Explaining new firm creation in Europe from a spatial and time perspective: A multilevel analysis based upon data of individuals,regions and countries

This paper seeks to better understand the significance of spatial context conditions and personal attributes for early‐stage entrepreneurship. We combine individual with regional and national level data using multilevel analysis to test our hypotheses. We differentiate between two phases in the entrepreneurial process as well as between general and ambitious entrepreneurship. First, we show that both the national and the regional context significantly impact individual entrepreneurial activities. Second, individual level characteristics exert the greatest overall influence, but the direction of this influence is not stable. Third, the impact of the three levels varies across the different phases in the entrepreneurial process as well as between different types of start‐ups. Fourth, we demonstrate that cross‐level interactions between individual characteristics and spatial context factors are important in explaining entrepreneurial activities.     

The size distribution of exporting and non‐exporting firms in a panel of Chinese provinces

This paper studies the size distribution of exporting and non‐exporting firms in a panel of Chinese provinces. The power law exponents of exporting firms are significantly less than those of non‐exporting firms. The average power law exponents fell from 0.74 in 1998 to 0.64 in 2007 for exporting firms, and from 1.03 in 1998 to 0.83 in 2007 for non‐exporting firms. Further analysis showed that credit constraints have a significant negative effect on the exponents of the size distribution of exporting firms, suggesting that smaller firms are more credit‐constrained than larger firms.     

釜内增韧PP橡胶组分序列结构13C-NMR谱数据的统计理论处理

首先用Bernoullian模型和Markov模型统计公式对10组釜内增韧PP橡胶组分(乙丙共聚物)13C-NMR谱数据进行分析比较,由其所得方差值可知,所有样品中的橡胶组分数据均不符合Bernoullian模型,而符合一级Markov模型;然后用Markov模型的统计公式计算,画出共聚物乙烯、丙烯的数量和质量分布曲线,由此可直观地比较各样品的组成分布;同时定量给出了共聚物数均和重均链段长度、链段分布指数及长链段的含量。     

Cr13型不锈钢小方坯连铸结晶器保护渣试验研究

针对Cr13型不锈钢在三机三流全弧形连铸机连铸150mm×150mm小方坯,研究连铸结晶器保护渣的配方,同时调整连铸拉坯工艺,降低因保护渣引起的铸坯表面缺陷。在生产使用现有保护渣的基础上,设计多个保护渣,经实验室性能测试及生产现场拉坯试验,确定适应生产的保护渣主要性能参数为:二元碱度R=0.81,半球点温度T半=1164℃,粘度(1300℃)=6.6P,而且该保护渣仍有改进的空间。     

Microstructure and wear behaviour of pulsed plasma nitrided AISI H13 tool steel

Recent years have witnessed an increasing usage of tool steels for industrial high strength applications. Plasma nitriding (PN) has been a flexible case hardening process compared with any other case hardening technologies. Literature has been found to be less secured in correlating generation and phase transformation of nitride phases with temperature and PN time and consequently their effect on different properties. In the present study, PN was applied with the aim to improve surface and near-surface property for three different PN time durations (6, 12 and 24?h). Surface properties of the plasma nitrided steel were evaluated by measuring hardness, roughness and wear resistance. Scanning electronic microscope (SEM) was used to reveal the microstructure and measure different layers after PN. X-ray diffraction enables identification of different phases along with its co-relation with different properties. PN with 6?h has been found superior based on wear resistant (0.0004?mg for 6?h compared to 12 and 24?h respectively 0.0005 and 0.0007?mg) & hardness (1107.66 HV for 6?h compared to 12 and 24?h respectively 1061.66 and 869.66 HV). Although microstructure revealed minimum case depth (minimum 107?µm for 6?h and maximum 220?µm for 24?h) and compound layer (6, 12 and 24?h are respectively 4, 6.5 and 11?µm), presence of unsaturated ?-Fe₃N phase play an important role in governing the final property.     

稳定同位素~(13)C分离用高效丝网波纹填料表面降膜流动研究

由于碳同位素分离系数小,分离难度大,需要采用高效规整填料实现~(13)C的分离。本文通过计算流体力学(CFD)数值模拟研究,采用流体体积函数(VOF)方法,研制了用于~(13)C分离的高比表面积丝网波纹规整填料(PACK-~(13)C),建立了PACK-~(13)C填料表面伴随有气相逆流的局部液体降膜流动模型,选用CO(l)-CO(g)为模拟计算物系,考察了板面结构、丝网目数等因素对液膜流动的影响,并对填料表面气液相界面进行追踪,探究了气液相界面波动对传质效率的影响,研究表明,改善填料壁面结构能够增强气液相界面波动,可以实现强化传质过程。填料表面局部降膜流动的研究方法,可应用于填料气液传质过程中涉及的多尺度流动及传质现象的可视化研究,为优化填料结构提供基础性理论指导。     

