Relaxational polarization and diffuse phase transitions of LA-substituted Pb (Zr,Ti)O3-ceramics

Dielectric properties of (Pb,La)Zr0.55Ti0.45O3 ceramics have been investigated as functions of temperature and frequency. The dielectric constant as a function of temperature can be represented over a considerable temperature interval by a quadratic law of the type: ε−1=ε−1max + C(T−TC')2. The permittivity versus temperature curves are strongly broadened around TC' (where is at a maximum) and the dielectric behaviour is at least partly of a relaxational nature. Interpretation of the dielectric behaviour (broadening, frequency dependence, quadratic law) seems possible by assuming a distribution of local Curie temperatures; the standard deviation of which can be correlated with the La concentration

The electric field-induced antiferroelectric to ferroelectric phase transition in some (Pb,La)Zr0.55Ti0.45O3 ceramics

Lanthanum substituted lead zirconate-titanate (PLZT) ceramics of composition 11.1/55/45 have been studied by measuring high electric field properties. Dc bias, dielectric and P-E hysteresis loop measurements have been employed to construct an E (electric field) versus T (temperature) phase diagram. At lower temperatures and small electric fields an antiferroelectric phase with tetragonal symmetry has been found. Applying high electric fields gives rise to a field-induced phase transition from the antiferroelectric to the ferroelectric state, however, without X-ray detectable change in crystal symmetry

We Heard Canada Was a Free Country : African American Migration in the Great Plains, 1890-1911

This study uses a transnational lens to examine the migration of Black Americans from Oklahoma to Canada in the early 20th century. Although scholars have documented this movement, they have not fully explored the vital and durable transnational connections among African American immigrants themselves. The use of family histories, newspaper articles, and immigration files show how black migrants searched for land and equality in Canada and attempted to build all black communities. Encouraged by the promises of Canadian immigration recruiters, black migrants left their homes and Jim Crowism in Oklahoma to settle in a “free country” and to realize the goals of American citizenship in a foreign land. But, Canada wanted white—not black—American settlers and immigration officials closed to African Americans the once porous boundary between the U.S. and Canada. Canadian authorities recognized the power of transnational connections among black migrants in promoting migration and settlement and, ironically, by effectively sabotaging that network, they ensured that African Americans had to abandon their quest for equality and opportunity in Western Canada

The fragmented city.

A dissertation submitted to the Faculty of Architecture University of the Witwatersrand in fulfillment of the requirements for the Degree of Masters in Urban Design.Johannesburg, like many other major cities in Third World nations is facing major urban influx, This urban influx is causing extreme pressure on the existing social and economic fabric. This in tum has resulted in fragmentation on a number of different levels - social, political, economic and physical. The spatial organisation of the city appears to be incapable of coping with this fragmentation. The means or method needs to be discovered where the fragmented elements of the city can be reunited and absorbed into the city system. (Abbreviation abstract)Andrew Chakane 201

A new engineering approach to predict the long-term hydrostatic strength of unplasticized poly(vinyl chloride) pipes

Extruded polymer pipes are qualified using pressurized pipe tests. With these tests the long-term hydrostatic strength is determined by subjecting the pipes to an internal pressure, while measuring the time-to-failure. Although these tests can be accelerated (at higher temperatures), they remain time consuming and require a spacious experimental setup. To circumvent this costly method a model based approach is proposed by which the long-term hydrostatic strength is predicted. Using short term measurements, the input parameters for this approach can be determined. In this engineering approach the effects of physical aging are included. The approach is capable to quantitatively predict the (long-term) failure time of pipe sections under internal pressure