Heat sample-surrounding

A variation, which results in a more simple apparatus, is the drop calorimeter. The test piece is heated (or cooled) externally, dropped into the calorimeter and the resultant change in temperature monitored. For the simplest measurements, the calorimeter need not be surrounded by an adiabatic jacket but in that case, corrections for the heat exchange with the surroundings must be applied. A procedure using a drop calorimeter has been standardized for thermal insulation in ASTM C35l". It is possible to combine the adiabatic and drop calorimeter methods by dropping a heated sample into an adiabatic chamber and this has been used for plastics12. 

The sample absorbs microwave radiation and converts the absorbed electromagnetic radiation to heat that the sample retains. Hence, the sample can be brought to a very high temperature without needing to heat the surroundings. Not all materials absorb microwave radiation. As with any matter-electromagnetic radiation interaction, three possibilities exist. The material can (1) reflect the radiation, (2) transmit the radiation with minimal attenuation, or (3) absorb the radiation. 

A vacuum of from 4-6 torr (0.53 - 0.80 kPa) is common, and the heat loss by the subliming water usually will keep the material frozen. During small-scale laboratory operations the sample holders are left in the open air, but in large-scale commercial applications, heat must be applied to provide the sublimation energy at the rate Just below that required to keep the material frozen. To sublime 1 g of ice at 0 C requires 666 calories (2.78 kJ). This heat is provided by warm-water trays placed under the sample trays, by radiation from heated walls surrounding the sample trays, or by microwave warming. Sublimation from commercial warm water-heated trays occurs at the rate of 0.1-1.0 kg HjO/hr/m. Few systems are cooled below -30 °F, because the vapor pressure is then too low for rapid sublimation. 

Generally, in the previously described heated sample holders, few attempts were made to control the atmosphere surrounding the sample as it was heated. A cover plate of Pyrex glass or quartz was employed but its main purpose was to prevent the sample from accidentally falling into the integrating sphere of the spectroreflectometer. In order to control the sample atmosphere, the sample holder shown in Figure 9.3 was constructed by Wendlandt and Dosch (16). 

On the whole, two types of calorimeters exist one is where the sample, located in a flat holder, is placed npon the sensitive part of the calorimeter, shaped as a horizontal plate. Thus, the sample may exchange a vertical heat flux between this temperature-sensitive plate. The other consists of a cylindrical sample surrounded by a temperature-sensitive cylinder, and the heat flux exchanged is radial. When this... 

The apparatus comprises a rod-shaped heat source surrounded by a tubular sample inside a cylindrical heat sink. The heat sink and the heat source have the same axis. The temperature of the heat sink is controlled, usually by circulating a liquid from a constant temperature bath, and a known power is supplied to the heat source. The temperature difference between the heat source and the heat sink is measured after steady state conditions have been established. The conductivity is obtained from Eq. 8. 

Specific heat measurements were made using a Perkin-Elmer DSC-IB differential scanning calorimeter. The sample size was generally 15 mg. and the data was obtained for a sample surrounded by a dry nitrogen atmosphere. Synthetic sapphire was used as a reference. In each run, carried out at a rate of 8°/min, a temperature range of 20°K was explored. D.S.C. scans were obtained at scanning rates of 32°C/min. 

The kinetic parameters of thermal decomposition were determined for the different samples as a function of the heating rates. The heating rate, surrounding environment, material composition, and temperature affects the thermal decomposition of materials. The rate-controlling step (reaction rate) in pyrolysis is the material and its physical size and ratio between the sample material and the surrounding chemical gas. 

One of the simplest and easiest soil sample preparation methods was described in a study of the uranium and thorium content in soil and plants grown near an abandoned lead-zinc-copper mine in Turkey (Sasmaz and Yaman 2008). Soil samples surrounding roots of plants were collected at 30-40 cm depths, dried at 100°C for 4 h, and ground using hand mortars. A 1 1 1 mixture of HCI-HNO3-H2O was added to the sample (6 mL for 1 g) and heated for 1 h so that sample constituents, except silica, were dissolved. The uranium concentration in the soil samples was 1.1-70.3 mg kg with lower values... 

Heat flux DSC usually consists of a cell containing reference and sample holders separated by a bridge that acts as a heat leak surrounded by a block that is a constant-temperature body (see Fig. 2.1). The block is the housing that contains the heater, sensors, and the holders. The holders are raised platforms on which the sample and reference pans are placed. The heat leak permits a fast transfer of heat allowing a reasonable time to steady state. The differential behavior of the sample and reference is used to determine the... 

Depending on the sensor type (size of the heated area), surrounding gas, and sample thickness, the available frequency... 

Measuring the gross heating value (mass) is done in the laboratory using the ASTM D 240 procedure by combustion of the fuel sample under an oxygen atmosphere, in a bomb calorimeter surrounded by water. The thermal effects are calculated from the rise in temperature of the surrounding medium and the thermal characteristics of the apparatus. 

For more efficient drying at elevated temperatures, the vacuum apparatus (Fig. 48(A)) is often used. The sample to be dried is placed in an inner tube surrounded by a heating jacket. 

In the ARC (Figure 12-9), the sample of approximately 5 g or 4 ml is placed in a one-inch diameter metal sphere (bomb) and situated in a heated oven under adiabatic conditions. Tliese conditions are achieved by heating the chamber surrounding the bomb to the same temperature as the bomb. The thermocouple attached to the sample bomb is used to measure the sample temperature. A heat-wait-search mode of operation is used to detect an exotherm. If the temperature of the bomb increases due to an exotherm, the temperature of the surrounding chamber increases accordingly. The rate of temperature increase (selfheat rate) and bomb pressure are also tracked. Adiabatic conditions of the sample and the bomb are both maintained for self-heat rates up to 10°C/min. If the self-heat rate exceeds a predetermined value ( 0.02°C/min), an exotherm is registered. Figure 12-10 shows the temperature versus time curve of a reaction sample in the ARC test. 

Usually, when heat flows into a system, its temperature rises. In this case, the temperature of the 50.0-g water sample might increase from 50.0°C to 80.0°C. When the hot plate in Figure 8.1 is shut off, the hot water gives off heat to the surrounding air. In this case, q for the system is a negative quantity. 

As is usually the case, the temperature of the system drops when heat flows out of it into the surroundings. The 50.0-g water sample might cool from 80.0°C back to 50.0°C. 

---