CaCu3Ti4O12 ceramics are prepared by the conventional solid-state reaction method with various sintering temperatures. Microstructures are examined by scanning electronic microscopy, and it is found that they could be categorized into three different types. Dielectric properties and complex impedances are investigated in the frequency range of 40 Hz—110 MHz over a temperature range of 25—280℃. The room temperature dielectric constant increases with sintering temperature. With increasing the measuring temperature, ceramics with different microstructures show diverse electrical properties. However, some common features exist among the electrical properties. For all of the ceramics, dielectric dispersion shows a low-frequency response and two Debye-type relaxations, and there exist three semicircles in the complex impedance plane at high temperatures. The observed electrical properties are ascribed to the detailed internal polycrystalline microstructure, i.e., to come from the contributions of the domains, the grain boundaries and defects inside grains such as domain boundaries, etc.
The paper presents a modified method which bases on the traditional two-step floating catchment area method and the gravitational method. the traditional two-step FCA method deals with distance and time obstruction by bisection method, any point within the threshold range have the same accessibility. Such approaches have been criticized for ignoring the differences which are caused by different distance factors. The M2FCA method addresses the internal spatial distribution problem by deriving fire area data from a smaller unit, hexagon cell. The potential cross border demand-supply interaction is taken into consideration by using circles of reasonable radius around each hexagon cell centroid as the basic spatial units, which can encompass areas on either side of an administrative border. After division of maximum threshold range which fr re brigade could reach in time, the M2FCA method considers the demand point within the threshold have different access to the fr re fighting service capacity during the process of fire-fIghting spatial accessibility measurement study of Ch engbaqu (former) Beijing city. The result shows that the improved M2FCA could reveal more detailed differences of spatial distribution in larger region, it could measure fire-fighting spatial accessibility precisely, it could accurately reveal the real fire-fighting service capacity people obtain, and predicate the areas without fire-fighting service, therefore, M2FCA could provide the basis for relevant authority to make decisions for planning and selecting sites.
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