Matrix density effects on the mechanical properties of SiC/RBSN composites

The room temperature mechanical properties were measured for SiC fiber reinforced reaction-bonded silicon nitride composites (SiC/RBSN) of different densities. The composites consisted of approx. 30 vol percent uniaxially aligned 142 micron diameter SiC fibers (Textron SCS-6) in a reaction-bonded Si3N4 matrix. The composite density was varied by changing the consolidation pressure during RBSN processing and by hot isostatically pressing the SiC/RBSN composites. Results indicate that as the consolidation pressure was increased from 27 to 138 MPa, the average pore size of the nitrided composites decreased from 0.04 to 0.02 microns and the composite density increased from 2.07 to 2.45 gm/cc. Nonetheless, these improvements resulted in only small increases in the first matrix cracking stress, primary elastic modulus, and ultimate tensile strength values of the composites. In contrast, HIP consolidation of SiC/RBSN resulted in a fully dense material whose first matrix cracking stress and elastic modulus were approx. 15 and 50 percent higher, respectively, and ultimate tensile strength values were approx. 40 percent lower than those for unHIPed SiC/RBSN composites. The modulus behavior for all specimens can be explained by simple rule-of-mixture theory. Also, the loss in ultimate strength for the HIPed composites appears to be related to a degradation in fiber strength at the HIP temperature. However, the density effect on matrix fracture strength was much less than would be expected based on typical monolithic Si3N4 behavior, suggesting that composite theory is indeed operating. Possible practical implications of these observations are discussed

Composite fermions in bands with N-fold rotational symmetry

We study the effect of band anisotropy with discrete rotational symmetry $C_N$ (where $N\ge 2$) in the quantum Hall regime of two-dimensional electron systems. We focus on the composite Fermi liquid (CFL) at half filling of the lowest Landau level. We find that the magnitude of anisotropy transferred to the composite fermions decreases very rapidly with $N$. We demonstrate this by performing density matrix normalization group calculations on the CFL, and comparing the anisotropy of the composite fermion Fermi contour with that of the (non-interacting) electron Fermi contour at zero magnetic field. We also show that the effective interaction between the electrons after projecting into a single Landau level is much less anisotropic than the band, a fact which does not depend on filling and thus has implications for other quantum Hall states as well. Our results confirm experimental observations on anisotropic bands with warped Fermi contours, where the only detectable effect on the composite Fermi contour is an elliptical distortion ($N = 2$).Comment: 6 pages + bibliography, 5 figure

Environmental effects on the tensile strength of chemically vapor deposited silicon carbide fibers

The room temperature and elevated temperature tensile strengths of commercially available chemically vapor-deposited (CVD) silicon carbide fibers were measured after 15 min heat treatment to 1600 C in various environments. These environments included oxygen, air, argon and nitrogen at one atmosphere and vacuum at 10/9 atmosphere. Two types of fibers were examined which differed in the SiC content of their carbon-rich coatings. Threshold temperature for fiber strength degradation was observed to be dependent on the as-received fiber-flaw structure, on the environment and on the coating. Fractographic analyses and flexural strength measurements indicate that tensile strength losses were caused by surface degradation. Oxidation of the surface coating is suggested as one possible degradation mechanism. The SiC fibers containing the higher percentage of SiC near the surface of the carbon-rich coating show better strength retention and higher elevated temperature strength

Connection between Fermi contours of zero-field electrons and ν=12\nu=\frac12 composite fermions in two-dimensional systems

We investigate the relation between the Fermi sea (FS) of zero-field carriers in two-dimensional systems and the FS of the corresponding composite fermions which emerge in a high magnetic field at filling $\nu = \frac{1}{2}$, as the kinetic energy dispersion is varied. We study cases both with and without rotational symmetry, and find that there is generally no straightforward relation between the geometric shapes and topologies of the two FSs. In particular, we show analytically that the composite Fermi liquid (CFL) is completely insensitive to a wide range of changes to the zero-field dispersion which preserve rotational symmetry, including ones that break the zero-field FS into multiple disconnected pieces. In the absence of rotational symmetry, we show that the notion of `valley pseudospin' in many-valley systems is generically not transferred to the CFL, in agreement with experimental observations. We also discuss how a rotationally symmetric band structure can induce a reordering of the Landau levels, opening interesting possibilities of observing higher-Landau-level physics in the high-field regime.Comment: 7 pages + references, 7 figures. Added many-body DMRG calculatio

Effects of large disorder on the Hofstadter butterfly

Motivated by the recent experiments on periodically modulated, two dimensional electron systems placed in large transversal magnetic fields, we investigate the interplay between the effects of disorder and periodic potentials in the integer quantum Hall regime. In particular, we study the case where disorder is larger than the periodic modulation, but both are small enough that Landau level mixing is negligible. In this limit, the self-consistent Born approximation is inadequate. We carry extensive numerical calculations to understand the relevant physics in the lowest Landau level, such as the spectrum and nature (localized or extended) of the wave functions. Based on our results, we propose a qualitative explanation of the new features uncovered recently in transport measurements.Comment: 15 pages, 13 figures, several pictures have been shrunk to comply with the size requirement

