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31.
Spectral assessment of mesh adaptations for the analysis of the dynamical longitudinal behavior of railway bridges 总被引:2,自引:0,他引:2
Summary Extensive studies, [1], concerning the longitudinal behavior of long railway bridges due to braking forces have been done
by measurements in situ, [2], and by statical, [3, 4], as well as dynamical simulations. Thereby, the only consistent numerical
realization with respect to the measured data was the dynamical one. However, the consecutive discretizations in space and
time with time-dependent system matrices are extremely time consuming due to the moving loads and varying stiffness of the
ballast under, and in front of, the moving train. Therefore, every effort should be made to optimize the discretization in
the space domain. This paper presents a strategy for assessing the quality of finite elements in space and for applying an
adaptive mesh-refinement for this special engineering problem. The method is characterized by a spectral assessment, comparing
a certain set of eigenvalues of the actual discretization with those of a very fine and rather exact numerical model. The
error estimator introduced in this paper controls a whole set of global eigenvalues with corresponding natural vibration modes
in order to assess certain types of shape functions. Thus, the procedure estimates local modifications on the one hand and
p-properties on the other by means of global indication.
Received 30 August 1999; accepted for publication 26 September 2000 相似文献
32.
宿州城市土壤重金属污染特征及其健康风险评价 总被引:2,自引:0,他引:2
选取宿州市不同功能区(工业区、商业区、车站附近、居民区以及城市绿地)采集表层土壤样品55个,利用XRF测定出土壤中重金属Cu、Zn、Pb、Cr、Mn、Ni的含量,并借助健康风险评价模型,分别评价了土壤重金属在不同暴露途径下的健康风险。结果表明,Cu、Zn、Pb和Cr的平均含量均超出安徽省土壤背景值,说明宿州城市土壤中已受到不同程度的重金属污染;工业区、商业区和车站附近土壤重金属含量普遍较高,Cu、Zn、Pb与交通运输、工业生产关系密切,Cr主要来源于工业废弃物排放,Ni则受到生活污染和交通污染的双重影响;土壤重金属的非致癌风险指数(HI)和致癌风险指数(Risk)均低于危害水平界限,未形成明显的健康风险。 相似文献
33.
在南方某癌症高发县5个乡镇10个村进行布点取样,采集深层地下水与浅层地下水共计20个水样。采用吹扫捕集与气相色谱-质谱联用方法测定水样中13种挥发性有机物(VOCs),检出二氯甲烷、三氯甲烷、苯和四氯化碳4种VOCs,其浓度分别为0.36—13.52、0.41—18.71、0.57—11.75μg/L和1.56—214.62μg/L。1个水样中苯和16个水样中四氯化碳超过GB 5749-2006规定的限值。采用优化的USEPA风险评价模型,对VOCs进行人体健康风险评价,其非致癌风险指数0.0073—0.7100,致癌风险水平1.98×10-6—1.99×10-4,全部水样的致癌风险水平超过10-6水质监控值,1个水样的致癌风险水平超过10-4的可接受水平。四氯化碳对非致癌风险指数和致癌风险水平贡献最大。 相似文献
34.
采用热脱附-气相色谱/质谱联机技术(TD-GC/MS)分析了芦苇杆粉末在不同环境温度下的挥发物释放行为。结果表明:芦苇杆粉末在40℃及160℃的挥发物中,主要有益成分分别为主要有害成分的4.43倍和5.13倍,环境友好性良好;在60℃及90℃的挥发物中,主要有益成分比例也比主要有害成分高,90℃时的有益成分为有害成分的2.45倍,环境友好性较好。以芦苇杆粉末为基材,在木质素软化温度点和木质材料碳化温度点之间(150—180℃)实施高压无胶模塑成形工艺制备木质粉末基复合材料必将体现出良好的环境友好性。 相似文献
35.
Tania Crucitti 《Accreditation and quality assurance》2001,6(8):379-381
Quality assurance is an important aspect of laboratory management. One of the activities involved is the participation in
external quality assessment (EQA) schemes by the clinical laboratory. These EQA schemes should be organised according to well-defined
guidelines, such as the ESO/IEC Guide 43. The present work provides an inventory of the EQA schemes organised in Europe. The
schemes are related to the fields of bacteriology, parasitology and virology. For each field various surveys have been organised.
Data such as the number of participants, frequency of survey and number of samples in a survey are presented. The nature and
way of manufacturing the control material is specified. 相似文献
36.
A. Maroto Ricard Boqué Jordi Riu F. Xavier Rius 《Accreditation and quality assurance》2002,7(3):90-94
The bias of an analytical procedure is calculated in the assessment of trueness. If this experimental bias is not significant,
we assume that the procedure is unbiased and, consequently, the results obtained with this procedure are not corrected for
this bias. However, when assessing trueness there is always a probability of incorrectly concluding that the experimental
bias is not significant. Therefore, non-significant experimental bias should be included as a component of uncertainty. In
this paper, we have studied if it is always necessary to include this term and which is the best approach to include this
bias in the uncertainty budget. To answer these questions, we have used the Monte-Carlo method to simulate the assessment
of trueness of biased procedures and the future results these procedures provide. The results show that non-significant experimental
bias should be included as a component of uncertainty when the uncertainty of this bias represents at least a 30% of the overall
uncertainty.
Received: 29 May 2001 Accepted: 10 December 2001 相似文献
37.
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39.
This study describes Singapore students’ (N = 607) performance on two tasks in a recently developed Mathematics Processing Instrument (MPI). The MPI comprised tasks sourced from Australia's NAPLAN and Singapore's PSLE. This study also examines students’ use of the model method to solve the two tasks. The model method is a visual problem-solving heuristic prevalently used in Singapore classrooms. The study found that students who solved the tasks using a visual method predominantly used the model method as a visual problem-solving strategy. Another interesting observation was the hindrance of successful problem solving caused by the persistence of prototypical images of model drawings. Implications include encouraging teachers to get their students to identify problem situations where the model method will both work and not work well, and making the role of the generator in the model method explicit in the mathematics textbooks. 相似文献
40.
Melissa C. Gilbert 《School science and mathematics》2014,114(7):339-348
This study examined the productive disposition of pre‐algebra students who demonstrated similar knowledge of the focal content but varied in other academic behaviors expected in the Common Core State Standards for Mathematics (CCSSM). Specifically, the study considered students' attention to precision when critiquing a peer's work. The comprehensive definition of productive disposition used included task values (interest, utility), an ability belief (efficacy), three personal achievement goals, and negative emotions. As hypothesized, the 61 students who provided a more precise critique reported higher productive disposition (in particular, significantly higher mastery‐approach personal achievement goals and less frequent negative emotions) than the 79 students who provided a basic critique. These findings illustrate how productive disposition can inform assessments of mathematical competence within the CCSSM recently implemented across the United States. 相似文献