全文获取类型
收费全文 | 3601篇 |
免费 | 13篇 |
国内免费 | 1篇 |
专业分类
电工技术 | 2篇 |
综合类 | 4篇 |
化学工业 | 85篇 |
金属工艺 | 19篇 |
机械仪表 | 5篇 |
建筑科学 | 47篇 |
矿业工程 | 1篇 |
能源动力 | 7篇 |
轻工业 | 46篇 |
水利工程 | 1篇 |
无线电 | 23篇 |
一般工业技术 | 57篇 |
冶金工业 | 3297篇 |
原子能技术 | 1篇 |
自动化技术 | 20篇 |
出版年
2023年 | 3篇 |
2021年 | 4篇 |
2020年 | 4篇 |
2019年 | 3篇 |
2018年 | 8篇 |
2017年 | 6篇 |
2016年 | 4篇 |
2015年 | 6篇 |
2014年 | 4篇 |
2013年 | 14篇 |
2012年 | 14篇 |
2011年 | 23篇 |
2010年 | 25篇 |
2009年 | 13篇 |
2008年 | 13篇 |
2007年 | 18篇 |
2006年 | 17篇 |
2005年 | 8篇 |
2004年 | 11篇 |
2003年 | 15篇 |
2002年 | 9篇 |
2001年 | 15篇 |
2000年 | 15篇 |
1999年 | 124篇 |
1998年 | 1068篇 |
1997年 | 662篇 |
1996年 | 386篇 |
1995年 | 226篇 |
1994年 | 165篇 |
1993年 | 211篇 |
1992年 | 23篇 |
1991年 | 41篇 |
1990年 | 28篇 |
1989年 | 35篇 |
1988年 | 26篇 |
1987年 | 26篇 |
1986年 | 19篇 |
1985年 | 17篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1982年 | 13篇 |
1981年 | 17篇 |
1980年 | 23篇 |
1979年 | 4篇 |
1978年 | 11篇 |
1977年 | 73篇 |
1976年 | 136篇 |
1973年 | 6篇 |
1972年 | 3篇 |
1955年 | 6篇 |
排序方式: 共有3615条查询结果,搜索用时 0 毫秒
81.
In this paper, we consider the manpower allocation problem with time windows, job-teaming constraints and a limited number of teams (m-MAPTWTC). Given a set of teams and a set of tasks, the problem is to assign to each team a sequential order of tasks to maximize the total number of assigned tasks. Both teams and tasks may be restricted by time windows outside which operation is not possible. Some tasks require cooperation between teams, and all teams cooperating must initiate execution simultaneously. We present an integer programming model for the problem, which is decomposed using Dantzig–Wolfe decomposition. The problem is solved by column generation in a branch-and-price framework. Simultaneous execution of tasks is enforced by the branching scheme. To test the efficiency of the proposed algorithm, 12 realistic test instances are introduced. The algorithm is able to find the optimal solution in 11 of the test instances. The main contribution of this article is the addition of synchronization between teams in an exact optimization context. 相似文献
82.
Patients with idiopathic and symptomatic restless legs syndrome (RLS) suffer from "dyskinesia while awake" or "daytime myoclonus" when at rest preceded by sensory symptoms. In order to characterise the RLS either as reflex movement or as voluntary movement we measured movement-related cortical potentials in 5 idiopathic and 8 uraemic RLS patients. Movements from both legs were polygraphically recorded concomitantly with cortical activity 2000 msec before to 500 msec after onset of EMG activity. These data were compared with a voluntary simulation of each patient's movement pattern and with 5 age-matched controls performing dorsiflexion of the right, left and both feet. Cortical activity preceding daytime myoclonus was absent in RLS patients whereas self-initiated leg movements in patients elicited onset times (1180-1380 msec) and amplitudes of Bereitschaftspotential (readiness potential) not significantly different from readiness potentials in control subjects (P > 0.05). Lack of movement-related potentials in myoclonus and/or dyskinesias during daytime in RLS patients is compatible with an involuntary mechanism of induction and points towards a subcortical or spinal origin of RLS. 相似文献
83.
Hypervolemia with hypertension often occurs 36-72 hours following massive blood and fluid replacement for hypovolemic shock. This syndrome of "fluid overload" has been attributed to the rapid intravascular flux of previously sequestered fluid in patients with impaired diuresis. This hypothesis was tested in 35 injured patients who received a mean of 9.3 L of blood and 17.4 L of salt during resucitation. The renal parameters measured soon after resuscitation included: 1) renal clearance of inulin (GFR), para-amino hippurate (ERPF), milliosmoles, sodium, and free water; 2) inulin space, renal vascular resistance (RVR), O2 consumption, renin, renal blood flow (RBF), and response to furosemide. Eighteen patients developed hypertension, hypervolemia, and respiratory insufficiency. When compared to the 17 normovolemic, non-hypertensive patients, the 18 hypervolemic patients had significantly increased RVR, with a significant decrease in RBF despite an increase in plasma volume and cardiac output. Furosemide produced less diuresis and natriuresis in the hypertensive patients. The balance between hypovolemia and "fluid overload" seemed percarious in the hypertensive patients. Peripheral renin and catecholamine levels were normal in both groups. Patients with post-traumatic "fluid overload" appear to have a combination of hypervolemia, respiratory insufficiency, hypertension, increased cardiac output, decreased extracellular fluid space, and decreased renal perfusion. These findings suggest that decreased interstitial fluid space compliance rather than "fluid overload" is the underlying factor leading to respiratory insufficiency. The therapeutic aspects of these findings are discussed. 相似文献
84.
