H.V. Alexandru

A macroscopic model for the habit of crystals grown from solutions

Abstract A macroscopic model for the habit of growth of a crystal of an arbitrary shape is presented. The normal habit is defined as a first approximation of the real one. The model predicts the reappearance and disappearance of a given crystallographic face or edge of the grown crystal. The growth velocity is expressed in terms of the order of reaction and supersaturation. Some applications for the theoretical calculation of growth parameters and experiments on the growth of Seignette salt and ADP crystals are described.

KDP prismatic faces : kinetics and the mechanism of their growth from solutions

Complex equipment for crystal growth in the dynamic regime was used to measure simultaneously the growth kinetics of three large KDP crystals at around 55°C, in ∼ 9 l of solution at pH = 4.7. The same pattern of evolution was recorded for the prismatic faces of all crystals. The (heterogenous) nucleation mechanism of growth changed to a dislocation mechanism at σ ≈ 7% and a “dead” growth zone was found at σ < 1.2–1.4%. The edge free energy was estimated as γ/kT ≈ 0.4 in the nucleation region. In the transient region σ ≈ 1.2–2.0%, a retardation effect due to impurities is suggested.

Melt growth and characterization of pure and doped meta-dinitrobenzene crystals

Abstract Pure and doped crystals of meta-dinitrobenzene have been grown using a Bridgman–Stockbarger method. We have investigated the influence of the dopant (oxine, resorcinol) on the optical properties of the bulk and film samples obtained using crystalline fragments from the melt grown ingots. Transmission variations reflecting the quality of the material are connected with the purity level and the growth conditions.

Growth kinetic of prismatic faces of ammonium dihydrogen phosphate crystal in solutions

A transitory period of growth becomes apparent during kinetic measurements of the prismatic faces of ADP crystals. The empirical power law equation R = Kσn, n = 3.71, fits the kinetic data found in a long-run growth experiment. After the fast regeneration of the seeds at high supersaturation, within a layer of 600–800 μm, the growth efficiency decreases 3 to 6 times versus the initial value. Subsequently, the growth rates of the macrocrystals become comparable with the growth rate measured by Chernov et al. (1986) for strong (microscopic) center of dislocations. These particular features have been explained in the frame of the BCF (1951) complex dislocation mechanism of growth.

KDP kinetics and dislocation efficiency of growth

Abstract A macroscopic growth kinetic study has been carried out by taking advantage of simultaneous observations on a number of crystals (three in our experiments), growing under the same conditions. A large number of measurements on kinetic data for R 〈0 0 1〉 (σ) were found in three separate experiments of KDP growth in a dynamic regime, using the temperature lowering method. Macroscopic growth rates were compared with those from the literature. Arrhenius corrections were made at 40°C. The large scatter of experimental data, particularly for σ ≈0.02–0.05 is discussed in relation with e R the growth efficiency. Growth rate of the pyramidal faces has distinct parabolic dependencies on supersaturation, for several efficiencies of growth. Burgers vector and the linear dimension of the dominant centres of dislocations during the growth are discussed.

Ba(Zn1/3Ta2/3)O3 Ceramics for Microwave and Millimeter-wave Applications

The Ba(Zn1/3Ta2/3)O3 (BZT) ceramic samples were prepared by solid‐state reaction and sintered in the range 1550–1650°C for 2 h. Several methods—X‐ray diffraction (XRD) and scanning electron microscopy (SEM)—were used for structural and morphological characterization. The unit cell distortion and the presence of the secondary phase content were studied by XRD. A long‐range order with a 2:1 ratio of Ta and Zn cations on the octahedral positions of the perovskite structure was noticed with the increase of the sintering temperature. SEM investigations revealed polyhedral well‐faceted grains and large grain size distribution. The dielectric properties in the microwave range were measured at room temperature and at 1 kHz on a large temperature interval (±150°C). The dielectric parameters were correlated with morphological and structural properties. Ceramic samples were annealed at 1410°C for 30 h to improve the microwave properties. The dielectric constant of BZT samples measured at 6 GHz and at 1 kHz was between 27 and 28 on the whole temperature range, that is, typical values for BZT material. The temperature coefficient of the resonance frequency at 6 GHz exhibits positive values less than 6 ppm/°C.