Politics by Other Means: Economic Expertise, Power, and Global Development Finance Reform

This dissertation investigates how economic expertise influences development governance by examining how state economists establish methods for decision-making in global development finance. It contributes to debates over expert power by taking a science studies approach to address two problems in existing theories and accounts of experts. First, social reformers, heterodox planning theorists, and development critics from both the left and the right treat rationality and politics asymmetrically. When experts fail, politics has triumphed. When experts succeed, the credit goes to rationality, not politics. Second, within this asymmetrical approach, investigations and explanations of expert power neglect a principal conduit of expert influence: their methods. This dissertation turns the focus to economists’ efforts to establish their methods as governing rationales and the effects these methods engender. Doing so allows us to approach particular forms of state rationality such as neoliberalism or managerialism not as processes of depoliticization, of intellectual rationality prevailing over political interests and values, but as explicit political accomplishments with both the power to bring about political effects and the susceptibility to being challenged. State economists’ efforts to establish three paradigmatic development economic methods in particular—governance indicators, growth diagnostics, and randomized controlled trials—and these methods’ effects on power relations, decision-making, and the distribution of resources were assessed using an embedded case study design of their use for decision-making in administering a new development finance fund, the United States Millennium Challenge Account. A mixed methods approach using interviews, documents, and various datasets found that economists could not realize the power of their intellectual rationality without exercising power thought to be the reserve of politicos. Economists had to employ various strategies of power both to gain autonomy from bureaucratic authorities and overcome opposition from expert groups holding alternative rationalities. This involved enrolling bystanders and opponents in their entrepreneurial efforts to establish methods. The more opposition economists faced, the more power they had to exercise and allies they had to enroll. Once enrollment was successful, economists’ status was elevated and their methods became indispensable to particular decision-making processes. These new ways of making decisions introduced different biases that elevated economists’ concerns, objectives, and ways of knowing. They also impacted the distribution of development finance in ways that exacerbated inequality in at least the short to medium term. This dissertation’s focus on economists’ political work and methods has implications for planning practice because it opens up new political possibilities. Rather than treating state expertise and public participation as antagonistic, zero-sum confrontations, planners can pursue democratic values by both “opening up the state” and “getting inside” methods. If orthodox economists had to overcome opposition from groups of opposing experts with competing rationalities then other experts can likewise use political strategies to establish their methods as governing rationales. Even in situations where this is not possible or desirable, understanding methods’ political effects can instigate reflective practice and possible change

Enterprise Information Systems Integration and Business Process Improvement Initiative: An Empirical Study

Since the mid-and late 80\u27s, business process improvement (BPI) has become one of the leading methodologies to deliver corporations with high quality products and services. Businesses are seeking not simply to automate existing operations, but to improve and redesign business processes and capture customers\u27 expectations for products, and service delivery. Extensive communication and inter-connectivity arising from adoption of standards and integrated services digital networks (ISDN) has become a major force affectingbusinesses in fundamental ways (Madnick, 1990; Boar, 1993). The second avenue through which businesses are identifying new opportunities is the availability of databases (Madnick, 1990). By linking inter-organizational, inter-functional, and inter-personal levels of the processes through IS networks, businesses are not only automating their activities, they are also reshaping and improving their business processes (Hammer and Champy, 1993). By accessing enterprise-wise information from databases, IS integration is providing numerous opportunities to coordinate organizational activities by facilitating communication and information exchange across departments without the need to go up and down the vertical chain of command. The use of information networks to access relevant information from databases has been of enormous importance to eliminate duplicate activities, prevent errors from occurring, cycle time reduction in product development, and customer responsiveness (Davenport, 1993). The need of a well planned database management system is one of the important requirements for BPI. In most organizations, data architecture has evolved as a result of applications databases in various departments rather than as a well planned data management strategy. Therefore, the resolution of data management problems becomes quite difficult (Goodhue, Quillard, and Rockart, 1988). The access to timely, accurate and consistent information is crucial in business process improvement. IS integration, through communication networks and database systems, enables organizations to create and sustain process improvement through timely retrieval of consistent and accurate information. Process improvement can be measured by the extent desired specified results are produced right thefirst time (i.e., outcomes with zero defect), the extent various processes minimize the consumption of the business resources, and the extent business processes are easily modified to meet or exceed customers\u27 expectations for products and service delivery. The current study is aimed at developing and empirically testing the relationships between IS integration and BPI. As presently there are only a handful studies that empirically test the relationship between information systems and BPI, this study is an important step for furthering the scope of present stage of the IS literature