From measurements of maximum and minimum motor nerve propagation velocity and neuronal excitability we conclude that there is a functional loss of motor units and distal nerve "dying back" in persons affected with unilateral acute cerebral vascular lesions. The study also suggests that transynaptic degeneration affects the lower motor neurone function on both sides. 相似文献
85.
M Hara YM Liu L Zhen IS Cohen H Yu P Danilo K Ogino JP Bilezikian MR Rosen 《Canadian Metallurgical Quarterly》1997,96(10):3704-3709
BACKGROUND: The classic calciotropic hormone parathyroid hormone (PTH) and its paracrine factor parathyroid hormone-related protein (PTHrP) both increase heart rate. METHODS AND RESULTS: We used standard electrophysiological techniques to study the effects of PTH and PTHrP on isolated rabbit sinus node, isolated canine Purkinje fibers, and disaggregated rabbit sinus node myocytes. Sinus node maximum diastolic potential, activation voltage, and amplitude were unchanged by PTH or PTHrP (P>.05). However, the slope of phase 4 and the automatic rate were increased at PTH and PTHrP > or = 10 nmol/L (P<.05). Comparable results were seen in canine Purkinje fibers. We then used the perforated-patch technique to study the I(f) pacemaker current in sinus node. PTH 12.5 nmol/L and PTHrP 12.5 to 18 nmol/L increased I(f) at -65 mV by 68+/-41% (n=5) and 69+/-50% (n=5), respectively. Actions of both agents were reversible. The increase in I(f) appeared to result from a change in maximal conductance and not a shift in the voltage dependence of activation. CONCLUSIONS: These observations provide, for the first time, direct electrophysiological support for the chronotropic actions of PTH and PTHrP. They suggest that classic hormones and paracrine factors can have multiple functions and that in the case of PTH and PTHrP, a newly recognized action is to alter automaticity directly. 相似文献
86.
Here we analyze the problem of determining whether experimentally measured spontaneous miniature end-plate currents (MEPCs) indicate that quanta are composed of subunits. The properties of MEPCs at end plates with or without secondary clefts at the neuromuscular junction are investigated, using both stochastic and deterministic models of the action of a quantum of transmitter. It is shown that as the amount of transmitter in a quantum is increased above about 4000 acetylcholine (ACh) molecules there is a linear increase in the size of the MEPC. It is possible to then use amplitude-frequency histograms of such MEPCs to detect a subunit structure, as there is little potentiation effect above 4000 ACh molecules. Autocorrelation and power spectral analyses of such histograms establish that their subunit structure can be detected if the coefficient of variation of the subunit size is less than about 0.12 or, if electrical noise is added, about 0.1. Positive gradients relate the rise time and half-decay times of MEPCs to their amplitude, even in the absence of potentiating effects; these gradients are shallower at motor nerve terminals that possess secondary clefts. The effect of asynchronous release of subunits is also investigated. The criteria determined by this analysis for identifying a subunit composition in the quantum are applied to an amplitude-frequency histogram of MEPCs recorded from a small group of active zones at a visualized amphibian motor-nerve terminal. This did not provide evidence for a subunit structure. 相似文献
87.
88.
89.
Untersuchungen zur Dauerfestigkeit carbonitrierter Proben aus den Einsatzstählen 18CrNiMo7‐6und 20MnCr5 下载免费PDF全文
C. Stöberl H. Bomas B. Clausen F. Hoffmann H.‐W. Zoch 《Materialwissenschaft und Werkstofftechnik》2015,46(6):533-549
The endurance limit of carbonitrided specimens of 18CrNiMo7‐6 and 20MnCr5 case hardening steels were examined under bending conditions. Four variants were manufactured differing in heat treatment and surface conditions. Besides conventional gas carburised variants, carbonitrided specimens with a low and a raised nitrogen content of 0.2 and 0.45% at the surface as well as a carbonitrided and ground variant were examined. The specimens were tested with stress ratios R = –1 and 0. Depending on the nitrogen content cyclic loading evokes compressive residual stresses at the specimens' surfaces, caused by elastic‐plastic deformation. Specimens of the ground variant show additionally a decrease of the residual austenite content when cycled. The induced compressive residual stresses are substantially responsible for high endurance limits at both stress ratios and are the key for understanding the beneficial effect of carbonitriding in this regard. 相似文献
90.
As ventilation filters accumulate particles removed from the airstream, they become emitters of sensory pollutants that degrade indoor air quality. Previously we demonstrated that an F7 bag-type filter that incorporates activated carbon (a “combination filter”) reduces this adverse effect compared to an equivalent filter without carbon. The aim of the present study was to examine how the amount of activated carbon (AC) used in combination filters affects their ability to remove both sensory offending pollutants and ozone. A panel evaluated the air downstream of four different filters after each had continuously filtered outdoor suburban air over a period of 6 months. Interim assessments (mid-term evaluation) were performed after 3 months. During both assessments, four unused filters, identical in type to the loaded filters, were also evaluated. The evaluated filters included a conventional F7 fiberglass filter and three modifications of a bag-type fiberglass combination filter: the “Heavy” corresponded to a commercially available filter containing 400 g of carbon per square meter of filter area, the “Medium” contained half as much carbon (200 g/m2), and the “Light” contained a quarter as much carbon (100 g/m2). Each filter was weighed at the beginning of the soiling period and after 3 and 6 months of service. Additionally, up- and down-stream ozone concentrations and filter pressure drops were measured monthly. Following 6 months of service, the air downstream of each of the combination filters was judged to be significantly better than the air downstream of the 6-month-old F7 filter, and was comparable to that from an unused F7 filter. Additionally, the combination filters removed more ozone from the air than the F7 filter, with their respective fractional removal efficiencies roughly scaling with their carbon content. 相似文献