首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   11334篇
  免费   601篇
  国内免费   2556篇
化学   13339篇
晶体学   102篇
力学   41篇
综合类   121篇
数学   95篇
物理学   793篇
  2024年   19篇
  2023年   201篇
  2022年   1069篇
  2021年   809篇
  2020年   729篇
  2019年   575篇
  2018年   513篇
  2017年   489篇
  2016年   638篇
  2015年   561篇
  2014年   532篇
  2013年   1026篇
  2012年   829篇
  2011年   632篇
  2010年   428篇
  2009年   602篇
  2008年   564篇
  2007年   566篇
  2006年   465篇
  2005年   407篇
  2004年   361篇
  2003年   304篇
  2002年   482篇
  2001年   184篇
  2000年   205篇
  1999年   191篇
  1998年   167篇
  1997年   130篇
  1996年   118篇
  1995年   114篇
  1994年   98篇
  1993年   79篇
  1992年   66篇
  1991年   63篇
  1990年   37篇
  1989年   61篇
  1988年   40篇
  1987年   15篇
  1986年   16篇
  1985年   14篇
  1984年   17篇
  1983年   10篇
  1982年   17篇
  1981年   5篇
  1980年   8篇
  1979年   9篇
  1978年   5篇
  1976年   6篇
  1975年   4篇
  1973年   3篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Previous research has shown that a region of the midbrain, the periaqueductal gray matter (PAG), is critical for vocalization. In this review, we describe the results of previous investigations in which we sought to find out how PAG neurons integrate the activity and precise timing of respiratory, laryngeal, and oral muscle activity for natural-sounding vocalization using the technique of excitatory amino acid microinjections in cats. In these studies, all surgical procedures were carried out under deep anaesthesia. In the precollicular decerebrate cat two general types of vocalization, classified as voiced and unvoiced, could be evoked by exciting neurons in the lateral part of the intermediate part of the PAG. The patterns of evoked electromyographic activity were strikingly similar to previously reported patterns of human muscle activity. Coordinated patterns of activity were evoked with just-threshold excitation leading to the conclusion that patterned muscle activity corresponding to the major categories of voiced and voiceless sound production are represented in the PAG. In a parallel series of human and animal experiments, we also determined that the speech and vocalization respiratory patterns are integrated and coordinated with afferent signals related to lung volume. These data have led to the proposal of a new hypothesis for the neural control of vocalization: that the PAG is a crucial brain site for mammalian voice production, not only in the production of emotional or involuntary sounds, but also as a generator of specific respiratory and laryngeal motor patterns essential for human speech and song  相似文献   
102.
双折射双频激光器频差特性分析   总被引:5,自引:0,他引:5  
郭继华  张书练 《光学学报》1996,16(6):16-720
用激光原理和晶体光学原理分析了腔内加旋光晶体输出光的频差特性,分析表明:当晶片旋转时,理论计算的频差永远小于晶体两本征模通过晶片位相差所决定的频差,有一特殊因子G^2起作用。G^2的物理意义是激光本征模在晶体两本征模上的投影比平方。这表明一个激光本征模在通过晶体时是部分以快模形式,部分以慢模形式,实际位相延迟介于两者之间,由于两者都介于快模和慢模之间,因而实际频差小于由快模和慢模决定的频差,理论计  相似文献   
103.
双折射双频激光器偏振特性的分析   总被引:5,自引:1,他引:5  
用激光原理和晶体光学原理分析了腔内加旋光晶体的激光器输出光的偏振特性,实验和分析表明,激光器输出两束不同频率的垂直线偏振光,激光器两端输出光的偏振面有一夹角a,且此夹角与晶轴和传光方向的夹角θ有关。理论计算结果在双折射双频激光器的实验结果符合很好。  相似文献   
104.
This paper seeks to determine the optimal settings for the deposition parameters, for TiO2 thin film, prepared on non-alkali glass substrates, by direct current (dc) sputtering, using a ceramic TiO2 target in an argon gas environment. An orthogonal array, the signal-to-noise ratio and analysis of variance are used to analyze the effect of the deposition parameters. Using the Taguchi method for design of a robust experiment, the interactions between factors are also investigated. The main deposition parameters, such as dc power (W), sputtering pressure (Pa), substrate temperature (°C) and deposition time (min), were optimized, with reference to the structure and photocatalytic characteristics of TiO2. The results of this study show that substrate temperature and deposition time have the most significant effect on photocatalytic performance. For the optimal combination of deposition parameters, the (1 1 0) and (2 0 0) peaks of the rutile structure and the (2 0 0) peak of the anatase structure were observed, at 2θ ∼ 27.4°, 39.2° and 48°, respectively. The experimental results illustrate that the Taguchi method allowed a suitable solution to the problem, with the minimum number of trials, compared to a full factorial design. The adhesion of the coatings was also measured and evaluated, via a scratch test. Superior wear behavior was observed, for the TiO2 film, because of the increased strength of the interface of micro-blasted tools.  相似文献   
105.
