• Applied Chemistry for Engineering
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• v.10 no.7
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• pp.1014-1019
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• 1999

Copolyamic acid PMDA/6FDA-PDA(PAA) and homopolyamic acids PMDA-PDA(PAA) and 6FDA-PDA(PAA) were synthesized from 1,2,4,5-benzenetetracarboxylic dianhydride(PMDA) and 2,2'-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride(6FDA) as the dianhydride and 1,4-phenylenediamine (PDA) as the diamine. Residual stresses were detected in-situ during thermal imidization of the co- and homopolyimide precursors as a function of processing temperature over the range of $25{\sim}400^{\circ}C$ using thin film stress analyzer(TFSA), and morphological structures were investigated by WAXD. In comparison, the resultant residual stress of polyimide films composed of different compositions decreased with the increasing content of PMDA unit in the chain and was about 5 Mpa in compression mode for PMDA-PDA. In this study, the synthesis of random PMDA/6FDA-PDA copolyimide could be completed and compensate for the difficulty of process due to high $T_g$ of PMDA-PDA and relatively higher stress of 6FDA-PDA. It showed that we can make a low level stress copolyimied having excellent mechanical properties by incorporating appropriate rod-like rigid structure PMDA-PDA unit into 6FDA-PDA polyimide backbone which generally shows higher stress due to rotational hinges such as bulky di(trifluoromethyl). Specially, PMDA/6FDA-PDA(0.9:0.1:1.0) satisfied excellent mechanical property and low level stress as an inter layer showing low dielectric constant.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.17 no.1
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• pp.7-15
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• 2004

The ferroelectric SBT thin films as a material of capacitors for non-volatile FRAMs have some problems that its remanent polarization value is relatively low and the crystallization temperature is quite high abovc 80$0^{\circ}C$. Therefore, in this paper, SBTN solution with S $r_{0.9}$B $i_{2.1}$T $a_{1.8}$N $b_{0.2}$$O_{9}$ composition was synthesized by sol-gel method. Sr(O $C_2$ $H_{5}$)$_2$, Bi(TMHD)$_3$, Ta(O $C_2$ $H_{5}$)$_{5}$and Nb(O $C_2$ $H_{5}$)$_{5}$ were used as precursors, which were dissolved in 2-methoxyethanol. SBTN thin films with 200 nm thickness were deposited on Pt/Ti $O_2$/ $SiO_2$/Si substrates by spin-coating. UV-irradiation in a power of 200 W for 10 min and rapid thermal annealing in a 5-Torr-oxygen ambient at 76$0^{\circ}C$ for 60 sec were used to promote crystallization. The films were well crystallized and fine-grained after annealing at $650^{\circ}C$ in oxygen ambient. The electrical characteristics of 2Pr=11.94 $\mu$C/$\textrm{cm}^2$, Ps＋/Pr＋=0.54 at the applied voltage of 5 V were obtained for a 200-nm-thick SBTN films. This results show that 2Pr values of the UV irradiated and rapid thermal annealed SBTN thin films at the applied voltage of 5 V were about 57% higher than those of no additional processed SBTN thin films. thin films.lms.s.s.

• Journal of the Korean Magnetics Society
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• v.17 no.1
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• pp.47-50
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• 2007

[ $ZnCr_2O_4$ ] shows geometrically frustrated magnet. Recently, $CoCr_2O_4$ has been investigated for multiferroic property and dielectric anomalies by spin-current model. Polycrystalline $CoCr_2O_4$ and $CoCrFeO_4$ compounds was prepared by wet-chemical process. Crystallographic and magnetic properties of $CoCr_2O_4$ and $CoCrFeO_4$ were investigate by using the x-ray diffractometer(XRD), vibrating sample magnetometer(VSM), superconducting quantum interference device magnetometer(SQUID), and $M\"{o}ssbauer$ spectroscopy. The crystal structure was found to be single-phase cubic spinel with space group of Fd3m. The lattice constants of $CoCr_2O_4$ and $CoCrFeO_4$ $a_0$ were determined to be 8.340 and 8.377 ${\AA}$, respectively. The ferrimagnetic transition temperature for the both samples were observed at 97 K and 320 K. The $M\"{o}ssbauer$ absorption spectra at 4.2 K show that the well developed two sextets are superposed with small difference of hyperfine field($H_{hf1}=507\;and\;H_{hf2}=492\;kOe$). Isomer shift values($\delta$) of the two sextets are found to be 0.33 and 0.34 mm/s relative to the Fe metal, respectively, which are consistent with the high spin $Fe^{3+}$ charge state.