Electrothermal Monte Carlo Simulations of InGaAs/AlGaAs HEMTs

Results from the application of our electrothermal simulator to n-type 0.15 μm gate In_0.15Ga_0.85As-Al_0.28Ga_0.72As HEMT structures are presented. The simulator involves an iterative procedure which alternately solves the Heat Diffusion Equation (HDE) and executes a Monte Carlo electronic transport algorithm. The net thermal flux generated during each Monte Carlo stage, calculated from the net rate of phonon emission, is fed into the thermal solution; the resulting temperature map is then used in the following Monte Carlo iteration. The HDE is solved through application of a novel analytical thermal resistance matrix technique which allows calculation of temperatures solely within the region of interest while including the large-scale boundary conditions. A novel charge injection scheme is applied for the treatment of side ohmic contacts, which avoids anomalous generation of thermal flux in adjacent regions. The characteristic ‘thermal droop’ is found in the I-V characteristics of the simulated device. Associated temperature distributions are shown to be spatially non-uniform with peak values and spatial locations dependent upon bias and the length of the containing die. Electron drift velocities and energies along the HEMT channel exhibit the largest shift on the inclusion of thermal self-consistency below the drain end of the gate, not at the location of the temperature peak.     

Urban Simulacra: London

As the digital revolution deepens and pervades every aspect of daily life, virtual realities begin to penetrate one another in a multiplicity of ways. The amount of sensing data being compiled on the city grows, enabling the construction of virtual realities that can, in turn, be transformed for diverse purposes. Here, Michael Batty and Andrew Hudson-Smith from the Centre for Advanced Spatial Analysis, University College London, outline how they went about the construction of a virtual city in central London. A conventional 3-D-GIS/CAD model was used as the basis on which to build a digital realm in which designers are cast as avatars and populations as agents, so as to define new ways in which to understand and plan the city. Copyright © 2005 John Wiley & Sons, Ltd.     

Apparent thermal conductivity of glass-fibre insulant: effects of compression and moisture content

The line source thermal probe technique was validated and used to measure the apparent thermal conductivities of commercially available dry and moist glass-fibre blankets under various compressions. For the dry insulant, a minimum apparent thermal conductivity of 0·039 W m^?1 K^?1 occurred at ～ 20°C for optimal values of bulk density—and consequently of volume voidage—of 0·45 kg m^?3 and 0·985, respectively. Observations for the apparent thermal conductivities of moist insulants did not agree with theoretical predictions but were corroborated by the experimental results of other investigators.     

Natural-cooling techniques for residential buildings in hot climates

In most developing countries in the hot regions of the world during the last 50 years there have been many buildings constructed that are highly energy wasteful. The interiors of these buildings have to be mechanically air-conditioned in order to achieve thermal-comfort conditions. Consequently vast financial expenditures have and are being spent upon the installation and operation of the electricity-generating plants needed to supply the maximum power demand during summer. In many of these countries, air-conditioning accounts for more than 70% of the national electrical power consumption. However, less than a century ago, the inhabitants of these countries lived in buildings that incorporated only natural-cooling (i.e. far cheaper) techniques for achieving thermal comfort. In this paper, these tranditional beneficial techniques have been reviewed and the recent advances in both hybrid and active natural-cooling systems described.     

Polymerization of acrylonitrile in the presence of vinylsulphone dyes

Information regarding the effect of vinylsulphone dyes on the kinetics of polymerization of acrylonitrile is presented in detail. Some of the physical properties of the products, polyacrylonitrile/vinylsulphone dye copolymers, are discussed, although the subject of their thorough characterization is reserved for a later publication. Considerable detail is given concerning the experimental procedures, to ensure ease of understanding of the techniques involved. ⁶⁰Co γ rays were employed as the polymerization initiator whilst the effect of γ radiation on the chromophore was noted, where relevant.     

Structural order in alkali-degraded PAN as seen in studies of low-temperature thermal conductivity

The thermal conductivity, κ, of polyacrylonitrile (PAN), subjected to varying degrees of degradation, has been measured in the temperature range 2–100 K as a means of studying the structural order of the samples. Although, at higher temperatures, the variation in κ throughout the range of specimens is only ～ 30%, at 2 K there is an order of magnitude increase in conductivity between the undegraded and most highly degraded samples. Such a change is indicative of a pronounced decrease in crystallinity, and this interpretation is supported by estimates of crystallinity as seen through X-ray analysis.     

Competition in the Built Environment: Scaling Laws for Cities,Neighbourhoods and Buildings

Built environments at any spatial scale are represented as sets of like objects—cities, neighbourhoods, buildings but also components such as streets, parks, etc.,—which are distributed spatially according to certain rules that we are only just beginning to detect and measure. Unlike normally distributed attributes of the population, these urban elements often scale according to forces that determine large numbers of small elements and a small number of large ones. While many objects evolve from small to large, not all objects can be large as resources as well as physical limits determine the distribution of their sizes. The theory behind such size distributions is referred to as scaling and the shape of such distributions is quite well-defined by various power and exponential laws. As objects grow and evolve, or are even designed, their form changes qualitatively due to the forces of competition and the constraints on space. Here we explore these ideas for cities, neighbourhoods in towns, and buildings, specifically high buildings, revealing that there are both important differences as well as similarities in their form, function, and structure.     

Structural design and performance of a rear support walking frame

Rear support walking frames provide predetermined vertical support for patients with dysfunctional lower limbs that have limited active control; the support is provided through a spring-loaded boom hinged on an upright stanchion mounted at the rear of a wheeled frame within which the patient ambulates. The application of these frames for total-body-involved cerebral palsy patients, in combination with a walking orthosis, has highlighted a number of practical problems that need to be addressed for the system to become fully viable. A composite material prototype walking frame has been developed that permits the patient to be transferred by a single carer without the need to use inappropriate manual handling techniques. The frame has improved structural properties, with stiffness in the sagittal and coronal planes increasing by between 50 and 100 per cent. Evaluation with patients showed that the greater structural stiffness permitted the objective of improved continuity of walking to be achieved. The strength of the frame is such that it can accommodate patients of up to 80 kg, more than twice that possible in the earlier system. Since the structural yield point is approximately twice the maximum working load, the device should not be prone to unacceptable fatigue characteristics. Despite the use of carbon composite materials (which have brittle failure characteristics), the mode of failure is of progressive collapse and is therefore inherently safe. The successful outcome of prototype testing has justified production development. Work is now proceeding on a design that incorporates further improvements in structural performance and ease of manufacture.