全文获取类型
收费全文 | 88篇 |
免费 | 5篇 |
专业分类
化学 | 10篇 |
力学 | 14篇 |
综合类 | 1篇 |
数学 | 52篇 |
物理学 | 16篇 |
出版年
2024年 | 1篇 |
2022年 | 3篇 |
2021年 | 3篇 |
2019年 | 2篇 |
2016年 | 4篇 |
2015年 | 1篇 |
2014年 | 3篇 |
2013年 | 3篇 |
2012年 | 2篇 |
2011年 | 7篇 |
2010年 | 2篇 |
2009年 | 7篇 |
2008年 | 7篇 |
2007年 | 7篇 |
2006年 | 4篇 |
2005年 | 4篇 |
2004年 | 6篇 |
2003年 | 1篇 |
2002年 | 2篇 |
2001年 | 1篇 |
2000年 | 3篇 |
1998年 | 2篇 |
1997年 | 3篇 |
1996年 | 1篇 |
1994年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有93条查询结果,搜索用时 31 毫秒
51.
Highly polluted forest/tilled soils and stream sediments from a mining and smelting area were subjected to single-extraction procedures to determine the extractable contents of Cd, Cu, Pb, and Zn. The results obtained from four widely used operationally defined single extraction tests were compared: deionised water, 0.01 mol L−1 CaCl2, 1 mol L−1 NH4NO3 and 0.005 mol L−1 DTPA. The analytical data were coupled with measurement of the pH and Eh in extracts, mineralogical investigations and thermodynamic modelling using the PHREEQC-2 code. The changes in the pH of the equilibrated suspensions significantly influenced the metal extractabilities, with higher values in the lower pH regions. Although the DTPA procedure generally extracted the highest amounts of metals, it was found to be unsuitable for highly organic acidic forest soils, where anionic metal-DTPA complexes are assumed to be re-adsorbed on the positively charged surfaces of soil organic matter and oxides. The NH4NO3 extraction was also unsuitable due to the high ionic strength (1 mol L−1), limiting the use of the thermodynamic speciation model and the formation of the Cu(NH3)2+ complex, leading to acidification of the suspension. 0.01 mol L−1 CaCl2 can be proposed as the most appropriate extraction medium, suitable for speciation modelling and analytical determinations using ICP-techniques and having an ionic strength similar to that of the soil solution. The metals are present in free ionic forms or chlorocomplexes in the CaCl2 extracts, preventing their re-adsorption on the positively charged surfaces of soil solids (organic matter, Fe- and Mn-oxides) in acidic and circum-neutral conditions. 相似文献
52.
Monitoring the effect of chemicals on biological communities. The biofilm as an interface 总被引:1,自引:0,他引:1
Sabater S Guasch H Ricart M Romaní A Vidal G Klünder C Schmitt-Jansen M 《Analytical and bioanalytical chemistry》2007,387(4):1425-1434
Biofilms can be regarded as early warning systems for detection of the effects of toxicants on aquatic systems, because they
have been successfully used for detection of other environmental stressors (e.g. pH, salinity, organic pollution). A variety
of methods is used for detection of the effects of toxicants by use of biofilms. The methods range from structurally-based
to functionally-based, and from in vitro-based to systemic approaches. Physiological approaches may be appropriate for detection
of acute effects. Among these methods, photosynthesis is more related to the effect of toxicants affecting algal communities,
directly or indirectly, and extracellular enzyme activity is less specific. Selecting one or the other may depend on the suspected
direct effect of the toxicant. Integrated studies have revealed the relevance of toxicants to top-down or bottom-up regulation
of the biofilm community. Persistent or chronic effects should affect other biofilm indicators, for example growth or biomass-related
factors (e.g. chlorophyll), or community composition. Among these, community composition might better reflect the effects
of the toxicant(s), because this may cause a shift from a sensitive to a progressively tolerant community. Community composition-based
approaches do not usually adequately reflect cause–effect relationships and require complementary analysis of properties affected
in the short-term, for example physiological properties. The current array of methods available must be wisely combined to
disentangle the effects of chemicals on biofilms, and whether these effects are transient or persistent, to successfully translate
the chemical action of toxicants into the effect they might have on the river ecosystem. 相似文献
53.
