M. A. Gaffar

Effect of Doping and Irradiation on Optical Parameters of Triglycine Sulphate Single Crystals

Crystals of triglycine sulphate (TGS) doped with orthonitroaniline (ONA) irradiated with different doses of gamma-radiation were used to investigate gamma-radiation effect on electrical and optical parameters. The absorption coefficient and the reflectance were measured and hence the extinction coefficient, the optical refractive index and the dielectric constants (er′, er″) of unirradiated and irradiated TGS crystals were calculated. Values of the allowed indirect optical energy gap Egopt. of TGS doped with ONA were calculated as a function of gamma-dose. Value of Egopt. decreases from 4.72 eV to 4.25 eV with increasing gamma-doses from 0 up to 3 Mrad. The values of the static dielectric constant er(0) and the effective electrical conductivity sigma0 at room temperature and their dependence on gamma-dose were also calculated.

Ferroelectric behaviour of triglycine sulphate crystals containing low concentrations of metal ions near the transition temperature

Several ferroelectric properties of triglycine sulphate single crystals both pure and with different doping substitutions in the structure have been studied in the vicinity of the critical point Tc. The parameters engaged in the statistical dipolar theory of ferroelectrics are derived, from dielectric constant and hysteresis loop measurements, allowing a distinction between atomic and dipolar polarisation. The numerical values of the parameters obtained in this way for pure and doped crystals are compared. The internal biasing field is estimated using the data of the pyroelectric measurements. The basic pyroelectric material figure of merit is also calculated and its temperature dependence is reported. The temperature dependence of the coercive field has been measured and found to obey a power law Ec=A(Tc-T)s. The piezoelectric modulus and the electromechanical coupling coefficient have been also measured and their temperature dependence is investigated. The J-E characteristic in both the ferroelectric and the paraelectric phases is examined; variations in the behaviour according to the type of dopant are observed. The effect of polarising pure and doped TGS samples with a DC field of intensity 102 kV m-1 on measured properties is also examined.

Investigation of developed precipitates in Al–1·1 wt-%Mg2Si balanced alloy by DSC and SEM techniques

Abstract The effect of temperature on the sequence of the hardening precipitates in Al–1·1 wt-%Mg2Si balanced alloy has been investigated by means of differential scanning calorimetry, scanning electron microscopy and hardness measurements. The non-isothermal DSC thermograms exhibited seven reaction peaks. Out of these seven peaks, five are exothermic (representing precipitation) and two are endothermic (representing dissolution). The activation energy associated with the individual precipitates is calculated. The activation energy of nucleation of GP zones (54·3 kJ mol−1) is close to the migration energy of Si in Al (52·7 kJ mol−1). The activation energy associated with the precipitation of β″ is determined as 77·6 kJ mol−1 and that for the formation of β′ precipitates is 145·3 kJ mol−1. The latter value is close to that for Si diffusion in Al (124 kJ mol−1) and that of Mg diffusion in Al (131 kJ mol−1). It can be concluded that the precipitation of β′ particles might be characterised by both the diffusion of Mg and Si atoms in Al to form β′ precipitates.

Absorption spectra and optical parameters of lithium–potassium sulphate single crystals

Abstract The optical transmittance and reflectance near the fundamental absorption region along the c- and a-axes of lithium potassium sulphate single crystal (LKS) are measured at room temperature. From the data the absorption coefficient (α) and the optical band gap (Eopt.g) were deduced. The type of transition was determined. The steepness parameter (σ), the temperature dependence of the energy gap and the exciton energy (E0) were also calculated. The extinction coefficient, the refractive index and both the real and imaginary parts of the dielectric permittivity were calculated as functions of the photon energy.

Specific heat and electrical resistivity of pure and doped lithium-ammonium sulphate single crystals

Abstract The temperature dependence of the specific heat Cp and the electrical resistivity of ϱ of pure lithium-ammonium sulphate (LAS) single crystals along the three principal crystallographic directions in investigated in the 300–500 K temperature range. The transition energy ΔQ and the number of both elementary (N0) and thermally excited (Ne) dipoles are calculated. It appears that only a small fraction of the total number of dipoles is capable of being thermally excited in the ferroelectric phase. The correlation between the Cp data and the spontaneous polarization Ps is verified. The J−E characteristics indicate the possibility of space charge effects at low measuring fields. Anomalous behaviours before and at the transition point is observed. Thermal annealing is found to be necessary for reproducible results. The temperature dependence of ϱ along the polar axis yields the values ΔE = 0.54 and 1.48 eV and ΔE = 1.95 eV for the energy activating the charge transport mechanisms in the ferro- and the paraelectric phases, respectively. A “pre-transition” phenomenon is observed while measuring both Cp and ϱ along the a- and b-axes. The mechanism of electrical conduction in the measuring range is discussed. Along the polar axis Cp of LAS crystals doped with either Cu2+, Co2+, Ni2+ or Mn2+ is measured in the same temperature range. Above Tc, the temperature range through which log Cp is linearly proportional to (T−Tc) is wider than that predicted theoretically. The transition energy, the fraction N e N 0 and Ps are also calculated for doped crystals.

