Sensitivity, thermal

Thermal sensitivity data are now primarily obtained using the thermal analysis method of differential scanning calorimetry (DSC) (see Chapter 2). Thermal sensitivity is the most reproducible, and best understood, of the various types of sensitivity tests, owing to the relatively minor effect of the specific test conditions on the value that is obtained. Thermal sensitivity is the result of the chemistry that occurs in a given mixture upon heating, and it is relatively independent of factors such as homogeneity, particle size, and percent composition. It can be very dependent, however, on the addition or omission of a component in a particular composition, especially if that component plays a role in the initial exothermic reaction that occurs upon heating of the material. 

Source L. M. Aikman et al., Improved Mixing, Granulation, and Drying of Highly Energetic Pyromixture. Propellants, Explosives, Pyrotechnics, 12, 17, 1987. 

Thermal sensitivity therefore requires that a sample be heated to a temperature where two conditions are met  

The oxidizer reaches a temperature where sufficient evolution of oxygen occurs. 

The fuel reaches a temperature where it is in a chemical state capable of reacting with the oxygen released by the oxidizer. For a metal fuel, this will typically occur as the metal approaches its melting point For a carbon-based fuel, this will occur when the compound reaches its decomposition temperature (typically between 200 and 350°C). 

Thermal sensitivity is an important criterion with pyrolants as it determines both safety and functionality in terms of rehability of ignition and propagation. Prehmi-nary indicators for the thermal stability are any means of thermal measurements such as Differential Scanning Calorimetry (DSC) (see Chapter 5), Differential Thermal Analysis (DTA) and Thermogravimetry (TG) or Differential Thermogravimetry (DTG). More precisely, the Heat Flow Calorimetry (HFC) measurements at a fixed temperature are able to probe the intrinsic stability of an energetic material [16]. 

To assess the overall thermal sensitivity of an energetic material in Germany, a number of tests related to the ignition towards standard ignition sources such as cerium-iron alloy sparks, bickford fuse flame jet, gas flames, red hot steel bar and match flame are conducted. In addition, the response of the material towards ignition under various levels of enclosure is tested, the so-called Koenen test [17]. 

The thermal tests (except for Koenen test) for three metal-fluorocarbon pyrolants are listed in Table 19.3. 

The thermal sensitivity of MTV and Magnesium/Poly(carbon monofluoride)/ Viton MPV is significant, and the substances are classified easily ignitable. Ignition temperatures for MTV formulations are listed Table 19.4. 

Farnell et al. investigated the ignitability of pyrotechnic compositions by solvent vapour flames [18]. In addition, they also looked at the ignition delay of compositions moistened with the solvent The ignition delays of mixtures of 5 g FW — 306 with 4cm acetone and alcohol each are 83 and 174 s, respectively. 

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Thermal reforming Thermal sensitization Thermal stability Thermal transfer Thermal-transfer printing Thermal treatment Thermal wave imaging Thermate Thermate-TH2 Thermate-TH3 Therm-Chek... 

Hydroperoxides are photo- and thermally sensitive and undergo initial oxygen—oxygen bond homolysis, and they are readily attacked by free radicals undergoing induced decompositions (eqs. 8—10). 

Typical substrates for siUcone release coatings are supercalendered kraft paper, glassines, and thermally sensitive films such as polyethylene and polypropylene. Ideal curing conditions are 150°C or lower, and line speeds are as fast as 460 m /min. Key properties for release coatings are cure speed, integrity of cure, and stable release values. 

The stiffness of pure titanium can be increased slightly by alloying alloys such as Ti—6A1—4V may be specified for partial dentures requiring additional rigidity. Titanium appHances do not tarnish or corrode in the mouth, have no metallic taste, and are easy to clean because plaque and calculus do not adhere to them. The relatively low thermal conductivity of titanium (relatively close to that of tooth enamel) gives these appHances a seemingly natural feel in the mouth and minimises thermal sensitivity (175). 

Thermal sensitivity is the potential for a material to explode under a thermal stimulus. Test methods are outlined in CCPS G-13. 

A thermistor is a thermally sensitive, semiconductor solid-state device, which can only sense and not monitor (cannot read) the temperature of a sensitive part of equipment where it is located. It can operate precisely and consistently at the preset value. The response time is low and is of the order of 5-10 seconds. Since it is only a temperature sensor, it does not indicate the temperature of the windings or where it is located but only its preset condition. 

