• B P Chandra

Articles written in Pramana – Journal of Physics

• Triboluminescence, a new tool to investigate fracture-initiation time of crystals under stress

The present paper reports that triboluminescence (TBL) does not appear at the instant of impact of the load but a certain time lag is required for its appearance which depends on the value of the stress applied to the crystal. Since TBL appears in sugar crystals during the creation of new surfaces, the fracture-initiation time of the crystal has been taken to be the delay time in observing TBL pulse after the application of stress. The dependence of fracture-initiation time,tfσ, of crystals on the stress, σ, may be expressed astfσ=to exp (− ασ), whereto and α are constants. The values of the lattice energy, and the change in lattice energy per unit stress, of sugar crystals have been calculated from TBL measurements and they have been found to be 21·2 kcal mole−1 and 0·41 × 10−8 kcal mole−1 dyne−1 cm2 respectively.

• Mechanoluminescence of ferroelectric crystals

Mechanoluminescence spectra of triglycine sulfate, guanidine aluminium sulfate, rochelle salt, diglycine manganous chloride dihydrate, lithium ammonium tartate monohydrate and potassium dihydrogen phosphate crystals resemble the discharge spectra of nitrogen gases. Glycine silver nitrate, thiourea, ammonium sulfate and azobenzene crystals, the structure of which is centrosymmetric at room temperature, do not show mechanoluminescence. Following the role of crystal structure, the mechanoluminescence excitation is discussed on the basis of the piezo-electrification of the new surfaces created during fracture of the crystals.

• Crystalloluminescence and temporary mechanoluminescence of As2O3 crystals

Crystalloluminescence and temporary mechanoluminescence of As2O3 crystals are investigated. The crystalloluminescence spectra are similar to the photoluminescence and mechanoluminescence (of fresh crystals, in CO2 atmosphere) spectra. The mechanoluminescence spectra of freshly grown crystals taken in air consist of the superposition of the photoluminescence and nitrogen emissions. The mechanoluminescence spectra of old crystals of As2O3 consist of only the nitrogen emission. The total number of crystalloluminescence flashes is linearly related to the total mass of the crystals grown. The mechanoluminescence intensity increases with the mass of the crystals. The mechanoluminescence intensity decreases with the age of the crystals and the rate of decrease increases with increasing temperature of the crystals. Different possibilities of crystalloluminescence and mechanoluminescence excitations in As2O3 crystals are explored and it is concluded that crystalloluminescence and mechanoluminescence are of different origins.

• Mechanoluminescence and high pressure photoluminescence of (Zn, Cd) S phosphors

The mechanoluminescence spectra of (Zn, Cd)S phosphors shift towards shorter wavelength side as compared to the photoluminescence spectra, however, the photoluminescence spectra shift towards shorter wavelength side with increasing pressure with less pressure coefficient. This finding eliminates the thermal population mechanism and suggests the electrical excitation mechanism for the mechanoluminescence excitation. The decay of mechanoluminescence after the deformation (Zn, Cd)S phosphors may be controlled by the recombination rate of holes and electrons, i.e. by the finite times required for the liberation of the electrons from the traps and for the electron transport, and consequently the decay of mechanoluminescence may be similar to the decay of photoluminescence.

• Mechanoluminescence of coloured KCl crystals

The gamma-irradiated KCl crystals exhibit mechanoluminescence during elastic, plastic and fracture deformation. The mechanoluminiscence (ml) intensity varies linearly with the number of newly-created dislocations and decreases with successive application and release of uniaxial pressure. The totalml intensity increases with applied pressure as well as with the temperature of the crystals. On the basis of the movement of the dislocations, the pressure and temperature dependence ofml is discussed.

• Mechanoluminescence excitation in alkali halide crystals and colouration decay in microcrystalline powders

The mechanoluminescence (ML) of NaCl, NaBr, NaF, LiCl and LiF crystals ceases at 105, 58, 170, 151 and 175°C respectively. Both the temperatureTc at whichML disappears and the temperatureTs required to induce a particular percentage of colouration decay in a given time, decreases with increasing nearest neighbour distance in alkali halide crystals. This perhaps suggests that similar processes cause the disappearance ofml in alkali halide crystals and the colouration decay in their microcrystalline powders. It is shown that mobile dislocations may cause the leakage of surface charge and the decay of colouration in microcrystalline powders.

• Tetrahedral manganese (II) complexes—intense and unique type of mechanoluminophores

Intense and unique type of mechanoluminescence (ML) is found in tetrahedral manganese (II) complexes. During the excitation of ML by the impact of a piston onto the crystal, the ML intensity initially increases with time, attains a maximum value and then decreases. After retardation of the piston, the decay rate of ML is faster during crystal deformation; however, its value decreases after cessation of the deformation and becomes equal to the decay rate of phosphorescence. The ML disappears below the melting point. Since the crystals of tetrahedral manganese (II) complexes are centrosymmetric, the local non-centrosymmetric sites near the defects are attributed to be responsible for the mechanoluminescence excitation.

