algorithms. In this paper, two ideas are proposed to speep up the well-known hierarchical subdivision method. First, a new hierarchy traversal scheme is described to reduce the number of raybounding volume intersection tests for secondary rays. Then, a plane-sweep method is proposed to make fewer intersection checks for eye rays. Experiments and discussions are presented to prove the feasibility of our methods. 相似文献
92.
In this paper, we propose queueing strategies employing the service interval-based priority (sip) which can provide delay-bounded, and loss-free services, while maximizing bandwidth utilization in the atm network. We also describe a variation of the sip, the residual service interval-based priority (rsip) which can achieve almost full utilization by assigning priorities dynamically on the basis of the residual service interval. We store the realtime cells belonging to different connections in logically separated queues, and for each queue, we set a parameter called service interval, during which only one cell is allowed to be transmitted. The sip server takes and transmits the head-of-line (hol) cell of the queue which has the smallest service interval, while the rsip server selects the queue with the smallest residual service interval. When there is no eligible real-time cell, it transmits non-real-time cell, thus enabling a maximized bandwidth utilization. Employing the above queueing strategies, we analyze the delay characteristics deterministically with the leaky bucket bounded input traffic and then dimension the optimal service interval. In dimensioning the service interval and buffer space of each real-time service queue, we consider burstiness of traffic in conjunction with delay constraints, so that bandwidth utilization can get maximized. In addition, we consider the issues of protection from malicious users, average bandwidth utilization, and coupling between the delay bound and the bandwidth allocation granularity. 相似文献
93.
Theoretical calculations of gain, refractive index change, differential gain, and threshold current for GaAs-AlGaAs quantum-wire lasers grown in V-shaped grooves are presented. The theoretical model is based on the density-matrix formalism with intraband relaxation, and the subband structure is calculated within the effective bond-orbital model. For the quantum-wire geometry treated, agreement with the observed subband spacings is found. Because of the small overlap of the optical field with the active region for a single quantum wire, lasing threshold is reached only when several subbands are filled 相似文献
94.
95.
Robotic synthesis is making possible the synthesis of large, systematically designed sets of compounds. We analyze a 512-compound set that is a 2(9) full factorial experimental design using a recursive partitioning algorithm, FIRM, and a high-dimension visualization tool, TempleMVV. These techniques are used to quickly and easily identify the main trends in the data set and also identify unusual observations. We show that analytical and visualization methods can be used synergistically to analyze a large, complex, high-dimensional data set. We also show that a fractional factorial design of 128 compounds would give essentially the same information. 相似文献
96.
Tzong-Lin Wu Hung-Chun Chang 《Lightwave Technology, Journal of》1995,13(9):1926-1934
Based on vectorial formulations which combine the surface integral equation method and the finite-element method, a novel numerical approach is proposed for calculating the dispersion coefficients of dual-mode elliptical-core fibers with arbitrary refractive index profiles. By differentiating the original formulations involving the propagation constant β and the guided mode fields Hx and Hy once and twice with respect to the normalized frequency V, the new formulations for {dβ/dV, dHx/dV, dHy/dV} and for {d2β/dV2, d2 Hx/dV2, d2Hy/dV2 } are obtained respectively. Once {β, Hx, Hy } is solved through the eigenvalue procedure which dominates the computing time, only a few matrix manipulations are required to obtain {dβ/dV, dHx/dV, dHy/dV} and {d2β/dV2, d2Hx/dV2 , d2Hy/dV2}. Some numerical examples are examined to see the influence of different refractive index distributions with dips on the dispersions of the four nondegenerate LP 11 modes for elliptical-core fibers 相似文献
97.
A CMOS fully differential buffer amplifier with accurate gain and clipping control is presented. The gain is made variable by controlling the amount of the feedback around the power amplifier by means of an additional gain control loop. A new clipping technique is used to control the clipping level of the amplifier. The amplifier is realized in a 1.2 μm CMOS process with a single 5 V power supply. Measurements confirm the presented techniques 相似文献
98.
99.
Neoxanthin, a major carotenoid pigment of spinach, is found in the Chloroplast membrane and has an unknown function in plants. Neoxanthin inhibited the production of superoxide anions in an artificial xanthine and xanthine oxidase system and depressed DNA synthesis in methylcholanthrene (MCA)-initiated C3H10T1/2 fibroblasts. in two-stage carcinogenesis experiments, neoxanthin at 0.2 micrograms/0.2 ml inhibited the formation of tumors that were induced sequentially by 7,12-dimethylbenz[a]anthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate (TPA) in the buccal pouch of Syrian Golden hamsters. To assess the ongoing process of carcinogenesis, the activity of ornithine decarboxylase (ODC), required for cell proliferation, was analyzed. Neoxanthin inhibited the activity of ODC when animals were treated with neoxanthin one hour before the application of TPA in two-stage carcinogenesis. However, neoxanthin did not inhibit ODC activity when animals were treated with neoxanthin one hour before the application of DMBA in two-stage carcinogenesis, and there was no subsequent tumor formation. In a short-term anti-initiation experiment, neoxanthin inhibited the covalent binding of isotope-labeled DMBA to DNA by 53%. These results indicate that neoxanthin inhibits the initiation stage and the promotion stage in two-stage carcinogenesis. This suggests that neoxanthin may act as a potential chemopreventive agent. 相似文献
100.
BL Chang 《Canadian Metallurgical Quarterly》1995,32(6):556-567
Construct validity of the concepts of pain, self-care deficit and impaired mobility was examined in a study of 120 medical-surgical patients. Factors derived from a factor analysis of the empirical signs and symptoms were: pain-factor and self-care deficit/impaired mobility-factor. The pain-factor clearly showed discriminant validity by differentiating between those with and without Clinical Nurse Specialist (CNS)-identified diagnosis of pain. An overlap between the defining characteristics of the diagnoses of self-care deficit and impaired mobility has been suggested in the present sample of patients. Limitations of the study and recommendations for further research are described. 相似文献