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   621篇
  免费   29篇
  国内免费   2篇
电工技术   5篇
化学工业   149篇
金属工艺   27篇
机械仪表   11篇
建筑科学   19篇
矿业工程   3篇
能源动力   5篇
轻工业   55篇
水利工程   2篇
石油天然气   1篇
无线电   47篇
一般工业技术   85篇
冶金工业   183篇
原子能技术   9篇
自动化技术   51篇
  2023年   5篇
  2022年   9篇
  2021年   8篇
  2020年   6篇
  2019年   9篇
  2018年   12篇
  2017年   4篇
  2016年   14篇
  2015年   16篇
  2014年   15篇
  2013年   14篇
  2012年   18篇
  2011年   18篇
  2010年   21篇
  2009年   14篇
  2008年   14篇
  2007年   26篇
  2006年   16篇
  2005年   15篇
  2004年   20篇
  2003年   20篇
  2002年   25篇
  2001年   17篇
  2000年   13篇
  1999年   10篇
  1998年   52篇
  1997年   30篇
  1996年   19篇
  1995年   19篇
  1994年   14篇
  1993年   21篇
  1992年   13篇
  1991年   4篇
  1990年   4篇
  1989年   4篇
  1988年   4篇
  1987年   4篇
  1985年   9篇
  1984年   9篇
  1983年   8篇
  1982年   8篇
  1981年   7篇
  1980年   4篇
  1978年   5篇
  1977年   5篇
  1976年   11篇
  1970年   3篇
  1966年   3篇
  1956年   4篇
  1935年   3篇
排序方式: 共有652条查询结果,搜索用时 15 毫秒
51.
STUDY DESIGN: Dissection and observation of the dorsal root ganglion of the second cervical spinal nerve bilaterally. OBJECTIVES: To determine the position of the C2 dorsal root ganglion and the heights of the C2 ganglion and its corresponding foramen. SUMMARY OF BACKGROUND DATA: The dorsal root ganglion has been well studied in the middle and lower cervical spine and in the lumbar spine. In no previous study are the position and height of the C2 dorsal root ganglion and its corresponding foramen described. METHODS: Fifteen cadaveric cervical spines were exposed posteriorly. The C2 nerve roots and ganglia were dissected between the arch of the atlas and the lamina of the axis. The heights of the C2 ganglion and foramen were measured. The location of the C2 ganglion was also macroscopically observed. RESULTS: The heights of the C2 ganglion and foramen are 5.7 +/- 0.8 mm and 7.7 +/- 1.2 mm, respectively. The C2 ganglion occupies 76% of the foramen height. All C2 dorsal root ganglia are confined within foramens between the arch of the atlas and the lamina of the axis. CONCLUSIONS: The C2 dorsal dorsal root ganglia are all proximally placed and occupy most of the foramen height, which may render the C2 ganglion vulnerable to entrapment.  相似文献   
52.
Many clinical and research situations require maintenance of isocapnia, which occurs when alveolar ventilation (V'A) is matched to CO2 production. A simple, passive circuit that minimizes changes in V'A during hyperpnoea was devised. It is comprised of a manifold, with two gas inlets, attached to the intake port of a nonrebreathing circuit or ventilator. The first inlet receives a flow of fresh gas (CO2=0%) equal to the subject's minute ventilation (V'E). During hyperpnoea, the balance of V'E is drawn (inlet 2) from a reservoir containing gas, the carbon dioxide tension (PCO2) approximates that of mixed venous blood and therefore contributes minimally to V'A. Nine normal subjects breathed through the circuit for 4 min at 15-31 times resting levels. End-tidal PCO2 (Pet,CO2) at rest, 0, 1.5 and 3.0 min were (mean+/-SE) 5.1+/-0.1 kPa (38.1+/-1.1 mmHg), 4.9+/-0.1 kPa (36.4+/-1.1 mmHg), 5.0+/-0.2 kPa (37.8+/-1.6 mmHg) and 5.0+/-0.2 kPa (37.6+/-1.4 mmHg) (p=0.53, analysis of variance (ANOVA)), respectively; without the circuit, Pet,CO2 would be expected to have decreased by at least 2.7 kPa (20 mmHg). Six anaesthetized, intubated dogs were first ventilated at control levels and then hyperventilated by stepwise increases in either respiratory frequency (fR) from 10 to 24 min(-1) or tidal volume (VT) from 400 to 1,200 mL. Increases in fR did not significantly affect arterial CO2 tension (Pa,CO2) (p=0.28, ANOVA). Only the highest VT decreased Pa,CO2 from control (-0.5 +/- 0.3 kPa (-3.4 +/- 2.3 mmHg), p<0.05). In conclusion, this circuit effectively minimizes changes in alveolar ventilation and therefore arterial carbon dioxide tension during hyperpnoea.  相似文献   
53.
54.
In this paper we address the topics of scale-space and phase-based image processing in a unifying framework. In contrast to the common opinion, the Gaussian kernel is not the unique choice for a linear scale-space. Instead, we chose the Poisson kernel since it is closely related to the monogenic signal, a 2D generalization of the analytic signal, where the Riesz transform replaces the Hilbert transform. The Riesz transform itself yields the flux of the Poisson scale-space and the combination of flux and scale-space, the monogenic scale-space, provides the local features phase-vector and attenuation in scale-space. Under certain assumptions, the latter two again form a monogenic scale-space which gives deeper insight to low-level image processing. In particular, we discuss edge detection by a new approach to phase congruency and its relation to amplitude based methods, reconstruction from local amplitude and local phase, and the evaluation of the local frequency.  相似文献   
55.
