全文获取类型
收费全文 | 2978篇 |
免费 | 284篇 |
国内免费 | 210篇 |
专业分类
化学 | 1001篇 |
晶体学 | 7篇 |
力学 | 646篇 |
综合类 | 55篇 |
数学 | 722篇 |
物理学 | 1041篇 |
出版年
2024年 | 10篇 |
2023年 | 29篇 |
2022年 | 91篇 |
2021年 | 96篇 |
2020年 | 128篇 |
2019年 | 90篇 |
2018年 | 79篇 |
2017年 | 104篇 |
2016年 | 143篇 |
2015年 | 109篇 |
2014年 | 158篇 |
2013年 | 181篇 |
2012年 | 124篇 |
2011年 | 182篇 |
2010年 | 114篇 |
2009年 | 144篇 |
2008年 | 155篇 |
2007年 | 176篇 |
2006年 | 161篇 |
2005年 | 143篇 |
2004年 | 112篇 |
2003年 | 112篇 |
2002年 | 109篇 |
2001年 | 93篇 |
2000年 | 81篇 |
1999年 | 69篇 |
1998年 | 77篇 |
1997年 | 61篇 |
1996年 | 59篇 |
1995年 | 49篇 |
1994年 | 26篇 |
1993年 | 32篇 |
1992年 | 20篇 |
1991年 | 13篇 |
1990年 | 21篇 |
1989年 | 8篇 |
1988年 | 14篇 |
1987年 | 12篇 |
1986年 | 9篇 |
1985年 | 13篇 |
1984年 | 14篇 |
1983年 | 12篇 |
1982年 | 18篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1971年 | 2篇 |
1969年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有3472条查询结果,搜索用时 46 毫秒
901.
Xiaodong Zheng 《Proceedings of the American Mathematical Society》1998,126(12):3669-3679
Confidence interval construction for parameters of lattice distributions is considered. By using saddlepoint formulas and bootstrap calibration, we obtain relatively short intervals and bounds with coverage errors, in contrast with and coverage errors for normal theory intervals and bounds when the population distribution is absolutely continuous. Closed form solutions are also provided for the cases of binomial and Poisson distributions. The method is illustrated by some simulation results.
902.
Concentration of tracer at a producing well is a function of time and of the flow field that transports the tracer from one well to another. Small localized changes in the permeability and porosity fields result in small changes in the observed concentration history. Calculation of the effect of changes in the petrophysical parameter fields on observed concentration is an integral part of most approaches to solving the inverse problem, i.e. calculation of permeability and porosity fields that are consistent with observed tracer concentrations. This paper does not address the inverse problem directly, only the forward problem, but the results are presented as semi-analytic formulas which are intended to provide physical insight into the limitations of the inverse procedure. In particular, for a simple example of dipole flow between two wells in a nearly homogeneous infinite 2D porous medium, the width of the region of significant influence is shown to scale as d where is the dispersivity and d is half the interwell distance. Also, for fixed injection and production rates the influence on concentration of variation in porosity and in log-permeability are shown to be similar in magnitude. 相似文献
903.
Kathleen L. Caldwell Amir Makhmudov Robert L. Jones Joseph G. Hollowell 《Accreditation and quality assurance》2005,10(7):356-361
In 2001 the Centers for Disease Control and Prevention (CDC) established a program, Ensuring the Quality of Urinary Iodine
Procedures (EQUIP); to assist laboratories around the world and assess the accuracy of their urinary iodine (UI). CDC designed
EQUIP to issue unknown specimens to participating laboratories three times per year. Each laboratory was asked to analyze
unknown samples in duplicate on three different days. During the first five rounds of EQUIP, 41 laboratories participated,
measuring unknown samples and reporting their results to CDC. CDC used these results to prepare a statistical report for the
laboratories. Feedback to the laboratories provided external confirmation regarding performance. As a group, laboratory performance
improved; several laboratories made considerable improvement. Several laboratories that showed no improvement have ordered
new equipment or are arranging for additional training. EQUIP is a key tool used to support laboratory quality assurance in
an effort to eliminate iodine deficiency disorders (IDD) in the world.
相似文献
Kathleen L. CaldwellEmail: |
904.