As can be seen in the table above, the upper two results for heat transfer coefficients hp between particle and gas are about 10% apart. The lower three results for wall heat transfer coefficients, h in packed beds have a somewhat wider range among themselves. The two groups are not very different if errors internal to the groups are considered. Since the heat transfer area of the particles is many times larger than that at the wall, the critical temperature difference will be at the wall. The significance of this will be shown later in the discussion of thermal sensitivity and stability. 

The main categories of electrical/optical ceramics are as follows phosphors for TV, radar and oscilloscope screens voltage-dependent and thermally sensitive resistors dielectrics, including ferroelectrics piezoelectric materials, again including ferroelectrics pyroelectric ceramics electro-optic ceramics and magnetic ceramics. 

PDMS based siloxane polymers wet and spread easily on most surfaces as their surface tensions are less than the critical surface tensions of most substrates. This thermodynamically driven property ensures that surface irregularities and pores are filled with adhesive, giving an interfacial phase that is continuous and without voids. The gas permeability of the silicone will allow any gases trapped at the interface to be displaced. Thus, maximum van der Waals and London dispersion intermolecular interactions are obtained at the silicone-substrate interface. It must be noted that suitable liquids reaching the adhesive-substrate interface would immediately interfere with these intermolecular interactions and displace the adhesive from the surface. For example, a study that involved curing a one-part alkoxy terminated silicone adhesive against a wafer of alumina, has shown that water will theoretically displace the cured silicone from the surface of the wafer if physisorption was the sole interaction between the surfaces [38]. Moreover, all these low energy bonds would be thermally sensitive and reversible. 

Perfluoroalkylmagnesium compounds undergo many of the reactions of their hydrocarbon analogues Care must be exercised in controlling the reaction temperature because of their thermal sensitivity... 

Thermal Sensitivity. At 375—90° it decomps with burning in 6-8secs (Ref 2). In a 20mm tube combstn is difficult. It melts and boils before burning and only the heated material burns without propagation to unheated material. When unconfined it melts and boils without burning (Ref 1)... 

Application of Thermally Sensitive Binders As an Ordnance Disposal Method , SRI, Calif, Progress Rapt 2426-6, Contract No N00017-73-C4329, Mod P00001 (1974) 72) R.A. 

In a plasma-activated reaction, the substrate temperature can be considerably lower than in thermal CVD (see Ch. 5, Sec. 9). This allows the coating of thermally sensitive materials. The... 

Similarly, the thermal sensitivity of sulfur allotropes makes mass spectrometry of elemental sulfur and sulfur-rich compounds difficult especially with the conventional electron impact ionization. Nevertheless, valuable information has been obtained by this technique also. 

Haddon, W. E. and Harden, L. A., Advantages of particle beam sample introduction for analysis of thermally sensitive natural products by mass spectrometry , in Proceedings of the 39th ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN, May 19-24, 1991, pp. 1316-1317. 

Structure preservation at different temperatures was studied for the decay of in Sb(C6H5)j to Te. Here a thermal sensitivity of the Te—CgHs bond was revealed, which is presumably due to the intermediate state Te(C6H5)5 ... 

Another beneht of continuous mixing is the better temperature control. This enables the control of a chemical reaction, if any, and the application of thermal sensitive raw materials, which cannot be used in internal mixers. 

Evaporation, Heat exchange, condensing vapours U.sually not May become necessary when processing thermally sensitive materials or if fouling of heat-transfer surfaces is possible. 

In the vast majority of gas-solid reactions, gaseous or evaporated compounds react at the surface of a solid catalyst. These catalytic processes are very frequently used in the manufacture of bulk chemicals. They are much less popular in processing of the large molecules typical of fine chemistry. These molecules are usually thermally sensitive and as such they will at least partially decompose upon evaporation. Only thermally stable compounds can be dealt with in gas-solid catalytic processes. Examples in fine chemicals manufacture are gas-phase catalytic aminations of volatile aldehydes, alcohols, and ketones with ammonia, with hydrogen as... 

A reactor is run adiabatically when no heat is exchanged between the reaction zone and the surroundings. The reaction temperature can then only be controlled by quenching with a cold stream of the reaction mixture or by inter-stage heat exchangers. For highly thermally sensitive large molecules treated in the fine chemicals sector this is a very impractical mode of operation. Therefore, adiabatic reactors will not be discussed here. 

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