全文获取类型
收费全文 | 635篇 |
免费 | 28篇 |
国内免费 | 16篇 |
专业分类
化学 | 296篇 |
力学 | 60篇 |
综合类 | 20篇 |
数学 | 165篇 |
物理学 | 138篇 |
出版年
2024年 | 2篇 |
2023年 | 3篇 |
2022年 | 26篇 |
2021年 | 54篇 |
2020年 | 26篇 |
2019年 | 20篇 |
2018年 | 22篇 |
2017年 | 28篇 |
2016年 | 30篇 |
2015年 | 26篇 |
2014年 | 44篇 |
2013年 | 37篇 |
2012年 | 37篇 |
2011年 | 30篇 |
2010年 | 13篇 |
2009年 | 41篇 |
2008年 | 18篇 |
2007年 | 34篇 |
2006年 | 24篇 |
2005年 | 27篇 |
2004年 | 21篇 |
2003年 | 27篇 |
2002年 | 23篇 |
2001年 | 11篇 |
2000年 | 17篇 |
1999年 | 7篇 |
1998年 | 10篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1986年 | 3篇 |
1984年 | 2篇 |
排序方式: 共有679条查询结果,搜索用时 15 毫秒
81.
饮食中微量元素砷的分布规律与人体健康关系 总被引:3,自引:1,他引:2
采用催化动力学光度法测定了六大类(共三十种)样品中的总砷含量,探讨了砷在食品中的分布规律,建立了砷在人体中的安全系效和积累系效的效学评价公式,用于评价砷与人体健康的关系,这对人们选择食物有一定的指导意义。 相似文献
82.
Broader Societal Issues of Nanotechnology 总被引:10,自引:5,他引:5
Nanoscale science and engineering are providing unprecedented understanding and control over the basic building blocks of matter, leading to increased coherence in knowledge, technology, and education. The main reason for developing nanotechnology is to advance broad societal goals such as improved comprehension of nature, increased productivity, better healthcare, and extending the limits of sustainable development and of human potential. This paper outlines societal implication activities in nanotechnology R&D programs. The US National Nanotechnology Initiative annual investment in research with educational and societal implications is estimated at about $30 million (of which National Science Foundation (NSF) awards about $23 million including contributions to student fellowships), and in nanoscale research with relevance to environment at about $50 million (of which NSF awards about $30 million and EPA about $6 million). An appeal is made to researchers and funding organizations worldwide to take timely and responsible advantage of the new technology for economic and sustainable development, to initiate societal implications studies from the beginning of the nanotechnology programs, and to communicate effectively the goals and potential risks with research users and the public. 相似文献
83.
The prevalence of voice problems among patients consulting the primary health care unit of a small Swedish town during 1984 was investigated. A study of the records of 11,606 patients indicated that 102 of them consulted their doctor mainly because of voice problems. The period prevalence of voice problems in the population of 20,049 people was 0.5%. A follow-up examination 1 year later indicated that 44% of these patients still had voice problems. Among the patients with a voice disorder diagnosis made by means of indirect laryngoscopy in 1984, 72% still had a voice disorder diagnosis at the follow-up. It is pointed out that the doctor who is seeing a patient with voice problems should make a thorough examination including indirect laryngoscopy. It is also important to discuss the patient's smoking habits and professional vocal strain to prevent recurrence. 相似文献
84.
An overview of the special issue of the Journal of Nanoparticle Research on Occupational and Environmental Health of nanotechnology
is presented. Papers published in this special issue show considerable progress in understanding nanoparticle toxicity, monitoring,
generation, dustiness, filtration, and applications of nanoparticles. More research is needed to ensure safe handling of nanomaterials
as nanotechnology continues to develop at an incredible pace.
相似文献
Chuen-Jinn TsaiEmail: |
85.
Use of the wavelet transform (WT) to study the forced vibrations in a rod in order to detect the presence of a defect is proposed. The axial vibrations in an inhomogeneous rod, produced by the application of different forces, are simulated by the Network Simulation Method and the obtained response is analyzed with the WT. An analysis of the detection and location of defects for several applied forces and defects generated by locally changing density or stiffness of the rod is presented. A noise test was carried out to check the robustness of the technique in real situations. 相似文献
86.
Phospholipid lung surfactant and nanoparticle surface toxicity: Lessons from diesel soots and silicate dusts 总被引:3,自引:0,他引:3
William E. Wallace Michael J. Keane David K. Murray William P. Chisholm Andrew D. Maynard Tong-man Ong 《Journal of nanoparticle research》2007,9(1):23-38
Because of their small size, the specific surface areas of nanoparticulate materials (NP), described as particles having at
least one dimension smaller than 100 nm, can be large compared with micrometer-sized respirable particles. This high specific
surface area or nanostructural surface properties may affect NP toxicity in comparison with micrometer-sized respirable particles
of the same overall composition. Respirable particles depositing on the deep lung surfaces of the respiratory bronchioles
or alveoli will contact pulmonary surfactants in the surface hypophase. Diesel exhaust ultrafine particles and respirable
silicate micrometer-sized insoluble particles can adsorb components of that surfactant onto the particle surfaces, conditioning
the particles surfaces and affecting their in vitro expression of cytotoxicity or genotoxicity. Those effects can be particle surface composition-specific. Effects of particle
surface conditioning by a primary component of phospholipid pulmonary surfactant, diacyl phosphatidyl choline, are reviewed
for in vitro expression of genotoxicity by diesel exhaust particles and of cytotoxicity by respirable quartz and aluminosilicate kaolin
clay particles. Those effects suggest methods and cautions for assaying and interpreting NP properties and biological activities. 相似文献
87.
