Infinitesimal cohomology and the Chern character to negative cyclic homology

There is a Chern character from K-theory to negative cyclic homology. We show that it preserves the decomposition coming from Adams operations, at least in characteristic 0. This is done by using infinitesimal cohomology to reduce to the case of a nilpotent ideal (which had been established by Cathelineau some time ago).Comment: Included reference for identification of relative Chern and rational homotopy theory characters; some minor editing for clarit

A negative answer to a question of Bass

In this companion paper to arXiv:0802.1928 we provide an example of an isolated surface singularity $R$ over a number field such that $K_0(R) = K_0(R[t])$ but $K_0(R) \neq K_0(R[t_1,t_2])$. This answers, negatively, a question of Bass.Comment: The paper was previously part of arXiv:0802.192

Geographic data science

Data science methods and approaches address all stages of transition from data to knowledge and action. Visualization of this data is essential for human understanding of the subject under study, analytical reasoning about it, and generating new knowledge. Geographic data science deals with data that incorporates spatial and, often, temporal elements. The articles selected for this special issue represent a mix of theoretical approaches and novel applications of geographic data science

Patterning the Condenser-Side Wick in Ultra-Thin Vapor Chamber Heat Spreaders to Improve Skin Temperature Uniformity of Mobile Devices

Vapor chamber technologies offer an attractive approach for passive heat spreading in mobile electronic devices, in which meeting the demand for increased functionality and performance is hampered by a reliance on conventional conductive heat spreaders. However, market trends in device thickness mandate that vapor chambers be designed to operate effectively at ultra-thin (sub-millimeter) thicknesses. At these form factors, the lateral thermal resistance of vapor chambers is governed by the saturation temperature/ pressure gradient in the confined vapor core. In addition, thermal management requirements of mobile electronic devices are increasingly governed by user comfort; heat spreading technologies must be designed specifically to mitigate hot spots on the device skin. The current work considers these unique transport limitations and thermal requirements encountered in mobile applications, and develops a methodology for the design of vapor chambers to yield improved condenser-side temperature uniformity at ultra-thin form factors. Unlike previous approaches that have focused on designing evaporator-side wicks for reduced thermal resistance and delayed dryout at higher operating powers, the current work focuses on manipulating the condenser-side wick to improve lateral heat spreading. The proposed condenser-side wick designs are evaluated using a 3D numerical vapor chamber transport model that accurately captures conjugate heat transport, phase change at the liquid–vapor interface, and pressurization of the vapor core due to evaporation. A biporous condenser-side wick design is proposed that facilitates a thicker vapor core, and thereby reduces the condenser surface peak-to-mean temperature difference by 37% relative to a monolithic wick structure

On the transient thermal response of thin vapor chamber heat spreaders: governing mechanisms and performance relative to metal spreaders

Vapor chambers can offer a passive heat spreading solution for thermal management in electronics applications ranging from mobile devices to high-power servers. The steady-state operation and performance of vapor chambers has been extensively explored. However, most electronic devices have inherently transient operational modes. For such applications, it is critical to understand the transient thermal response of vapor chamber heat spreaders and to benchmark their transient performance relative to the known behavior of metal heat spreaders. This study uses a low-cost, 3D, transient semi-analytical transport model to explore the transient thermal behavior of thin vapor chambers. We identify the three key mechanisms that govern the transient thermal response: (1) the total thermal capacity of the vapor chamber governs the rate of increase of the volume-averaged mean temperature; (2) the effective inplane diffusivity governs the time required for the spatial temperature profile to initially develop; and (3) the effective in-plane conductance of the vapor core governs the range of the spatial temperature variation, and by extension, the steady-state performance. An experiment is conducted using a commercial vapor chamber sample to confirm the governing mechanisms revealed by the transport model; the model accurately predicts the experimental measurements. Lastly, the transient performance of a vapor chamber relative to a copper heat spreader of the same external dimensions is explored as a function of the heat spreader thickness and input power. The mechanisms governing the transient behavior of vapor chambers are used to explain the appearance of key performance thresholds beyond which performance is superior to the copper heat spreader. This work provides a foundation for understanding the benefits and limitations of vapor chambers relative to metal heat spreaders in transient operation and may inform the design of vapor chambers for improved transient performance

Working-Fluid Selection for Minimized Thermal Resistance in Ultra-Thin Vapor Chambers

The behavior of a vapor chamber is strongly coupled to the thermophysical properties of the working fluid within. It is well known that these properties limit the maximum power (heat load) at which a vapor chamber can operate, due to incidence of the capillary limit. At this limit, the available capillary pressure generated within the wick structure balances the total pressure drop incurred along the path of fluid flow within the wick. A common figure of merit prioritizes working fluids that maximize this capillary-limited operating power. The current work explores working fluid selection for ultra-thin vapor chambers based on a thermal performance objective, rather than for maximized power dissipation capability. A working fluid is sought in this case that provides the minimal thermal resistance while ensuring a capillary limit is not reached at the target operating power. A resistance-network-based model is used to develop a simple analytical relationship for the vapor chamber thermal resistance as a function of the working fluid properties, operating power, and geometry. At small thicknesses, the thermal resistance of vapor chambers becomes governed by the saturation temperature gradient in the vapor core, which is dependent on the thermophysical properties of the working fluid. To satisfy the performance objective, it is shown that the choice of working fluid cannot be based on a single figure of merit containing only fluid properties. Instead, the functional relationship for thermal resistance must be analyzed taking into account all operating and geometric parameters, in addition to the thermophysical fluid properties. Such an approach for choosing the working fluid is developed and demonstrated

On the Transient Thermal Response of Thin Vapor Chamber Heat Spreaders: Optimized Design and Fluid Selection

Vapor chambers provide highly effective heat spreading to assist in the thermal management of elec- tronic devices. Although there is a significant body of literature on vapor chambers, most prior research has focused on their steady-state response. In many applications, electronic devices generate inherently transient heat loads and, hence, it is critical to understand the transient thermal response of vapor cham- bers. We recently developed a semi-analytical transport model that was used to identify the key mech- anisms that govern the thermal response of vapor chambers to transient heat inputs (Int. J. Heat Mass Trans. 136 (2019) 995–1005). The current study utilizes this understanding of the governing mechanisms to develop design guidelines for improving the performance of vapor chambers under transient operating conditions. Two key aspects of vapor chamber design are addressed in this study: first, a parametric op- timization of the wall, wick, and vapor-core thicknesses; and second, the selection of the working fluid. A protocol is demonstrated for selecting these parameters given the external vapor chamber envelope di- mensions and boundary conditions. The study helps provide a framework for designing vapor chambers subject to transient heat loads, and to differentiate such design from the practices followed traditionally for steady-state operation

Numerical Studies of QGP Instabilities and Implications

Because the initial shape of the QGP in a heavy ion collision is anisotropic, the momentum distribution becomes anisotropic after a short time. This leads to plasma instabilities, which may help explain how the plasma isotropizes. We explain the physics of instabilities and give the latest results of numerical simulations into their evolution. Nonabelian interactions cut off the size to which the soft unstable fields grow, and energy in the soft fields subsequently cascades towards more ultraviolet scales. We present first results for the power spectrum of this cascade.Comment: Talk given at workshop on Quark-Gluon Plasma Thermalization, Vienna, 10-12 August 2005. 8 page