全文获取类型
收费全文 | 3296篇 |
免费 | 200篇 |
国内免费 | 158篇 |
专业分类
化学 | 1803篇 |
晶体学 | 46篇 |
力学 | 516篇 |
综合类 | 111篇 |
数学 | 188篇 |
物理学 | 990篇 |
出版年
2024年 | 8篇 |
2023年 | 48篇 |
2022年 | 114篇 |
2021年 | 121篇 |
2020年 | 93篇 |
2019年 | 83篇 |
2018年 | 94篇 |
2017年 | 112篇 |
2016年 | 103篇 |
2015年 | 80篇 |
2014年 | 101篇 |
2013年 | 319篇 |
2012年 | 188篇 |
2011年 | 120篇 |
2010年 | 143篇 |
2009年 | 230篇 |
2008年 | 224篇 |
2007年 | 208篇 |
2006年 | 209篇 |
2005年 | 136篇 |
2004年 | 127篇 |
2003年 | 100篇 |
2002年 | 114篇 |
2001年 | 101篇 |
2000年 | 62篇 |
1999年 | 45篇 |
1998年 | 43篇 |
1997年 | 42篇 |
1996年 | 40篇 |
1995年 | 40篇 |
1994年 | 44篇 |
1993年 | 35篇 |
1992年 | 28篇 |
1991年 | 21篇 |
1990年 | 16篇 |
1989年 | 14篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1986年 | 3篇 |
1985年 | 7篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 5篇 |
1977年 | 1篇 |
1975年 | 2篇 |
1957年 | 1篇 |
排序方式: 共有3654条查询结果,搜索用时 93 毫秒
31.
实验装置由PVC基管状流通钾离子选择电极和与它结合的流动注射分析体系两部分组成。对该体系的分析特点和影响测定的参数进行了讨论,建立了土壤浸提液中钾测定方法,样品分析结果和火焰光度法有良好一致性。 相似文献
32.
Study of various binding materials like potassium bromide, poly(vinyl alcohol), starch, silver and aluminum has been carried out using laser-induced breakdown spectroscopy (LIBS). The role of matrix effects using these five binders on LIBS signal intensity was investigated for better performance of LIBS technique as a quantitative analytical tool. For comparative study of different binders, the signal intensity of different Mg lines at 518.3, 517.2, 383.8 and 279.5 nm wavelengths were recorded for pellets prepared with known concentrations of Mg in these binders. The influence of laser energy on ablated mass under different binding materials and its correlation with LIBS signal intensity has been explored. Optical scanning microscopy images of the ablated crater were studied to understand the laser ablation process. The study revealed that the binding material plays an important role in the generation of LIBS signal. The relative signal intensity measured for a standard Mg line (at 518.3 nm) were 735, 538, 387, 227 and 130 for potassium bromide, starch, poly(vinyl alcohol), silver and aluminum as binders, respectively. This indicates clearly that potassium bromide is better as a binder for LIBS studies of powder samples. 相似文献
33.
34.
Synthesis and Crystal Structure of [{Cd(hmbdc)(H_2O)_3}·2H_2O]_n 总被引:1,自引:0,他引:1
A novel coordination polymer [{Cd(hmbdc)(H2O)3}·2H2O]n (hmbdc = 5-hydroxy- isophthalic acid) has been synthesized and characterized by elemental analysis, IR spectra and single-crystal X-ray diffraction. The crystal belongs to monoclinic, space group P21/c, with a = 9.599(3), b = 18.699(5), c = 7.557(2) , β = 108.198(4)°, V = 1288.6(6) 3, Z = 4, Mr = 382.60, Dc = 1.972 g/cm3, F(000) = 760, μ = 1.740, the final R = 0.0555 and wR = 0.0995 for 1732 observed reflections with I > 2σ(I). The structural analysis shows that the intermolecular hydrogen bonds and π-π interactions result in a three-dimensional supramolecular framework. 相似文献
35.
0IntroductionRareearthsarenowappliedwidelyinChina,whichcanimprovecropsyieldsandthetheirqualit-ies犤1犦.Thebeneficialeffectsmaybeduetothestimu-latoryeffectsoftheseelementsonthenutrientuptakebyplantsorontheincreasingofchlorophyllsynthesisintheplants犤2犦.Whilealotofresearcheshavebeendoneontheimprovednutritionofcropsafterapplica-tionofrareearths,muchlessattentionhasbeenpaidtothedeteriorationofsoilqualityduetotheapplicationofrareearthsforyears犤3犦.Scientistshavediscoveredthataccumula… 相似文献
36.
Barros N. Feijóo S. Simoni A. Critter S. A. M. Airoldi C. 《Journal of Thermal Analysis and Calorimetry》2000,63(2):577-588
The microcalorimetric method was used to calculate the metabolic enthalpy change per mol of glucose degraded by soil microorganisms,
ΔH
met. This parameter has been calculated by microcalorimetry for many organic, inorganic and biochemical reactions, but there
is only some information about its quantification for microbial growth reactions in soils. Values of ΔH
met were calculated for different soil samples collected in Galicia (Spain) and Campinas (Săo Paolo, Brazil). Exponential microbial
growth was stimulated in all soil samples by the addition of glucose and power-time curves were recorded. Results showed changes
in the values of ΔH
met calculated for all the soil samples, suggesting a dependence of this value with the microbial growth rate constant, with
the percentage of growth, with the initial number of microorganisms of soil samples, with the quantity of glucose added and
with the strain of bacteria growing in soil.