A microchip device to enhance free flow electrophoresis using controllable pinched sample injections
Micro free flow electrophoresis (µFFE) is a valuable technique capable of high throughput rapid microscale electrophoretic separation along with mild operating conditions. However, the stream flow separation nature of free flow electrophoresis affects its separation performance with additional stream broadening due to sample stream deflection. To reduce stream broadening and enhance separation performance of µFFE, we presented a simple microfluidic device that enables injection bandwidth control. A pinched injection was formed in the reported µFFE system using operating buffer at sample flow rate ratio (r) setting. Initial bandwidth at the entrance of separation chamber can be shrunk from 800 to 30 µm when r increased from 1 to 256. Stream broadening at the exit of separation chamber can be reduced by about 96% when r increased from 4 to 128, according to both theoretical and experimental results. Moreover, the separation resolution for a dye mixture was enhanced by a factor of 4 when r increased from 16 to 128, which corresponded to an 80% reduction in sample initial bandwidth. Furthermore, a similar enhancement on amino acids separation was obtained by using injection control in the reported µFFE device and readily integrated into online/offline sample preparation and/or downstream analysis procedures. 相似文献
54.
55.
56.
The advantages of open, vertical-field, magnetic resonance-guided, focused ultrasound surgery (MRgFUS) are attractive. The inverse technique using the bi-boundary conditions is proposed to design a uterine-oriented intraoperative RF coil with an ultrasound aperture for the MRgFUS system. In the current proposed scheme, the desired magnetic field of the RF coil was set to completely overlap the target organ. The current density distribution on the RF coil surface, accounting for the expected magnetic field, was solved using the inverse technique. The stream function was available through the ‘discretization’ of the current density distribution on the RF coil surface. The coil windings were obtained from the contour plot of the stream function. As a modification of previous designs, the bi-boundary conditions are proposed in the inverse technique for the existence of the ultrasound aperture. Based on the obtained coil windings, a prototype coil was constructed. MR imaging of the phantom and the human body was performed to show the efficacy of the prototype coil. The results of temperature measurement using the prototype coil in a 0.4-T MR system were satisfactory. The performance of the prototype coil improved compared with the previously reported design. 相似文献
57.
In this paper, we introduce a new class of PRSGs, called partitioned pseudorandom sequence generators(PPRSGs), and propose an RFID authentication protocol using a PPRSG, called S-protocol. Since most existing stream ciphers can be regarded as secure PPRSGs, and stream ciphers outperform other types of symmetric
key primitives such as block ciphers and hash functions in terms of power, performance and gate size, S-protocol is expected to be suitable for use in highly constrained environments such as RFID systems. We present a formal
proof that guarantees resistance of S-protocol to desynchronization and tag-impersonation attacks. Specifically, we reduce the availability of S-protocol to pseudorandomness of the underlying PPRSG, and the security of the protocol to the availability. Finally, we give
a modification of S-protocol, called S*-protocol, that provides mutual authentication of tag and reader.
相似文献
58.
59.
60.
The expectation and variance of the joint linear complexity of random periodic multisequences 总被引:4,自引:1,他引:3
The linear complexity of sequences is one of the important security measures for stream cipher systems. Recently, in the study of vectorized stream cipher systems, the joint linear complexity of multisequences has been investigated. By using the generalized discrete Fourier transform for multisequences, Meidl and Niederreiter determined the expectation of the joint linear complexity of random N-periodic multisequences explicitly. In this paper, we study the expectation and variance of the joint linear complexity of random periodic multisequences. Several new lower bounds on the expectation of the joint linear complexity of random periodic multisequences are given. These new lower bounds improve on the previously known lower bounds on the expectation of the joint linear complexity of random periodic multisequences. By further developing the method of Meidl and Niederreiter, we derive a general formula and a general upper bound for the variance of the joint linear complexity of random N-periodic multisequences. These results generalize the formula and upper bound of Dai and Yang for the variance of the linear complexity of random periodic sequences. Moreover, we determine the variance of the joint linear complexity of random periodic multisequences with certain periods. 相似文献