• A search for mechanoluminophors capable of pressure-induced thermal population of excited states

During the process of deforming a crystal, a high pressure is developed near the tip of mobile cracks, which may in turn produce a new ground state by thermal electron transfer. Upon sudden release of pressure, the electron can either relax to one atmosphere ground state or remain in the excited state potential well long enough to relax to one atmosphere and radiatively transfer back to the ground state. For analysing the pressure induced thermal population of the excited state, the mechanoluminescence(ML) and high pressure photoluminescence(PL) of several organic and inorganic crystals were measured. The study indicated that usual pressure coefficient of energy shift of the order of 50–100 cm−1/kbar and the stress at the crack-tip of the order of 5–10 kbar, are not sufficient to cause the thermal population of the excited state. If by any means the product of pressure coefficient and stress at the mobile crack-tip can be increased by 50 to 100 times, then the thermal population of the excited states may take place. Using the pressure coefficient of energy shift and the difference in ML and PL spectra, and using independently the change in relative intensities of the vibronic peaks, the pressure at the emitting mechanoluminescent crystal sites is evaluated and it is found to be of the order of several kbar which varies from crystal to crystal.

• Mobile interstitial model and mobile electron model of mechano-induced luminescence in coloured alkali halide crystals

A theoretical study is made on the mobile interstitial and mobile electron models of mechano-induced luminescence in coloured alkali halide crystals. Equations derived indicate that the mechanoluminescence intensity should depend on several factors like strain rate, applied stress, temperature, density of F-centres and volume of crystal. The equations also involve the efficiency and decay time of mechanoluminescence. Results of mobile interstitial and mobile electron models are compared with the experimental observations, which indicated that the latter is more suitable as compared to the former. From the temperature dependence of ML, the energy gaps between the dislocation band and ground state of F-centre is calculated which are 0.08, 0.072 and 0.09 eV for KCl, KBr and NaCl crystals, respectively. The theory predicts that the decay of ML intensity is related to the process of stress relaxation in crystals.

• Field emission theory of dislocation-sensitized photo-stimulated exo-electron emission from coloured alkali halide crystals

A new field emission theory of dislocation-sensitized photo-stimulated exo-electron emission (DSPEE) is proposed, which shows that the increase in the intensity of photo emission fromF-centres during plastic deformation is caused by the appearance of an electric field which draws excited electrons out of the deeper layer and, therefore, increases the number of electrons which reach the surface. The theory of DSPEE shows that the variation of DSPEE flux intensity should obey the following relation$$\frac{{\Delta J_e \left( \varepsilon \right)}}{{J_e \left( o \right)}} = \left[ {\frac{{Y_s }}{{d_F }}\exp \left( {\frac{\chi }{{kT}}} \right) - 1} \right]$$. The theory of DSPEE is able to explain several experimental observations like linear increase of DSPEE intensityJe with the strain at low deformation, occurrence of the saturation inJe at higher deformation, temperature dependence ofJe, linear dependence ofJe on the electric field strength, the order of the critical strain at which saturation occurs inJe, and the ratio of the PEE intensity of deformed and undeformed crystals. At lower values of the strain, some of the excited electrons are captured by surface traps, where the deformation generated electric field is not able to cause the exo-emission. At larger deformation (in between 2% and 3%) of the crystal, the deformation-generated electric field becomes sufficient to cause an additional exo-electron emission of the electrons trapped in surface traps, and therefore,t here appears a hump in theJe versusε curves of the crystals.

• Correlation between deformation bleaching and mechanoluminescence in coloured alkali halide crystals

The present paper reports the correlation between deformation bleaching of coloration and mechanoluminescence (ML) in coloured alkali halide crystals. When the F-centre electrons captured by moving dislocations are picked up by holes, deep traps and other compatible traps, then deformation bleaching occurs. At the same time, radiative recombination of dislocation captured electrons with the holes gives rise to the mechanoluminescence. Expressions are derived for the strain dependence of the density of colour centres in deformed crystals and also for the number of colour centres bleached. So far as strain, temperature, density of colour centres, Ea and volume dependence are concerned, there exists a correlation between the deformation bleaching and ML in coloured alkali halide crystals. From the strain dependence of the density of colour centres in deformed crystals, the value of coefficient of deformation bleaching D is determined and it is found to be 1.93 and 2.00 for KCl and KBr crystals, respectively. The value of (D+χ) is determined from the strain dependence of the ML intensity and it is found to be 2.6 and 3.7 for KCl and KBr crystals, respectively. This gives the value of coefficient of deformation generated compatible traps χ to be 0.67 and 1.7 for KCl and KBr crystals, respectively.

• # Pramana – Journal of Physics

Volume 96, 2022
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019