In this study, nanoparticle emission of TiO2 nanopowder coated on different substrates including wood, polymer, and tile, was evaluated in a simulation box and measured
with a Scanning Mobility Particle Sizer (SMPS) for the first time. The coating process for the substrate followed the instructions
given by the supply company. In the simulation box, UV light, a fan, and a rubber knife were used to simulate the sun light,
wind, and human contacting conditions. Among the three selected substrates, tile coated with TiO2 nanopowder was found to have the highest particle emission (22 #/cm3 at 55 nm) due to nanopowder separation during the simulation process. The UV light was shown to increase the release of particle
below 200 nm from TiO2 nanopowder coating materials. The results show that, under the conditions of UV lamps, a fan and scraping motion, particle
number concentration or average emission rate decreases significantly after 60 and 90 min for TiO2/polymer and TiO2/wood, respectively. However, the emission rate continued to increase after 2 h of testing for TiO2/tile. It is suggested that nanoparticle emission evaluation is necessary for products with nanopowder coating. 相似文献
88.
Measuring particle size-dependent physicochemical structure in airborne
single walled carbon nanotube agglomerates 总被引:1,自引:1,他引:1
Andrew D. Maynard Bon Ki Ku Mark Emery Mark Stolzenburg Peter H. McMurry 《Journal of nanoparticle research》2007,9(1):85-92
As-produced single-walled carbon nanotube (SWCNT) material is a complex matrix of carbon nanotubes, bundles of nanotubes (nanoropes),
non-tubular carbon and metal catalyst nanoparticles. The pulmonary toxicity of material released during manufacture and handling
will depend on the partitioning and arrangement of these components within airborne particles. To probe the physicochemical
structure of airborne SWCNT aggregates, a new technique was developed and applied to aerosolized as-produced material. Differential
Mobility Analysis-classified aggregates were analyzed using an Aerosol Particle Mass Monitor, and a structural parameter Γ
(proportional to the square of particle mobility diameter, divided by APM voltage) derived. Using information on the constituent
components of the SWCNT, modal values of Γ were estimated for specific particle compositions and structures, and compared
against measured values. Measured modal values of Γ for 150 nm mobility diameter aggregates suggested they were primarily
composed of non-tubular carbon from one batch of material, and thin nanoropes from a second batch of material – these findings
were confirmed using Transmission Electron Microscopy. Measured modal values of Γ for 31 nm mobility diameter aggregates indicated
that they were comprised predominantly of thin carbon nanoropes with associated nanometer-diameter metal catalyst particles;
there was no indication that either catalyst particles or non-tubular carbon particles were being preferentially released
into the air. These results indicate that the physicochemistry of aerosol particles released while handling as-produced SWCNT
may vary significantly by particle size and production batch, and that evaluations of potential health hazards need to account
for this.
Disclaimer: The mention of any company or product does not constitute an endorsement by the Centers for Disease Control and
Prevention. The findings and conclusions in this paper are those of the authors and do not necessarily represent the views
of the National Institute for Occupational Safety and Health. 相似文献
89.
Exposure to indoor air pollution (IAP) from the combustion of biomass fuels is an important cause of morbidity and mortality in developing countries. In the work discussed in this paper we evaluated the endocrine activity of soot particles from biomass fuels by using yeast bioassay. These pollutants could have -galactosidase activity with a relative potency (RP) about 10–7–10–9 that of estradiol. Soot particles from wood and straw combustion only partially induced -galactosidase activity whereas others produced fully inductive activity in the yeast assay system. These pollutants did not have estrogen antagonist and progesterone agonist activity within the defined concentration range. However, these pollutants require 2–4 orders of magnitude higher IC50 to inhibit the activity of progesterone in a similar dose-response manner to mifepristone. We therefore propose that the endocrine activity of some environmental pollutants may be because of inhibition of the progesterone receptor (hPR). GC–MS results showed that substituted polycyclic aromatic hydrocarbon (PAH) compounds, substituted phenolic compounds and derivatives, aromatic carbonyl compounds, and phytosteroids in these soot particles may be mimicking endogenous hormones. 相似文献
90.
The scattering behavior of P-waves in piezoelectric composites with 1–3 connectivity is studied. The method of wave function expansion is adopted for the theoretical derivations. Analytical expressions are obtained for the distributions of mechanical displacement in z-direction along the circumferences of piezoelectric cylinders. These solutions are used to study the influence of each element of the stiffness matrix and the piezoelectric matrix on the various resonant modes of vibration. Numerical results obtained indicate that perturbations of the elements c44 and e15 significantly affect resonant frequencies and amplitudes, perturbations of c11 and c12 have pronounced effects on resonant modes of high frequencies also. However, the resonant modes are not so sensitive to the perturbations of c13, e31 and e33. The dynamic characteristics of 1–3 connectivity piezoelectric composites exposed here are meaningful for the design and manufacture of sensor/actuator elements by this kind of composites as well as the on-line health monitoring of the mechanical properties variations of the composites itself. 相似文献