Vibration Spectra of Ferrocyanide Ions in the Paraelectric Phase of K4[Fe(CN)6] · 3H2O

Lattice, rotation and intermolecular vibrations of ferrocyanide ions in the paraelectric phase of potassium ferrocyanide trihydrate (KFCT) crystals are calculated using the correlation theorem based on the group theory. The correlation for the lattice vibrations of the ions between their site group symmetry C i and the factor group C 2h of the crystal yield six fundamental lattice vibrations allowed in the infrared spectrum. The same number for rotations are expected to be allowed in the Raman spectrum. The active number of intermolecular vibrations in Raman and infrared spectra are 30 and 34 vibrations, respectively. The FTIR spectrum of KFCT measured at room temperature (in the paraelectric phase) in the frequency range 4000-200 cm -1 gives the same modes of vibrations as those calculated theoretically. The effect of irradiating KFCT crystals with a γ-dose of 5 × 10 5 Gy on the IR spectrum indicates minor changes in absorption bands.

Critical behaviour of dielectric permittivity and spontaneous polarization of triglycine sulphate single crystals doped with organic molecules

Dielectric permittivity, e 22 , spontaneous polarization, P s , and coercive field, E c , of triglycine sulphate (TGS) crystals doped with d-l alanine, orthonitroaniline, paranitroaniline or aniline molecules have been measured from room temperature up to the transition point. A remarkable displacement of the hysteresis loops of polarization along the direction of the field axis and a broadened transition from the polar to the non-polar phase are detected. An increase in the value of P s and a decrease of both e 22 and E c are also observed. The changes in both e 22 and P s attributed to the creation by the dopants of an internal electric field, E b . The internal bias affects the Curie constant and e max in a way similar to that introduced by an external bias. The value of E b in the vicinity of the phase transition and the parameters of the statistical dipolar theory of ferroelectricity have been calculated.

Thermal analysis study for the phase determination and instable to metastable transformation of the Co–13Cu alloy

The differential thermal analysis (DTA) is utilized to determine the phase boundary and phase equilibria of Co–Cu alloy having a miscibility gap. Regions for various phase i.e., spinodal and coherent binodal, peritectic, magnetic and the transformation from the two phases to α phase transformations are distinctly determined for the Co–13 at%Cu alloy. The obtained results might indicate the continuity of the decomposition kinetics at the boundary between instable and the metastable regions but an activation barrier is needed. The activation energy for the magnetic transformation is determined to be 182.2 ± 2.1 kJ mol−1.

Investigations on the existence of a high-temperature phase transition in ammonium sulphate crystals

Abstract The existence of a high-temperature phase transition in ammonium sulphate (AS) crystals is discussed. The DC electrical conductivity and the dielectric permittivity in the temperature range 300–460 K are measured and the results are examined trying to detect the proposed phase transition. The results indicate no evidence for a ferroelectric phase transition, but the anomaly observed at 423 K simply is the result of variations in the conduction mechanism which could be attributed to an order-disorder type of phase transition.

Investigation of the pyroelectric and piezoelectric properties of triglycine sulphate single crystals containing organic molecules

The pyroelectric coefficient and the piezoelectric modulus of triglycine sulphate (TGS) single crystals doped with d-l-alanine, orthonitroaniline, paranitroaniline or aniline molecules has been measured from room temperature up to the transition point. The data are compared with those which the authors have previously obtained on pure TGS. A noticeable increase in the value of the pyroelectric coefficient of the doped samples over the whole temperature range is observed. A significant pyroelectric signal is still detected several degrees above the transition point of pure crystals. The spontaneous polarisation is calculated using data for the measured pyroelectric current. The piezoelectric modulus of the doped crystals reaches its maximum value at a temperature lower than the Curie point of pure TGS before it decreases steadily to zero. A relation between the piezoelectric modulus and the temperature just before the maximum value is deduced. The role of the internal bias created in the crystal after doping is also discussed.