The interpretation of variations of ΔH
met provides important qualitative and quantitative information. It reports data that allow to interpret from a qualitative point
of view, the increase in biomass as a consequence of the degradation of the organic matter in soil, to understand changes
in the percentages of soil organic matter and to know if the microbial population growing in differential soil samples is
homogeneous. Therefore, to report that value would be very important in ecological studies, but beforehand, it is necessary
to solve some problems that can appear in the experiments done to make the quantification
.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
37.
Arsenic (As) is a toxic trace element that occurs naturally in groundwater and soils. Understanding the reactions of arsenite
(As(III)) and arsenate (As(V)) with soil and mineral surfaces is critical for predicting the fate and transport of As in the
environment and developing better ways to remediate As-contaminated areas. This investigation uses X-ray absorption near edge
spectroscopy (XANES) to evaluate the solid phase oxidation state and mineral surface binding sites in three agricultural soil
samples from California, USA by fitting linear combinations of XANES spectra derived from several synthetic and well characterized
As(III)- and As(V)-treated model compounds (Fe and Al metal hydroxides and aluminosilicate illite clay mineral). The results
suggest that As(III) is either partially or completely oxidized to As(V) when reacted with soil in an aqueous, batch reaction.
The As(III)-treated Aiken soil was composed of 60% As(III) attached to surfaces similar to lepidocrocite (γ-FeOOH)) and 40%
As(V) attached to aluminosilicate (illite). The Fallbrook soil completely oxidized As(III) and the product was As(V) adsorbed
on Al hydroxide (gibbsite, γ-Al(OH)3) (62%), illite (16%), and lepidocrocite (γ-FeOOH) (22%). The reaction of As(III) with Wyo soil resulted in 42% As(III) adsorbed
on surface similar to goethite and 58% As(V) adsorbed on lepidocrocite. Arsenic(V) adsorption on soil resulted in stable As(V)
surface complexes that were well described by XANES spectra from As(V) adsorption complexes on gibbsite, illite, and lepidocrocite. 相似文献
38.
A series of lightly La-doped CdO thin films (1%, 5%, and 7%) have been prepared by a spin coater sol-gel technique on amorphous
glass and crystalline Si substrates. Those prepared films were studied by X-ray diffraction (XRD), UV-VIS-NIR absorption spectroscopy,
and dc-electrical measurements. The investigation shows that La doping grows slightly the CdO lattice parameter and decreases
the intrinsic energygap from 2.1 eV to 1.7 eV. The optical properties were easily explained in the framework of classical
Drude theory and thus all the corresponding parameters were determined. The electrical behaviour of the samples shows that
they are degenerate semiconductors until the atomic percentage of the La dopant was 7% then the sample was converted into
a non-degenerate semiconductor. Generally, it was observed that the conductivity and mobility of the carriers were decreased
by increasing the La content in the CdO film samples. 相似文献
39.
ICP-AES法测定锌阳极中的铝、镉、铁、铜、铅 总被引:2,自引:0,他引:2
通过分析高频功率、雾化压力、辅助气流量和泵速等试验条件,建立了ICP-AES法测定锌阳极中铝、镉、铁、铜、铅的方法。用该方法测定锌阳极中的铝、镉、铁、铜、铅,其RSD分别为0.17%、0.63%、2.7%、5.2%、2.5%,回收率分别为99.3%-101.2%,99.3%-100.3%、97.1%-102.2%、97.8%-102.9%。对锌阳极试样进行测定,该方法的测定结果与GB4951-85方法的测定结果基本一致。 相似文献
40.
为了解佛山市产母、新生儿血铅水平及其相关影响因素 ,采用电感耦合高频等离子体发射光谱法 ,于 2 0 0 2年 5~ 9月 ,取本院产科产母静脉血及其新生儿脐血 1 5 2例 ,进行血铅、钙、锌、铁、铜和锰水平测定 ,按妇幼保健管理卡内容 ,进行相关记录。结果表明 ,产母血铅水平90 1± 3 4 8μg/L ,BPb≥ 1 0 0 μg/L者占 2 0 3 9% ;新生儿脐血铅水平 89 2± 3 1 6μg/L ,铅中毒率为2 2 3 7% ;母子血铅值呈显著正相关 (r=0 85 7,P <0 0 1 ) ,产母血铅值与其血钙、铁值显著负相关 (r=0 2 66,-0 3 0 1 ,P <0 0 5 ) ;城郊产母及新生儿血铅水平显著高于城区。揭示市郊孕妇、新生儿血铅较高 ,血钙、锌、铁含量影响血铅水平 相似文献