Pure and doped triglycine sulfate crystals: growth and characterization.

Triglycine sulfate (TGS) crystal is an important ferroelectric crystal used on a large spectrum of radiation detection. Crystals from this family are used as targeted vidicon materials. Pure, (L + D) and L‐ or D‐alanine doped TGS crystals in the ferro‐ and paraelectric phase, 3–4 cm long, were grown by slow solvent evaporation or temperature reduction. The alanine segregation coefficient, being K∼0.01 dopant concentrations in the crystals, was less than 1%. The L‐ or D‐alanine doped crystals presented mirror symmetry and unipolarity. Permittivity and losses were registered online as a function of temperature or at constant temperature versus time (1 kHz, HIOKI, RLC automatic bridge). Essential parameters permittivity, losses, positive and negative polarization components, coercive field components, and the bias field of the doped samples were measured versus temperature. Hysteresis loops and their derivatives were measured with a specially designed Sawyer–Tower device. Generally, doped crystals presented much smaller permittivity and losses and a higher pyroelectric coefficient. Pure TGS samples show nonreproducible permittivity values and its relaxation was studied at constant temperature in the ferroelectric phase. Dielectric parameters have been automatically recorded online every 10 s, during ∼7 days. It was found that the relaxation time is not a real constant on such large time intervals. In a semi‐log scale, permittivity shows three stages, probably related to several mechanisms of relaxation.

Pyroelectric Properties of Alanine Doped TGS Single Crystalline Thick Films under Constant Electric Stress

Pyroelectric properties of triglycine sulfate (TGS) thick films, separately doped with L and D alanine were investigated. Internal bias field of about 1 kV/cm, induced by the two dopants, stabilize the polarization in the opposite direction on the ferroelectric axis. Pyroelectric current (under constant stress) was recorded with a computer controlled Keithley 6517 electrometer, crossing up and down the Curie point. A reverse external electric field was applied on doped materials during heating, crossing up the Curie point. It is shown that the pyroelectric coefficient can be increased about four times at room temperature under un optimized DC electric field applied on the pyroelectric wafer.

Ferroelectric properties of pure and doped triglycine sulphate crystal

Abstract Permitivity and losses of pure TGS crystal show a peculiar behaviour crossing up and down the Curie point between room temperature and 60 °C in two successive runs. The TGS:LD alanine doped crystal show peculiar hysteresis loops, composed of three elementary cycles due to a “free switching” component P′ and two other non-equivalent components PA and PB of opposite directions pinned by dopants. They change their shares in the hysteresis loop versus AC electric field and the pinned components retain up to 20% of the polarisation for pure TGS at saturation. The coercive and the bias fields of the two pinned components show different behaviour both for increasing AC field and temperature, suggesting a non-equivalent alanine substitution in the crystal lattice.

Dielectric spectroscopy in para-ferro transition of TGS

abstract Triglycine sulphate crystal (TGS) is one of the most studied ferroelectric materials, having a second order transition around 49°C. The complex dielectric constant we have measured on the frequency range 1–107 Hz and on a large temperature range, in ferroelectric phase after the para - ferro transition. Temperature was monitored from room temperature to 65°C, were for 30 min it was kept constant and then was lowered down at a rate of 0.6°C/min. Both component of permittivity were automatically registered. This study was focused on the dielectric relaxation processes. Three fundamental relaxation mechanisms (LF, MF, and HF) were found and analyzed with the Cole-Cole functions. The activation energy of these relaxation mechanisms have been estimated in the ferroelectric phase