Effect Of Axial Temperature Gradient Research Articles

Effect of axial temperature gradient on chromatographic efficiency under adiabatic conditions

The effect of axial temperature gradient on the chromatographic efficiency was studied under adiabatic conditions by a modeling approach. The equilibrium-dispersive model of chromatography was used for the calculations. The model was extended by taking into account the axial temperature gradient. The results show that due to the temperature gradient, there are retention and migration velocity gradients in the column. Since the retention factor, k, is not constant in the column, k cannot be calculated as the ratio of net retention and hold-up times. As a result of the gradual increase of migration velocity, the retention times of solutes decrease as the slope of temperature gradient increases. In addition, the band in the column have extra broadening due to larger migration velocity of the front of band. The width of bands becomes larger at larger change of temperature. In the same time, however, the release velocity of the compounds from the column is increasing as ΔT increases. Accordingly, an apparent peak compression effect makes the peaks thinner. As a result of the two counteracting effects (peak expansion, apparent peak compression) the column efficiency does not change significantly in case of axial temperature gradient under adiabatic conditions. The resolutions, however, decrease slightly due to the decrease of retention times.

Axial temperature gradient and mobile phase gradient in microcolumn high-performance liquid chromatography

The effect of axial temperature gradient (ATG) along a microcolumn on the separation performance at both isocratic and gradient elution mode was investigated. A thermostat system was designed to form an ATG along the packed column. Polycyclic aromatic hydrocarbons (PAHs) were separated on a 0.53 mm × 150 mm i.d. 5 μm C 18 microcolumn, with water and acetonitrile as mobile phase. The separation results obtained at mobile phase gradient (MPG) and ATG in microcolumn HPLC were compared with the results performed at ambient conditions. Extrapolated curves of peak width at half height ( w h ) versus ln k showed that w h is narrower at the same retention time when ATG was applied in addition to MPG. The column efficiency was enhanced 20–30% and the resolution was slightly reduced because of reduction of selectivity at elevated temperature at ATG condition. The RSD of retention time in ATG mode was less than 2.5%.

Effect of the axial temperature gradient on the formation of grown-in defect regions in Czochralski silicon crystals; reversion of the defect regions between the inside and outside of the Ring-OSF

The effect of axial temperature gradient ( G) on the formation of grown-in defect regions in Czochralski silicon (CZ-Si) crystals was investigated. A CZ-Si crystal was grown with the temperature gradient at the center ( G c) far higher than that at the outside ( G s) by tuning the hot zone of the pulling furnace. It was found, by changing the growth rate ( f p), that the shape of Ring-like region of oxidation-induced stacking faults (Ring-OSF), including the crystal axis and the diameter, became a W-band which has not been reported previously. This W-band results in the reversion of grown-in defect regions inside and outside of the Ring-OSF in the wafers. That is, the wafers included a dislocation-cluster region inside the Ring-OSF, while the void region was outside the Ring-OSF. The appearance of the W-band of Ring-OSF was readily explained using the values of f p/ G c and f p/ G s.

Growth of paratellurite crystals: effect of axial temperature gradient on the quality of the crystals

Paratellurite (TeO 2) crystals have been grown under different temperature gradients (10–15, 15–20, 20–25 and 50–60°C/cm) employing resistive and induction heating systems. The growth parameters were optimized for each gradient range to harvest good quality crystals. The problems of constitutional supercooling and cracking were avoided by employing suitable rotation and pulling rates. The effect of melt depth in the crucible on the crystals is discussed. The assessment of the crystalline quality by the chemical etching technique, the scanning electron microscopic (SEM) analysis on bubbles and inclusions in the crystals and the energy dispersive analysis by X-rays (EDAX) on defect areas are also reported.