Biomorphic Charcoal/TiO₂ Composites And Biomorphic TiO₂ Ceramics From Bio-templates

Manufacturing biomorphic porous ceramics from the templates of biomaterials has been focused on the international research. In this paper, moso bamboo, rattan（calamus simplicifolius）, branch wood, and mature wood of poplar（Populus×Euramericana cv.） were impregnated by butyl titanate sol Ti（OC4H9）4 through vacuum/positive pressure technology. Then biomorphic charcoal ceramics, charcoal/TiO₂（C/TiO₂） ceramics were produced from the templates of poplar, rattan and moso bamboo in a nitrogen atmosphere. Further C/TiO₂ ceramics were impregnated by butyl titanate sol, finally biomorphic Ti O₂ ceramics were manufactured from the templates of C/TiO₂ ceramics in an air atmosphere at more than 1000℃. The main conclusions are as follows:（1） The times of sol impregnation into woods were decreased effectively through the technology of atmosphere pressure/ vaccum/positive pressure/atmosphere pressure. Furthermore the probable mechanism about sol penetration and transmission in the hardwoods was illustrated from the bordered pit and fusiform fibers of poplar, fluid thermodynamics, and the experimental technology. So it was a referable technology of sol impregnation into the woods.（2） The TiO₂-phase formation of specimens was monitored by XRD analysis during heat treatment by pyrolyzing charcoal and N2 between 400 and 1200 °C. The graphitization degrees of all the wood ceramics sintered at 800 ℃ were negative value（-6.61%）, the results showed the graphitization degrees were lower. And the anatase structure was observed at 400 °C. The phase transformation shifted from anatase to rutile between 700 and 800 °C; consequently, the TiO₂-phase was completely rutile in structure by 900 °C. As the temperature increased, the diffraction peaks of TiO₂ became steeper; Ti O₂ phase transition temperature from anatase to rutile was significantly increased in nitrogen atmosphere compared with air atmosphere, owing to joint effect of inert atmosphere and reducing charcoal.（3） Compared with TG-DTG curves of the C/TiO₂ ceramics, thermal stability of C/TiO₂ ceramics and the infiltrated quantity of various templates were analyzed. As the results, the temperature of C/TiO₂ sintered were the higher, the thermal stability of ceramics were the better. The corresponding remnants（TiO₂） from the C/TiO₂ of mature poplar, poplar branch, rattan, the interior of the bamboo, the middle of the bamboo, and the exterior of the bamboo were approximately 73.83%, 76.11% 61.80%, 24.85%, 31.46% and 33.10%, respectively. So the difficult coefficient were sequenced from low to high, they were the C/TiO₂ of mature poplar, poplar branch, rattan, the interior of the bamboo, the middle of the bamboo, and the exterior of the bamboo. The temperature of fastest burning, the initial decomposition and the burned were only associated with the target temperature of sintered materials. Therefore it provided a reliable basis for the calcined C/TiO₂ ceramics using the templates from the wood branch or other types of biomaterials.（4） The dimensional stabilities were compared wood ceramics, C/TiO₂ ceramics, with the templates from the poplar, and rattan. The shrinkage rates（SR） of volume were more than 50% from the templates from the poplar, and rattan to C/TiO₂ ceramics, but the shrinkage rates of volume from branch wood was bigger than that of mature wood owing to their development. Moreover SR in the diameter were more than SR in the axial from branch wood to C/TiO₂ ceramics; SR from mature wood to C/TiO₂ ceramics sequenced from big to small were that of the radial, tangential, and axial, respectively. So the anisotropy of shrinkage was very evident. When rattans sintered to 800C/TiO₂, radial and axial SR were 21.8% and 11.7%, respectively. Hence, the radial SR for impregnated rattans to 800C/TiO₂ was almost twice that of the axial. Furthermore removal of the sapwood from both of the ceramic sections successfully prevented the thermo-stress of the anisotropic structure over the course of the sintering process from native rattan to biomorphic ceramic. Therefore, this suggests that the rattan templates should cut off the sapwood from shaped-cylinder complete transverse sections of rattan to obtain biomorphic C/TiO₂ ceramics from rattan.