Isothermal luminescence

Fig. 12. Decay of the isothermal luminescence of y-irradiated vitreous methylcyclohexane containing 10 2M biphenyl at various temperatures. , 77K +, 66K O, 4K. The curves are normalized to the initial intensity at t0 = 240 s after the end of irradiation. The data are from ref. 52.

Choubey A., Das S., Sharma S. K. and Manam J. 2010.Calculation for the trapping parameters of K3Na(S04)2 phosphor hy isothermal luminescence decay method. Mater. Chem. Phys. 120 472-475. 

In the usual TL experiments, the samples are irradiated at low temperature (when kT Eg) then kept in the dark at the excitation temperature Tex in order to quench the isothermal luminescence. A uniform heating with small constant velocity continues up to a temperature at which all the charges have been thermally excited out of traps and luminescence completely disappears. The intensity of TL emission does not remain constant at constant temperature but decreases with time and eventually ceases altogether (Rivera 2011 Chen and McKeever 1997). 

Abstract We describe mechanochromic and thermochromic photoluminescent liquid crystals. In particular, mechanochromic photoluminescent liquid crystals found recently, which are new stimuli-responsive materials are reported. For example, photoluminescent liquid crystals having bulky dendritic moieties with long alkyl chains change their photoluminescent colors by mechanical stimuli associated with isothermal phase transitions. The photoluminescent properties of molecular assemblies depend on their assembled structures. Therefore, controlling the structures of molecular assemblies with external stimuli leads to the development of stimuli-responsive luminescent materials. Mechanochromic photoluminescent properties are also observed for a photoluminescent metallomesogen and a liquid-crystalline polymer. We also show thermochromic photoluminescent liquid crystals based on origo-(/ -phenylenevinylene) and anthracene moieties and a thermochromic photoluminescent metallocomplex. 

Lum, luminescence (t = lifetime, Q = quantum yield) Magn, magnetic susceptibility Ads, adsorption isotherm. 

Fig. 2 (Left panel) Structure cf sensor 1. (Center panel) Changes in luminescence intensity of sensor I upon the addition cf cyanide. [(Inset) Binding isotherm monitored by the integrated luminescence intensity.] (Right panel) Changes in the time-resolved PL decay cf sensor I upon the addition of cyanide. [(Inset) Shifts in fractional intensity of the two hfetime components obtained from biexponential analysis.]...

This chapter discusses the intenelation between mechanical properties, molecular mobility and chemical reactivity of curing epoxy-amine thermosets, illustrated by examples of how the charge recombination luminescence (CRL), heat-capacity and rate constants of chemical reactions are influenced by gelation and vitrification during isothermal cure. A comparison of dynamic mechanical, CRL and modulated temperature DSC data shows that vitrification is accompanied by an increase in CRL and a decrease in heat-capacity, and that the heat-capacity and CRL continue to change after the viscoelastic properties have levelled out. It is also shown how the rate constant of an intermolecular secondary amine reaction, measured by near infirared spectroscopy, is sensitive to gelation, whereas the intramolecular rate constant instead is sensitive to vitrification. 

In this chapter the interrelation between mechanical properties, molecular mobility and chemical reactivity is discussed. Examples of how the changes in charge recombination luminescence, heat capacity and rate constants of chemical reactions can be related to the evolution of viscoelastic properties and the transitions encountered during isothermal cure of thermosetting materials are given. The possible application of the experimental techniques involved to in-situ cure process monitoring is also reviewed. 

Charge recombination luminescence was measured in a set-up described in detail elsewhere (75). Stoichiometric mixtures of DGEBF and DDM in aluminium pans were taken to the cure tenq)erature at 15 G/min and cured isothermally under nitrogen in a chamber covered by a quartz window. The sanq>le was intermittently irradiated with a Kulzer Duralex UV-3(X) fibre optic wand for 60 s. After each irradiation the shutter of the photomultiplier was opened with a delay of 5 s, and the initial intensity of emitted light, 7o, was recorded. 

Figure 3. Charge-recombination luminescence intensity during isothermal cure of DGEBF/DDM at three diffoent temperatures.

Figure 16 (A) buckling of an EBPVD TBCs after isothermal oxidation (B) an example of asymmetrical luminescence spectrum recorded through the point in (A). The two doublets are indicated by the sohd and dashed lines, where the solid and dashed lines correspond to intact TGO and debonded TGO, respectively.

Table 1.27 lists some general applications of chemiluminescence. Early CL studies were limited by a low S/N ratio. Thus, oxidation eould only be observed at high temperatures where other techniques provide more direct results. With the development of fast pulse analysers low light levels ean now be measured with great sensitivity (a few photons/s). Consequently, it has been claimed that low temperature luminescence measurements are useful in lifetime prediction [570]. Some major applications of the CL technique are identified as isothermal measurements in oxygen to study progress of oxidation, and in nitrogen to estimate the amounts of peroxides present... 

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