Stress-controlled, low-cycle, push-pull fatigue tests were performed on three variants of the bearing steel SAE 52100 with slightly different compositions and heat treatments. The experiments demonstrated differences in the cyclic plastic behaviour of differently hardened steels (bainitically-hardened and martensitically-hardened, respectively), whereas the two martensitic variants, which differ in composition, behaved very similarly. Bainitically-hardened SAE 52100 steel exhibited initial hardening followed by cyclic softening above a stress amplitude level of 1200 MPa. In contrast, the martensitically-hardened variants showed a pronounced cyclic hardening. The deformation behaviour of the martensitically-hardened bearing steel in a monotonic tensile test and during the first cycles can be well understood on the basis of the transformation of retained austenite. This process leads to an onset of plastic deformation at lower stresses compared to the bainitically-hardened bearing steel. As a result of the subsequent cyclic hardening of the martensitic variants, the CSS curves are almost identical for the differently hardened conditions under investigation. Additional tests under pulsating compression documented that a high negative mean stress enhances the cyclic plasticity.  相似文献   
56.
Recently, Gray and Ostby (Toxicol. Appl. Pharmacol. 133, 285-294, 1995) reported that in utero and lactational TCDD exposure causes striking abnormalities in the rat female reproductive system, including reduced fecundity and vaginal threads. The mechanism by which TCDD induces such abnormalities is unknown. Thus, we sought to determine: (1) whether TCDD reduced fecundity by destroying ovarian follicles and (2) whether the vaginal threads resulted from a TCDD-induced developmental defect during embryogenesis or abnormal vaginal opening at puberty. Pregnant Holtzman rats were treated with 1.0 microgram TCDD/kg or vehicle by a single oral dose on gestation day (GD) 11, 15, or 18. Female offspring were monitored for vaginal opening and terminated on postnatal days 2, 21, and 42. The reproductive tract was removed and evaluated for structural abnormalities. The number of primordial follicles also was determined for each ovary. TCDD exposure on GD 11, 15, or 18 did not change the day of vaginal opening, affect ovarian morphology, or reduce the number of primordial follicles. However, this exposure induced the cleft clitoris and vaginal thread originally described by Gray and Ostby (1995) in approximately 55-96% and 36-44% of the litters in our study, respectively. Histologically the thread presented as a thick cord of mesenchyme surrounded by epithelial cells. This defect was clearly visible in histological sections at birth and was noted in the closed vaginas of prepubertal animals. These data suggest that in utero and lactational exposure to TCDD does not reduce the size of the primordial follicle pool; however, it induces developmental abnormalities in the vaginal canal.  相似文献   
57.
Sanchez  E. Sommer  P. Menu  J. Iseli  C. 《Micro, IEEE》1987,7(6):29-40
Switzerland's first 16-bit processor design may lead to a system capable of easing HLL implementation. It has already led to a better understanding of complex ICs.  相似文献   
58.
New results for the enthalpy of mixing of liquid Fe-Mn alloys, the enthalpy of formation of γ-Fe-Mn solid solutions and the heat capacity of α-Fe-Mn and γ-Fe-Mn alloys obtained by isoperibolic calorimetry and differential thermal analysis (DTA) measurements have been used for the reassessment of the molar Gibbs energy of the various phases of the Fe-Mn system. Experimental information about martensitic transformation temperatures in Fe-Mn alloys published recently was incorporated in the updating of the occurring metastable equilibria. The present reevaluation results in a good fit with all available experimental data and is compared with the results of previous assessments.  相似文献   
59.
Currently there is limited information available on the accuracy and precision of relative isotopic abundance (RIA) measurements using high-resolution direct-infusion mass spectrometry (HR DIMS), and it is unclear if this information can benefit automated peak annotation in metabolomics. Here we characterize the accuracy of RIA measurements on the Thermo LTQ FT Ultra (resolution of 100,000-750,000) and LTQ Orbitrap (R = 100,000) mass spectrometers. This first involved reoptimizing the SIM-stitching method (Southam, A. D. Anal. Chem. 2007, 79, 4595-4602) for the LTQ FT Ultra, which achieved a ca. 3-fold sensitivity increase compared to the original method while maintaining a root-mean-squared mass error of 0.16 ppm. Using this method, we show the quality of RIA measurements is highly dependent on signal-to-noise ratio (SNR), with RIA accuracy increasing with higher SNR. Furthermore, a negative offset between the theoretical and empirically calculated numbers of carbon atoms was observed for both mass spectrometers. Increasing the resolution of the LTQ FT Ultra lowered both the sensitivity and the quality of RIA measurements. Overall, although the errors in the empirically calculated number of carbons can be large (e.g., 10 carbons), we demonstrate that RIA measurements do improve automated peak annotation, increasing the number of single empirical formula assignments by >3-fold compared to using accurate mass alone.  相似文献   
60.
Recent studies of the factors regulating neurogenesis in vertebrates reveal three emerging themes. First, the number of cellular stages involved in this process may be greater than has previously been appreciated. Second, homologues of genes that regulate neurogenesis in invertebrates appear to play analogous roles in development of vertebrate nervous systems. Third, extrinsic factors can act to regulate neuron number during neurogenesis by controlling survival and differentiation, and not simply proliferation, of neural progenitor cells.  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