（5） The dimensional stabilities were compared from C/TiO₂ ceramics to TiO₂ ceramics of the poplar, and rattan templates. As a result, the averages SR along the radial direction and axial direction were 9.00% and 8.79% from C/TiO₂ ceramics to TiO₂ ceramics of branch wood. Additionally, the average radial SR and axial SR were 9.7% and 9.9% from C/TiO₂ ceramics to TiO₂ ceramics of rattan. Moreover the average radial SR, axial SR, and tangential SR were 8.81%, 8.61%, and 8.31% from C/TiO₂ ceramics to TiO₂ ceramics of mature wood. Therefore the dimensional stabilities were isotropic from C/TiO₂ ceramics to TiO₂ ceramics.（6） The microstructures were displayed through scanning electron microscopy（SEM） in the wood ceramics, C/TiO₂ ceramics, and TiO₂ ceramics from poplar, rattan, moso bamboo. the average diameter of vessels, fiber cells, radial cells, and a group pits on transverse section of the mature wood ceramics at 800℃ were approximately 30 to 150, 3 to 15, 3 to 10, and around 1.5 μm, respectively. Moreover the similar porous existed in the wood ceramics of branch wood at 800℃, only the average thickness of cell walls in the branch wood ceramics was thinner than that of mature wood ceramics. The diameter of vessels, sieve tubes, and fiber cells on the rattan charcoal transverse section were approximately 200, 50, and 2 μm, respectively. A few apertures with a pore size of approximately 1 μm on the fiber cell walls were found. In addition, parenchyma tissue exhibited the largest pores（ 10 μm） and the smallest apertures（ 2 μm）. The pit sizes of the rattan charcoals along the axial section in the vessel wall were approximately 1 to 5 μm. The anatomical structure of the 800C/TiO₂ is shown. Most diameters of big vessels were reduced to approximate 3/4 of the original dimensions（< 150 μm） in the transverse sections. The average diameter of big vessels, small vessels, fiber cells, and parenchyma cells on transverse section of the moso bamboo ceramics at 800℃ were approximately 100, 10 to 30, 1 to 10, and around 25 μm, respectively. While the average diameter of parenchyma cells and fiber cells on axial section of the moso bamboo ceramics at 800℃ were approximately 20 to 60, and 1 to 10μm, respectively. However the average diameter of the pits in the parenchyma and fiber cell walls was approximately 1μm, further more their numbers is few. So the templates form moso bamboo was infiltrated difficultly. Biomorphic C/TiO₂ ceramic sintered at 800℃perfectly performed the biomorphic feature of wood ceramics both in transverse section and in axial section. Additionally the surprising mesoporous like honeycomb in the C/TiO₂ ceramics formed at approximately 800 °C in N2. However, the mesoporous unexpectedly disappeared from biomorphic ceramics. Moreover, a lot of micrometer pores derived from the axial section during the removal of the bio-templates and the formation of the TiO₂ rutile phase. The pores could be potential application of molecular sieve, and adsorption for gas. However biomorphic TiO₂ ceramics were difficultly from moso bamboo templates.（7） The bamboo fibers were segregated by the solution of 30% H2O₂ and CH₃COOH（the volume ratio was 1:1）. And then medical gauze was used to filter fiber and parenchyma. Finally the bamboo fiber and parenchyma were separated completely. Moreover the mass ratio of bamboo fiber and parenchyma was 49:34 through this technology. In addition the technology of the separation is effective method for the poplar fibers and bamboo fibers.（8） After impregnation with 20Mmol/L butyl titanate solution, the fibers of poplar, rattan, and moso bamboo were sintered at the same temperature in a nitrogen atmosphere to produce active carbon fiber/TiO₂ composites. The sequenced rate of carbonization from big to small was the bamboo fibers, poplar fibers, and rattan fibers, respectively. All kinds of the activated carbon fibers with TiO₂ were calcined beyond 1000 ℃, the carbonized rate decreased significantly, so the temperature had evidently influence on the yield of the activated carbon. According to GB/T12496.10-1999 the adsorptions of methylene blue were determinated in the activated carbon fiber with TiO₂. After 700 ℃ as the temperature increased, the adsorptions of all kinds of activated carbon for methylene blue can increase. But the adsorption of the activated carbon/TiO₂ composite sintered at around 1000 ℃ for methylene blue reached for the maximum amount. When the temperature increased further, the adsorption of that declined gradually.