Heat sinks dimensions

Table 8-3. Heat sink dimension, base plate 25 x 25 x 2 x mm ...

A second example of interest in the present context refers to the scaling of thermal effects. Any object (a chemical reactor such as a living body) that produces heat at a rate proportional to its volume ( <2r a Vr) and exchanges heat with a cooling device or with the ambient at a rate proportional to its lateral surface Sl and to the temperature difference with respect to the external heat sink (i.e., Qe = USe(Tt - Ta)) can maintain the same temperature, independently of its dimensions, only if the ratio USe/Vx is kept constant. In general, this condition cannot be satisfied, since the ratio SeJ V) is inversely proportional to the characteristic linear dimension, and the... 

Taking into account typical numbers for a and D, this underlines that the channel width should be considerably smaller than 1 mm (1000 pm) in order to achieve short residence times. Actually, heat exchangers of such small dimensions are not completely new, because liquid cooled microchannel heat sinks for electronic applications allowing heat fluxes of 790 watts/cm2 were already known in 1981 [46]. About 9 years later a 1 cm3 cross flow heat exchanger with a high aspect ratio and channel widths between 80 and 100 pm was fabricated by KFK [10, 47]. The overall heat transport for this system was reported to be 20 kW. This concept of multiple, parallel channels of short length to obtain small pressure drops has also been realized by other workers, e.g. by PNNL and IMM. IMM has reported a counter-current flow heat exchanger with heat transfer coefficients of up to 2.4 kW/m2 K [45] (see Fig. 3). 

An alumina substrate of dimensions 1 cmx 1 cmx 0.5 mm carries a device dissipating 20 W. If the substrate is bonded to a metallic heat sink, estimate the steady state difference in temperatures between the surface carrying the device and the heat sink. The thermal conductivity of alumina may be taken to be 35 Wm-1 K-1. [Answer 3°C]... 

A compact sensor of greatly reduced dimensions (outer diameter x length 36 x 46 mm) has been constructed and is shown in Fig. 2. In order to conveniently accommodate enzyme columns and to ensure isolation from ambient temperature fluctuations, a cylindrical copper heat sink was included. An outer Delrin jacket further improved the insulation. The enzyme column (inner diameter x length 3x4 mm), constructed of Delrin, was held tightly against the inner terminals of the copper core. Short pieces of well-insulated gold capillaries (outer diameter/inner diameter 0.3/0.2 mm) were placed next to the enzyme column as temperature-sensitive elements. Microbead thermistors were mounted on the capillaries with a heat-conducting epoxy. Two types of mini system has been constructed as discussed below. 

One of the major markets for wide band-gap materials is in electronics. Specifically, they are suitable for and have been used for heat sinks (diamond), short wavelength optoelectronic devices (GaP, GaN, SiC), high-temperature electronics (SiC, GaN), radiation resistant devices, and high-power/high-frequency electronic devices (diamond, GaN, SiC). " Recent research showed that Mn-doped GaN can be used for spintro-nic applications.f" Atomically flat technology developed by NASA for SiC and GaN WBG material can introduce a new dimension of application for WBG materials. 

The capillaries are made either of borosilicate glass or fused quartz. Pyrex glass has a cutoff at about 280 nm, so fused silica is often used. The capillary tube is coated with a polyimide to enhance flexibility by preventing the hydration of strained siloxy bonds. The columns vary in internal dia-meter from 10 to 100 pm and in length from 30 to 100 cm, with 50 to 100 cm being common. The capillary side wall is usually thick by comparison, 300 to 600 pm, to act as a heat sink. A small portion (1 to 2 mm) of the coating is burned away if a photometric detector is used. Figure 31-6 shows typical dimensions. 

The high viscosity of concentrated sulfuric acid causes a fairly high backpressure, which was mandatory for residence times as short as possible. The homemade pilot plant continuous reaction system consisted of a static mixer and a residence time loop with a total volume of565 mb (Fig. 15.19). The maximum temperature of the whole system was limited to 60 °C, because it was foimd that the complete heat of reaction was liberated within one second during the flow through the static mixer. To control the reaction temperature in this scale correctly dimensioned heat sinks are mandatory. With these precautions the same results were obtained as before. Using this equipment over 200 kg of4-(phenyl)morpholin-3-one were nitrated within 50 h. 

The equipment used in DTA studies is shown schematically in Fig. 16.16. The sample is loaded into a crucible, which is then inserted into the sample well (marked S). A reference sample is made by placing a similar quantity of inert material (such as AI2O3) in a second crucible. This crucible is inserted in the reference well, marked R. The dimensions of the two crucibles and of the cell wells are as nearly identical as possible furthermore, the weights of the sample and the reference should be virtually equal. The sample and reference should be matched thermally and arranged symmetrically with the furnace so that they are both heated or cooled in an identical manner. The metal block surrounding the wells acts as a heat sink. The temperature of the heat sink is slowly increased using an internal heater. The sink in turn simultaneously heats the sample and reference material. A pair of matched thermocouples is used. One pair is in contact with the sample or the sample container (as shown) the other pair is in contact with the reference. The output of the differential thermocouple, or AT, is amplified and sent to the data... 

The dimensions of the tested heat sinks are summarized in Table 8-3. 

Quantitative DTAs have been modified by experienced researchers in order to measure the thermal conductivity of polymer systems [6-8], The procedure outlined below is reservedfor experienced DSC users. The sample is placed in direct contact with the sample holder, so that the instrument can record the temperature and the heat flow on one side of the sample. A heat sink of known temperature is placed in contact with the other side of the sample. Knowing the sample dimensions, the temperature difference between the sample holder and the heat sink and the recorded heat flow, the thermal conductivity of the sample can be calculated as follows ... 

Switches and Relays Grounding contacts Thermal contacts Integrated circuits Cyclic electrical connection For shielding Heat sinks Small dimension complex systems ... 

Consideration of tunnel dimensions is critical for the reflow application. Short tunnel ovens may not permit a profile adequate for attaining prescribed reflow temperature-versus-time profiles for larger, thermally massive PCB assemblies. Tunnel height dimensions must also be adequate to accommodate the tallest components or component heat sinks. Tunnel width will limit the size of the board that can be introduced to it. 

As indicated in Figure 13, the sample is placed in a heater or furnace surrounded by a coolant or heat sink and insulation. For the measurement of linear thermal expansivity the bottom of the probe is perfectly flat, and the probe acts merely as a push rod. Connected to the upper end of the probe is an Ivdt and some mechanism for maintaining the probe-lvdt system vertical. The sample is heated at a controlled or programmed rate, and its change in linear dimension is measured by the displacement of the Ivdt core. The sample probe and outer tube are made of a low thermal-expansion material, usually fused quartz, and are... 

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