全文获取类型
收费全文 | 46217篇 |
免费 | 4920篇 |
国内免费 | 3101篇 |
专业分类
电工技术 | 2866篇 |
技术理论 | 1篇 |
综合类 | 4142篇 |
化学工业 | 7555篇 |
金属工艺 | 4464篇 |
机械仪表 | 1923篇 |
建筑科学 | 3124篇 |
矿业工程 | 1933篇 |
能源动力 | 1846篇 |
轻工业 | 3824篇 |
水利工程 | 1238篇 |
石油天然气 | 2631篇 |
武器工业 | 651篇 |
无线电 | 4256篇 |
一般工业技术 | 5011篇 |
冶金工业 | 3870篇 |
原子能技术 | 1079篇 |
自动化技术 | 3824篇 |
出版年
2024年 | 111篇 |
2023年 | 627篇 |
2022年 | 1246篇 |
2021年 | 1622篇 |
2020年 | 1712篇 |
2019年 | 1402篇 |
2018年 | 1358篇 |
2017年 | 1762篇 |
2016年 | 1906篇 |
2015年 | 1970篇 |
2014年 | 2928篇 |
2013年 | 3221篇 |
2012年 | 3586篇 |
2011年 | 3713篇 |
2010年 | 2801篇 |
2009年 | 2698篇 |
2008年 | 2417篇 |
2007年 | 2983篇 |
2006年 | 2556篇 |
2005年 | 2186篇 |
2004年 | 1864篇 |
2003年 | 1629篇 |
2002年 | 1330篇 |
2001年 | 1159篇 |
2000年 | 923篇 |
1999年 | 778篇 |
1998年 | 641篇 |
1997年 | 519篇 |
1996年 | 496篇 |
1995年 | 445篇 |
1994年 | 338篇 |
1993年 | 264篇 |
1992年 | 231篇 |
1991年 | 188篇 |
1990年 | 155篇 |
1989年 | 120篇 |
1988年 | 71篇 |
1987年 | 50篇 |
1986年 | 43篇 |
1985年 | 37篇 |
1984年 | 31篇 |
1983年 | 17篇 |
1982年 | 16篇 |
1981年 | 13篇 |
1980年 | 19篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1975年 | 9篇 |
1959年 | 8篇 |
1951年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Association of size‐fractionated indoor particulate matter and black carbon with heart rate variability in healthy elderly women in Beijing 下载免费PDF全文
W. Dong L. Pan H. Li M. R. Miller M. Loh S. Wu J. Xu X. Yang J. Shan Y. Chen F. Deng X. Guo 《Indoor air》2018,28(3):373-382
Associations between size‐fractionated indoor particulate matter (PM) and black carbon (BC) and heart rate variability (HRV) and heart rate (HR) in elderly women remain unclear. Twenty‐nine healthy elderly women were measured for 24‐hour HRV/HR indices. Real‐time size‐fractionated indoor PM and BC were monitored on the same day and on the preceding day. Mixed‐effects models were applied to investigate the associations between pollutants and HRV/HR indices. Increases in size‐fractionated indoor PM were significantly associated with declines in power in the high‐frequency band (HF), power in the low‐frequency band (LF), and standard deviation of all NN intervals (SDNN). The largest decline in HF was 19% at 5‐minute moving average for an interquartile range (IQR) increase (24 μg/m3) in PM0.5. The results showed that smaller particles could lead to greater reductions in HRV indices. The reported associations were modified by body mass index (BMI): Declines in HF at 5‐minute average for an IQR increase in PM0.5 were 34.5% and 1.0% for overweight (BMI ≥25 kg/m2) and normal‐weight (BMI <25 kg/m2) participants, respectively. Moreover, negative associations between BC and HRV indices were found to be significant in overweight participants. Increases in size‐fractionated indoor PM and BC were associated with compromised cardiac autonomic function in healthy elderly women, especially overweight ones. 相似文献
82.
83.
采用XRD、SEM、残余应力测试仪、维氏硬度计、测长机等手段对合金物相、微观组织、残余应力、硬度、线膨胀率进行表征,研究自然时效对GT35合金组织与性能的影响。结果表明,GT35合金在自然时效过程中,增强相TiC的形貌与粒径没有明显变化。当自然时效时间小于15天时,GT35合金的硬度、残余应力与线膨胀率逐渐增加,随后逐渐减小并趋于稳定。当自然时效超过90天后,GT35合金的组织与性能趋于稳定,其硬度、残余应力和线膨胀率分别为945.7 HV5、-184 MPa和-2.22×10-5。 相似文献
84.
研究了不同工艺参数对980 MPa级连续退火双相钢组织及力学性能的影响,利用光学显微镜、透射电镜(TEM)以及拉伸试验对双相钢的微观组织和力学性能进行测试及分析。结果表明:DP980钢的退火组织主要由铁素体、马氏体岛和少量的贝氏体组成,马氏体岛附近的位错密度较高。随着均热温度的升高,DP980钢的抗拉强度呈现先降低后升高的趋势,屈服强度与抗拉强度的趋势一致,伸长率先升高后降低。随着过时效温度的升高,DP980钢的抗拉强度和屈服强度降低,降低幅度较小,伸长率上升,但变化不明显,说明通过调整过时效温度来调控其力学性能的作用较小。 相似文献
85.
