| As an important material for X-ray analysis, Pentaerythritol(C(CH2OH)4, PET) crystal possesses high diffraction intensity, weak background signal and good thermal stability, so it has been applied extensively in X-ray analyses and detection fields, such as X-ray fluorescence spectra, laser plasma X-ray spectra, and so on. Its wavelength of X-ray for detecting is in the range of 0.55~0.8 nm, which is corresponding to the detected elements from Al13 to Zr40. In order to grow high quality PET crystal, the main influencing factors on the crystal growth have been studied systematically in this paper.At first, the dipentaerythritol in the raw material of PET has been removed completely by recrystallization for three times, then the determination of the solubility curves and the meta-stable zones of PET in water and 36% acetic acid solutions has been done respectively, it is indicated that the solubility coefficients of PET in water solution reduce gradually along with the temperature dropping, while in acetic acid solution, PET have greater solubility and wider metastable zone.PET crystals have been grown by slow-cooling method in solution, and the influences of growth temperature, seed crystal shape and solvent on growth have been studied simultaneously. PET crystals have been obtained respectively from water solutions with different start saturate temperatures: 70 ℃, 65 ℃, 60 ℃ and 50 ℃, the experiments of growth show that the lower degree of supersaturation is needed for crystal growth at higher temperature, and 65 ℃ as the start saturate temperature is more advantageous to grow larger crystals; flat seed with(001) face can reduce the degree of supersaturation demanded in crystal growth; in 36% acetic acid solution, even though lower degree of supersaturation can satisfy the crystal growth, and growth morphology changes at a certain extent in this growth system.XRD has confirmed that the crystals grown in the experiment are PET phase indeed, the IR spectra have shown that the acetic acid as a main composition of solvent in growth solution can not affect the structure of PET molecules in crystal, and LC-MS analyses have given a conclusion that there is no condensation reaction between the hydroxyls of pentaerythritol molecules to take place in growth period.Finally, the performance parameters related to the quality of the crystal material have been analyzed: transmission spectra and rocking curves of the PET crystals were measured respectively, the results show that crystal grown in water at 65 ℃ as start growth temperature is the best in transmittance and structural perfection, and the transmittance of the crystal grown at start temperature 50℃ in acetic acid is better than the crystal grown in water at the same temperature, while the crystal structural perfection of the former is a little bit poor; the DSC and TG analyses give the result that PET crystal has obvious absorption peak at 250 ℃, so it has good thermal stability; and the hardness tests show the microhardnesses of all the PET crystal samples are 60Kg/mm2 or so. |
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