Propylene carbonate (PC) is the most common organic solvent used in lithium batteries due to its high dielectric constant and high chemical stability with lithium metal. The favorable conductivity of PC |LiClO 4 (lithium perchlorate) electrolytes, with values of about 10 Ϫ3 S cm Ϫ1 , has been reported. [1][2][3][4] Several vibrational spectroscopy studies 1,4,5 revealed that a strong interaction between Li ϩ and PC occurs for the ring deformation of PC molecules at 712 cm Ϫ1 .The conductivity of PC |LiClO 4 electrolytes is about 10 Ϫ3 S cm Ϫ1 which is lower than the conductivity of aqueous electrolytes (about 10 Ϫ2 S cm Ϫ1 ). Therefore, mixed-solvent electrolytes with conductivities as high as 10 Ϫ2 S cm Ϫ1 have been developed by adding ethylene carbonate (EC) as a co-solvent. Recently, Li ϩ -solvent interactions in PC-EC solutions have been studied using both Raman 6 and IR 7 spectroscopies and nuclear magnetic resonance (NMR). 8 Strong Li ϩ -EC interactions 6-8 were observed at the peak bands 726 and 903 cm Ϫ1 due to ring breathing and ring bending modes, respectively.The conductivities of mixed-solvent electrolytes depend on the concentration of salt and the relative amounts of the two solvents. In this work, the conductivities of electrolytes with different compositions were studied using the ac impedance technique. Also, the interactions between Li ϩ and EC were characterized by Raman spectroscopy, and these results were used to determine the solvation number of Li ϩ . ExperimentalLiClO 4 (Aldrich) was dried in a vacuum oven at 120ЊC before being added to the solvent. Both PC and EC (Aldrich) were purified by reduced-pressure distillation, and K 2 MnO 4 was added to these solvents during distillation to eliminate impurities in the solvents. Both solvents were weighted separately before mixing. Different concentrations of LiClO 4 were added to the mixed solvents containing various molar ratios of EC/PC to form the electrolytes. The electrolyte compositions are listed in Table I. The electrolytes were stored in a dessicator with 4 Å molecule sieves before testing. The same volume of liquid electrolyte was used in each measurement with the identical quartz container.The Raman spectra were recorded with a IFS66 ϩ FRA 106/S type of Fourier transform IR-Raman spectrometer (Bruker Instruments Corporation, Germany). The laser power used in the test was 475 mW with an excitation line of about 1.064 m. The angle between the scattered light and the incident light was 180Њ. Each Raman spectrum included 128 scans, each with a resolution of 2 cm Ϫ1 . All Raman spectra were collected from liquid samples.The ionic conductivities of the electrolytes were measured by ac impedance spectroscopy using a frequency response analyzer (FRA S1260) and an IBM PC. A frequency range between 30 MHz and 100 mHz was used during the test. The real resistance of the liquid electrolytes was estimated from the resistance ZЈ at which the reactance ZЉ reached its minimum value. 9 The electrodes used in the measurements were stainless steel plate...

A simple lab‐scale weaving method was used to produce multidirectional fiber preforms to improve the delamination resistance and damage tolerance of composites. Mechanical properties measured in this study included short‐beam shear strength and damage tolerance of 2‐D and 3‐D woven composites. The constituents of fiber and matrix in these composites are Kevlar‐29 (Du Pont) and Epon 828 (Shell Chemical Co.) epoxy. Attention was directed to the differentiation of deformation and failure mechanisms in these composites as a function of material parameters and loading conditions. The sequence of failure mechanisms differ between 2‐D and 3‐D composites. Results indicated that 3‐D woven composites exhibited higher shear strength than 2‐D woven composites. Also, 3‐D composites absorbed more impact energy than 2‐D composites. Failure mechanisms of composites subjected to repeated impacts were observed intermittently by using scanning electron microscopy and light microscopy. The failure of 2‐D composites was due to a combination of shear and tensile mode, but in 3‐D composites the failure was dominated by tensile mode.