Commonly used flow rate measurement systems provide an accurate and stable output value of the quasi-stationary flow rate. In some pump types as e.g. single-blade pumps significant flow rate fluctuations may occur even in steady operation points due to rotor-stator interaction. For the analysis of the time-resolved flow rate a new measurement and evaluation method is presented based on an electromagnetic flow meter. Internal averaging of the flow meter is deactivated and the raw signal is evaluated directly with a sampling rate of 3 kHz. With ensemble-averaging in combination with an impeller position detection, interfering signals acting on the time-resolved measurement signal are filtered out. Accompanying numerical simulations of the pump flow circuit are carried out with a 1D method of characteristics and validated against well-established time-resolved pressure measurements of the pump flow. Experiment and simulation show a resembling trend of pressure as well as flow rate fluctuations over the entire operation range of the pump. Thus, by the combined utilisation of measurement and simulation technique, we assure the validity of the ensemble-averaged flow rate fluctuation results. We find that the flow rate fluctuations show a consistent phase shift to the pressure fluctuations that increases towards overload. The flow rate amplitude is an order of magnitude smaller than the amplitude of the pressure fluctuations. 相似文献
86.
Tadeusz Zając Agnieszka Klimek-Kopyra Ryszard Mazurek Andrzej Oleksy Jacek Strojny 《Journal of Natural Fibers》2020,17(8):1199-1211
ABSTRACT Nine sowing densities of linseed were compared in a two-year experiment. Self-regulation of the density of a linseed stand resulted in a differentiated branching of the plants. The number of capsules on a linseed shoot, as well as seeds in the capsule, and the number of seeds from the shoot were highly varied from year to year, and there is even a stronger differentiation in the weight of 1000 seeds. Linear relationships were noted between the mass of seeds from the linseed plant and the number of capsules. The yield of linseed was affected by both factors. An increasing sowing rate reduced the seed yield from the shoot and the plant. At lower seeding rates, the lateral shoots contributed in the seeds yield of the plant to a higher degree compared to the main ones. 相似文献
87.
基于Atmega16A单片机自主设计一套地压监测系统。整个过程完成了硬件的购置、软件的设计、数据采集程序的编写、系统管线布置方式的设计以及系统的调试等工作。本监测系统的设计及装置的开发具有灵敏度高、功耗低,信号传输可靠、实时、远程和多元化数据监测,后期其他类型数据监测可扩展性强,系统成本低等优点。采集数据的声发射能率分析结果显示,有两次较大的声发射率值与监测范围内两次较大冒顶事故都可以很好的对应。希望该系统的应用能为矿山地下开采过程中地压活动的监测和预警提供技术支持。 相似文献
88.
The purpose of this paper is to determine the effect of the air torque position (ATP) damper hysteresis on the accuracy of air velocity measurements and the adequacy of the ATP damper mathematical model tested in the present study. A total of three dampers with non-cascading blades were examined: a damper with a single flat blade, a damper with two opposed flat blades, and a damper with two flat blades - one of which is a measuring blade and the other is fixed in the horizontal direction. Furthermore, a total of two damper installation locations in a duct system were examined as well: at the duct exit with a straight duct section preceding the damper and within the duct itself with straight duct sections both preceding and following the damper. The ATP damper hysteresis was found to exert a significant effect on the accuracy of air velocity measurements and the adequacy of the ATP damper mathematical model tested if the ATP damper is more open, i.e. with lower blade angles of attack. Moreover, the ATP damper installed at the duct exit was found to be more susceptible to hysteresis effects than the damper located within the duct. 相似文献
89.
为了使7075铝合金薄壁件表面喷丸加工应力得到松弛和均化,运用ANSYS建立平台式振动时效有限元模型,通过振动模态和谐响应分析,获得最佳激振频率和试样装夹位置。在此基础上,将薄壁框架件置于平台上进行振动时效处理,以验证仿真分析结果,评价其对试样表面应力作用效果。结果表明:在亚共振频率为112 Hz时,试样在平台零振幅和最大弯曲位置处分别对称布置,时效后表面均形成应力松弛,松弛率分别为29.5%和33.3%,不均匀波动分别为20.3%和20.7%,而沿振动方向松弛程度不均匀,说明激振力造成了材料表面组织屈服,且屈服程度与应力强度有关。提出再时效-位置交叠方法,使应力松弛不均匀度分别下降到9.9%和15.9%,应力均化得到改善,该研究工作为薄壁类零件表面应力均匀化提供了参考。 相似文献
90.
Chatter is one of the most critical problems that causes poor surface quality and restriction of machining efficiency. Spindle speed variation (SSV) is a well-known technique for suppression of regenerative chatter. However, in the authors’ understanding, the chatter suppression effect diminishes when the spindle speed difference between the present and previous cutting moments is small. Furthermore, the stability changes largely according to the spindle speed variation profile which changes with the set condition of SSV parameters, e.g., nominal spindle speed, variation period and variation amplitude. Therefore, SSV parameters should be adequately set to avoid this limitation and to exert its effect throughout the entire duration of cutting. However, there is no clear methodology to determine the optimal condition. This paper presents the characteristics of chatter growth during SSV focusing on the change of chatter frequency, which lead to novel indices to evaluate the chatter stability when cutting with SSV. To verify the validity of the indices, time-domain simulations and the cutting experiments with triangular spindle speed variation (TSSV) are carried out. The influence of SSV parameters on the chatter stability is investigated from the simulation and experimental results. The limitations of widely utilized SSV profiles are discussed. 相似文献