影响胰岛素荧光光谱的几个物理因素的测定和分析   总被引:4,自引:0,他引:4  
胰岛素是调节细胞代谢或生长的重要因子,也是多种细胞体外培养的促生长因子。本文通过荧光光谱,测定和分析了电场、浓度等物理因素对胰岛素荧光光谱的影响。证明电场、浓度和温度都能影响胰岛素荧光光谱,从而影响胰岛素多聚体的构象,并可能影响其功能。  相似文献   
106.
Structural and optical properties of 1 at % Al-doped Zn1−xMgxO (x=0–8%) powders prepared by sol–gel method were systematically investigated by means of X-ray diffraction, scanning electron microscopy, ultraviolet–visible absorbance measurement, photoluminescence and Raman scattering spectra. All the powders retained the hexagonal wurtzite structure of ZnO. The band gap and near band emission energies determined from absorbance and photoluminescence spectra increased linearly with increasing Mg content, respectively, which implied that the Mg worked effectively on ZnO band gap engineering, irrespective of Al codoping. However, according to the PL and Raman scattering studies, for the sample of x=8%, the Al doping efficiency was decreased by higher Mg codoping. On the other hand, the effect of Mg codoping on photocatalytic degradation of methylene orange was explored experimentally. The substitution of Mg ions at Zn sites shifted the conduction band toward higher energies and then enhanced the photocatalytic activity, while the incorporation of interstitial Mg ions and decreased Al doping efficiency for higher Mg doping sample (x=8%) reduced the photocatalytic activity.  相似文献   
107.
The sweep frequency ultrasound (SFU) and single frequency countercurrent ultrasound (SFCU) pretreatments were modeled and compared based on production of angiotensin I-converting enzyme (ACE) inhibitory peptides from garlic hydrolysates. Two mathematical models were developed to show the effect of each variable and their combinatorial interactions on ACE inhibitory activity. The optimum levels of the parameters in SFU were determined using uniform design, which revealed these as follows: total ultrasonic time 1.5 h, on-time of pulse 18 s and off-time of pulse 3 s. Under optimized conditions, the experimental values of SFU and SFCU were 65.88% and 67.78%, which agreed closely with the predicted values of 63.44% and 67.33%. The SFU and SFCU pretreatments both resulted in higher ACE inhibitory activity compared with untreated garlic (p < 0.05). However, there were no significant differences in the ACE inhibitory activities and IC50 values obtained from SFCU and SFU pretreatments under optimum conditions (p > 0.05).  相似文献   
108.
Decoquinate (1), an old and inexpensive coccidiostat, exhibited potent antimalarial activity, however, its antischistosomal activity against Schistosoma japonicum has not yet been evaluated. Based on decoquinate, a series of decoquinate derivatives was designed, synthesized, evaluated as a new class of antischistosomal agents against S. japonicum adult worms in vitro. Among them, compound 15 killed 100% of adult S. japonicum in 72 h at the concentration of 10 μmol/L in vitro, exhibited stronger wormkilling activity than PZQ in vitro and could serve as a promising lead compound to develop new antischistosomal agents.  相似文献   
109.
This study evaluated high pressure processing (P1 – 400?MPa/5?min; P2 – 550?MPa/2?min) and thermal pasteurization (TP – 70°C/30?s) effects on sweet cherry juice's microbiological and physicochemical parameters, during four weeks of refrigerated storage. All treatments reduced the microbiological load to undetectable levels not affecting total soluble solids and titratable acidity. The pH increased with all treatments, however, it decreased during storage. Phenols were differently affected: TP increased them by 6%, P1 had no effect while P2 decreased them by 11%. During storage, phenols in control and TP samples decreased by 26% and 20%, P1 samples decreased them by 11% whereas P2 showed no variation. TP had no effect on anthocyanins, while pressure treatments increased them by 8%. Anthocyanins decreased during storage, particularly in the control and P1 (decreasing 41%). All treatments had no effect on antioxidant activity until the 14th day, thereafter high pressure processing samples showed the highest antioxidant activity.  相似文献   
110.
作者合成了6氟3(取代)苯基4(1H)喹啉酮的5氯衍生物与7氯衍生物,二者极性差异小,难于分离,以混合物形式存在.所合成的化合物均未见文献报道,本文通过1HNMR上的化学位移、积分面积以及偶合常数确证了混合物中两组份的结构.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号