UPPER-BOUND SOLUTIONS FOR BEARING CAPACITY OF STRIP FOOTINGS OVER ANISOTROPIC NONHOMOGENEOUS CLAYS

All available solutions for the problem of bearing capacity on clays of anisotropic and nonhomogeneous strength are based on the assumption of either circular or Prandtl-type failure mechanisms. Although these solutions are rigorous within the concept of limit analysis or limit equilibrium methods, the formulations are too mathematical and exceedingly cumbersome and the bearing capacity value can only be determined through numerical optimization for each given combination of soil and failure mechanism parameters. By means of the upper bound approach of limit analysis, and adopting translational failure mechanisms, this paper presents analytical solutions for the bearing capacity of surface strip footings on clays of strength anisotropic and linearly increasing with depth. Closed-form expressions for bearing capacity factors in the case of smooth and rough footings, have been derived by considering two newly introduced kinematically admissible translational failure mechanisms with varying wedge angles. It was remarkable to find that the two mechanisms would render closed-form expressions for the bearing capacity factor if and only if the deformed region underneath the footing is set to be bounded by two vertical discontinuity surfaces. The derived formulas are expressed in terms of degree of strength anisotropy and a nondimensional parameter that reflects strength nonhomogeneity. Besides being the only closed-form solutions yet available for the bearing capacity of strip footings on clays with anisotropic and nonhomogeneous strength, the derived expressions have been found to not only provide upper bound values for the bearing capacity factor that compared favorably with available solutions, but also yield the best upper bound values when strength increase with depth becomes predominant.     

Cournot oligopolistic competition in spatially separated markets: The Stackelberg equilibrium

Consider a two-stage non-cooperative Cournot game with location choice involving r firms. There are n spatially separated markets located at the vertices of a network. Each firm, first selects the location of a facility and then selects the quantities to supply to the markets in order to maximize its profit. Non-zero conjectural variation at the second stage in the model by Sarkar et al. (1997) is studied. When the demand in each market is sufficiently large, equilibrium in the quantities offered by each firm in the markets exists. Furthermore, each firm chooses to locate its facility at the vertices.Partially financed by Ministerio de Ciencia y Tecnología (Spain) and FEDER, grant BFM2002-04525-C02-01.Received: August 2002 / Accepted: May 2003     

Production of hydrogen by unmixed steam reforming of methane

Unmixed steam reforming is an alternative method of catalytic steam reforming that uses separate air and fuel–steam feeds, producing a reformate high in H₂ content using a single reactor and a variety of fuels. It claims insensitivity to carbon formation and can operate autothermally. The high H₂ content is achieved by in situ N₂ separation from the air using an oxygen transfer material (OTM), and by CO₂ capture using a solid sorbent. The OTM and CO₂ sorbent are regenerated during the fuel–steam feed and the air feed, respectively, within the same reactor. This paper describes the steps taken to choose a suitable CO₂-sorbent material for this process when using methane fuel with the help of microreactor tests, and the study of the carbonation efficiency and regeneration ability of the materials tested. Elemental balances from bench scale experiments using the best OTM in the absence of the CO₂ sorbent allow identifying the sequence of the chemical reaction mechanism. The effect of reactor temperature between 600 and on the process outputs is investigated. Temperatures of 600 and under the fuel–steam feed were each found to offer a different set of desirable outputs. Two stages during the fuel–steam feed were characterised by a different set of global reactions, an initial stage where the OTM is reduced directly by methane, and indirectly by hydrogen produced by methane thermal decomposition, in the second stage, steam reforming takes over once sufficient OTM has been reduced. The implications of these stages on the process desirable outputs such as efficiency of reactants conversion, reformate gas quality, and transient effects are discussed.     

Influence of climatic ageing on the mechanical properties and the microstructure of low‐density polyethylene films

Blown extruded films of low‐density polyethylene (LDPE) have been subjected to climatic ageing in a sub‐Saharan facility at Laghouat (Algeria) with direct exposure to sun. Samples were characterized by complementary techniques after prescribed amounts of time up to 8 months. It was shown by tensile testing that the mechanical properties are quite sensitive to ageing: (i) the elastic modulus increases and saturates, (ii) the tensile stress increases slightly, and (iii) the rupture energy decreases dramatically after 4 months weathering. Fourier‐transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (¹³C NMR) were performed to identify the evolution of the polymer microstructure. The FTIR spectra reveal the initial presence of vinylidene groups that exhaust rapidly after 4 months ageing. Also, it detects the progressive multiplication of vinyl groups and oxidation products of many kinds. The NMR technique revealed specifically the carbon–carbon configurations in the polymer chains. By contrast to the original film that contained almost exclusively butyl chain branches, the aged specimens presented shorter ramifications, namely ethyl branches. Also, the presence of quaternary atoms was detected after long ageing times. The discussion of these complementary results in the light of current literature makes possible to identify the leading mechanisms that control the decay of LDPE film properties. Although these mechanisms are numerous and complex, they can be schematically summarized within three main classes: oxidation, scission, and crosslinking. Each class is discussed in details. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008