Sample heating

Small probed regions down to 1-2 pm are possible using microscope lenses. Lasers can supply as much pump power as needed to compensate for weaker signals, but a limit is reached when sample heating or nonlinear optically induced processes become significant. 

Because the laser beam is focused on the sample surface the laser power is dissipated in a very smaU area which may cause sample heating if the sample is absorbing and may cause break-down if the sample is susceptible to photodecomposition. This problem sometimes may be avoided simply by using the minimum laser power needed to observe the spectrum. If that fails, the sample can be mounted on a motor shaft and spun so that the power is dissipated over a larger area. Spinners must be adjusted carefully to avoid defocusing the laser or shifting the focal spot off the optic axis of the monochromator system. 

Sample Heating Weight H/C Yield BET L. (002) Reversible Irrev. 

The single-point BET surface area measurement was used to check for open pores. The results for some soft and hard carbon samples heated at 700°C and 1000°C are presented in Table 2 for comparison. The hard carbon samples studied here have about ten times more open porosity than the soft carbons. 

Fig. 17. Voltage versus capacity for the second cycle of the KS pitch samples heated at different temperatures as indicated.

All samples heated at 700 and 800°C show significant hysteresis that is, lithium is inserted in the materials near zero volts and removed at about one volt. We have shown that the amount of lithium which can be inserted in 700°C materials is directly proportional to their hydrogen (H) content. Table 4 shows that materials heated to 700 and 800°C retain substantial hydrogen. Upon heating to 900 C, the hydrogen is predominantly eliminated and so is the hysteresis. The samples then show substantial recharge capacity at low voltages. 

Electron irradiation (100 keV) of the sample, heated to 800°C, yields MWCNTs (20-100 nm in length) attached to the surface. Such nanotube growth does not take place if natural graphite, carbon nanoparticles or PTFE are subjected to electron irradiation. The result implies that the material may be a unique precursor for CNTs and may constitute a new preparation method. 

IVInOj sample Heat treatment temp. (°C) Initial volume of 50g (mL) Pore volume by N, desorption (mL/g 1 ) EPV ( mL/g 1) %based on N2 pore volume Remarks ... 

V-dimethylaminomethylene derivatives of primary amides Add 250 fjL 1 of Methyl-8 reagent to less than 1 mg of sample. Heat at 60° for 20-30 min. 

Preparation of methoxime derivatives Add 0.5 ml of MOX reagent to the sample. Heat at 60° for 3 hours. Evaporate the reaction mixture to dryness with clean, dry nitrogen. Dissolve in the minimum amount of ethyl acetate. Some solids will not dissolve. 

Add 150 fi 1 of acetic anhydride and 100 fil of pyridine to less than I mg of sample. Heat at 60° for 30 min. Evaporate to dryness with clean, dry nitrogen. Dissolve residue in 25 ju.1 of DMF or other suitable solvent. 

The techniques referred to above (Sects. 1—3) may be operated for a sample heated in a constant temperature environment or under conditions of programmed temperature change. Very similar equipment can often be used differences normally reside in the temperature control of the reactant cell. Non-isothermal measurements of mass loss are termed thermogravimetry (TG), absorption or evolution of heat is differential scanning calorimetry (DSC), and measurement of the temperature difference between the sample and an inert reference substance is termed differential thermal analysis (DTA). These techniques can be used singly [33,76,174] or in combination and may include provision for EGA. Applications of non-isothermal measurements have ranged from the rapid qualitative estimation of reaction temperature to the quantitative determination of kinetic parameters [175—177]. The evaluation of kinetic parameters from non-isothermal data is dealt with in detail in Chap. 3.6. 

Such differences in the secondary structure behavior with respect to temperature can be explained by suggesting that molecular close packing of proteins in the film is the main parameter responsible for the thermal stability. In fact, as in the case of BR, we have close packing of molecules even in the solution (membrane fragments) there are practically no differences in the CD spectra of BR solution at least tiU 75°C (denaturation takes place only for the sample heated to 90°C). RC in solution begins to be affected even at 50°C and is completely denatured at 75°C, for the solution contains separated molecules. 

Low-intensity light sources should give efficient irradiation of thin liquid layers [21]. Sample heating is reduced and so is radical recombination. In addition, oxygen enrichment of solutions before and after micro reactor passage can be handled differently and is no longer a major safety problem [21]. 

To study the nucleation and growth of Au nanoclusters in silica within the above theoretical frame, we implanted fused silica slides with 190keV-energy Au ions, at room temperature and current densities lower than 2 pA/cm, to reduce sample heating [49,50]. The implantation conditions were chosen to have, after annealing, a subsurface buried layer of Au nanoparticle precipitation of about... 

The TPSR technique has also been used by Konduru and Chuang [160] in order to investigate N20 and NO decomposition pathways on Cu-ZSM-5. The infrared monitoring of the adsorbed species during the sample heating under NO showed that the Cu+(NO) intensity parallels the rate of N20 formation. This TPSR result allowed the authors to suggest that NO adsorbed on Cu+ acts as precursor for N20 formation. 

Copper metal surface area was determined by nitrous oxide decomposition. A sample of catalyst (0.2 g) was reduced by heating to 563 K under a flow of 10 % H2/N2 (50 cm min"1) at a heating rate of 3 deg.min 1. The catalyst was then held at this temperature for 1 h before the gas flow was switched to helium. After 0.5 h the catalyst was cooled in to 333 K and a flow of 5 %N20/He (50 cm3mirr ) passed over the sample for 0.25 h to surface oxidise the copper. At the end of this period the flow was switched to 10 % H2/N2 (50 entitlin 1) and the sample heated at a heating rate of 3 deg.min"1. The hydrogen up-take was quantified, from this a... 

Figure 7.4 STM chamber in situ sample heating system.

A sample heated in a sealed tube exploded violently at 70°C. Unconfined, the peroxide decomposed (sometimes explosively) immediately after melting at 79°C. 

A large scale explosion involving initiation of wet sodium picrate by impact was investigated [1], A small sample heated to above 250°C exploded with sufficient violence to destroy the DTA thermocouple [2],... 

A small sample heated to 160°C decomposed exothermically, reaching 250°C. It had previously been distilled at 9570.65 mbar without decomposition. 

Small samples heated rapidly in sealed tubes to around 100°C exploded. 

A sample heated above its m.p. (88° C) exploded violently. See other acyl azides... 

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