Maria Belli Stephen L. R. Ellison Ales Fajgelj Ilya Kuselman Umberto Sansone Wolfhard Wegscheider 《Accreditation and quality assurance》2007,12(8):391-398
A metrological background for the selection and use of proficiency testing (PT) schemes for a limited number N of laboratories-participants (less than 20–30) is discussed. The following basic scenarios are taken into account: (1) adequate
matrix certified reference materials (CRM) or in-house reference materials (IHRM) with traceable property values are available
for PT use as test items; (2) no appropriate matrix CRM is available, but a CRM or IHRM with traceable property values can
be applied as a spike or similar; (3) only an IHRM with limited traceability is available. The discussion also considers the
effect of a limited population of PT participants N
p on statistical assessment of the PT results for a given sample of N responses from this population. When N
p is finite and the sample fraction N/N
p is not negligible, a correction to the statistical parameters may be necessary. Scores suitable for laboratory performance
assessment in such PT schemes are compared.
Presented at the 3rd International Conference on Metrology, November 2006, Tel Aviv, Israel. 相似文献
905.
Metrological designs of experiment for proficiency testing (PT) with a limited number of participants are discussed. The designs are based on development of in-house reference materials (IHRMs) with traceable assigned values and fit-for-purpose uncertainties, used in a PT scheme as the measurement standards. When adequate certified reference materials (CRMs) are available, a comparative approach for IHRM development, using simultaneous analysis of IHRM and CRM test portions in pairs, is proposed for PT objectives. In the case where adequate CRMs are not available – for example, in the field of analysis of unstable aqueous systems – the function of the measurement standard for PT can be fulfilled by a synthetic IHRM prepared gravimetrically using non-adequate, non-aqueous CRMs or pure substances. The CRM or the pure substance is used as a spike for fortification of a natural water sample, while the natural water sample is used as a working IHRM for the spike determination. In this case the traceability chain is longer, since two IHRMs are added for one quantity determination, but it remains unbroken.Presented at the Second International Conference on Metrology –Trends and Applications in Calibration and Testing Laboratories, 4–6 November 2003, Eilat, Israel 相似文献
906.
建立了加速溶剂萃取与气相色谱-三重四极杆质谱联用(GC-MS/MS)法对枸杞中20种有机磷农药残留进行测定。样品经酸化乙腈-二氯甲烷(体积比4∶1)提取,Cleanert TPH固相萃取柱净化,旋转蒸发浓缩后氮吹至干,采用GC-MS/MS检测,并对萃取溶剂、温度、冲洗体积等实验条件进行了优化。结果表明,20种药物在1~500μg/kg范围内线性良好,相关系数(r~2)为0.999 0~0.999 8;在20、40、100μg/kg 3个加标水平下的回收率为70.0%~82.9%,相对标准偏差(RSD,n=5)为0.6%~8.5%,定量下限为1~15μg/kg。该方法样品前处理简单、定量准确,适用于枸杞中有机磷农药残留的检测。 相似文献
907.
908.
《Chemphyschem》2003,4(1):14-30
Although the physics of Raman spectroscopy and its application to purely chemical problems is long established, it offers a noninvasive, nondestructive, and water‐insensitive probe to problems in the life sciences. Starting from the principles of Raman spectroscopy, its advantages, and methods for signal enhancement, the bulk of the review highlights recent applications. Structural investigations of a hormone receptor, testing the biocompatibility of dental implants, probing soil components and plant tissue alkaloids, and localization of single bacteria are just four problems in which Raman spectroscopy offers a solution or complements existing methods. 相似文献
909.
建立了加速溶剂萃取(ASE)-超高效液相色谱(UPLC)-串联质谱法(MS/MS)测定茶叶中拟除虫菊酯类农药残留的方法。ASE萃取温度为80 ℃,萃取压力为10.34 MPa,以正己烷-丙酮(2∶1,v/v)为溶剂静态萃取5 min,循环一次。萃取液浓缩后经GCB/NH2-Florisil柱净化,UPLC分离,MS/MS正离子扫描(ESI+)、多反应离子监测(MRM)模式进行分析,外标法定量。线性回归分析表明:10种拟除虫菊酯的浓度与其峰面积的线性关系显著,相关系数(r)均不小于0.9995,检出限(LOD)在0.5~5.0 μg/kg之间,定量限(LOQ)在1.6~16.6 μg/kg之间;在定量限、0.4 mg/kg以及最高残留限量(MRL,无MRL的加入1 mg/kg)3个水平进行添加回收试验(n=7),回收率为68.7%~103.8%,RSD为0.8%~13.2%。该方法前处理简单,耗时短,灵敏度和准确度高,可满足茶叶中痕量拟除虫菊酯类农药残留测定的要求。 相似文献
910.