기상에서 나노미터 크기의 티타니아 입자들로 구성된 응집체의 구조재배열

장희동, Sheldon K. Friedlander*

한국자원연구소 자원활용소재연구부, UCLA 화학공학과*

Restructuring of Aggregated Titania Nanoparticles in the Gas Phase

Hee Dong Jang and Sheldon K. Friedlander*

Division of Minerals Utilization and Materials, Korea Institute of Geology, Mining and Materials, 30 Kajungdong, Yusungku, Taejeon, 305-350, KOREA

*Department of Chemical Engineering, University of California at Los Angeles, 405 Hilgard Ave., Los Angeles, CA90095, U.S.A.

INTRODUCTION

Inorganic particles smaller than several tens of nanometers in diameter (nanoparticles) are  of interest for the synthesis of new materials because of their low melting point, special optical properties, and unusual mechanical properties compared with their bulk material counterparts[1]. Such nanoparticles are routinely produced by gas phase reactions of precursor vapors because this method allows control of process conditions, particle size, particle crystal structure and purity[2]

Nanoparticles from gas phase reactions are nearly always produced in the form of chain aggregates that result from collisions between smaller aggregates in the Brownian motion. The chain aggregates are held together by bonds whose strength may range from van der Waals attractions to stronger ones such as ionic or covalent linkages. Aggregate structures can be described by Df which provides morphological information of a statistical nature. It has been shown that aerosol aggregates become more compact when heated as a result of the rearrangement of the primary particles(Figure 1).  Restructuring of the aggregates depends on the energies that hold the primary particles together and the bending of branches and side chains. Weber and Friedlander[3] hypothesized that restructuring was a temperature activated process driven by the deviation of the free energy of the aggregate structure(originally of low fractal dimension) from the free energy of the corresponding compact object(Df=3).

The role of chain aggregates in producing the high ductility and possible elasticity of nanophase ceramic materials was not considered in previous studies4,5,6. Recently, Friedlander et al.[4] discovered a remarkable elastomeric behavior of chain aggregates of titania nanoparticles through observations made under the transmission electron microscope.

The purpose of the present study is to investigate the structural properties of aggregates of titania nanoparticles including the fractal dimension, Df, and the activation energy for restructuring, and to compare the results with metal nanoparticles such as silver and copper.

EXPERIMENTS

The experimental apparatus consists of an aggregate generator, a restructuring tube, and the particle size measurement and collection systems (Figure 1). Titania nanoparticles were produced by the thermal decomposition of titanium tetraisopropoxide (TTIP, Aldrich Co. 97%) vapor in the gas phase. The stoichiometry for the reaction is given by

Ti(OC3H7)4  □□□  TiO2 + 4C3H6 + 2H2O

The aerosol generator consisted of a quartz tube(I.D.=19mm, length=120cm) and horizontal electric furnace(Thermolyne, Model 21100) set to 800℃ for thermal decomposition.TTIP vapor was fed from an evaporation bath maintained at 38℃ and introduced continuously into the aerosol generator  using nitrogen(99.999%) as a carrier gas. The mole fraction of TTIP in the aerosol generator was 6x10-6 mol/l, and total gas flowrate was 3 l/min.

Chain aggregates that formed at the end of the reaction chamber were collected on a 300 mesh copper grid(Ted Pella, Inc., Model 01820).  With a transmission electron microscope(TEM, JEOL, JEM 100CX) shapes, sizes and crystal structure of the aggregates were observed. The size distribution of titania aggregates was measured by a differential mobility analyser(DMA, TSI Model 3071) and a condensation particle counter(CPC, TSI Model 3010).

To investigate aggregate restructuring, an aerosol stream of 1 l/min from the aerosol generator was introduced into the first DMA at near room temperature after bipolar charging of the aggregates in a 85Kr source(2.5 mCi, TSI Model 3077). Also introduced were 5 l/min of N2 sheath gas. A particular size class of aggregates, negatively charged and nearly uniform in mobility diameter, was selected in the DMA and passed through a heating section consisting of a quartz tube(I.D.=19mm, length=120cm) and a tube furnace. The residence time for aggregate restructuring was controlled by varying the length of the heating section and the gas flow rate. The change in mobility diameter of the aggregate due to  restructuring was measured with a second DMA in combination with a CPC. The restructured aggregates leaving the heating section were sampled and observed with the TEM to find the change in the structure of the aggregates.

RESULTS AND DISCUSSION

Primary titania nanoparticles produced at 800℃ in the reaction zone by thermal decomposition of TTIP vapor collided to form chain aggregates by cluster/cluster aggregation. The average size of the primary particles was about 7nm in diameter. The crystalline phase of the titania particles was found to be anatase from an analysis of the electron diffraction pattern. The mobility diameter of the chain aggregates ranged from 40nm to 400nm, and the number average size of the aggregates was about 110nm. The total number of aggregates per cc of gas was 5.2x106 measured with the CPC.

The temperature for restructuring was increased from 20℃ to 1,000℃, and the residence time in the heating section from 1.70sec to 7.66sec to find the the change in the mobility diameter. Temperatures above 1,000℃ were not included because sintering of nanoparticles was expected[5].  When the residence time exceeded 3.83sec, the mobility diameter of the aggregates began to decrease at 700℃.

The change in Df with temperature at a residence time of 5.10sec was observed with the TEM. The aggregates were composed of more than 100 nanoparticles. As the restructuring temperature increased from 20℃ to 1,000℃, the titania aggregates became more compact, and the mobility diameter smaller. Bond strengths between the primary particles within an aggregate differ. Aggregate subunits probably move around the weakest bonds to reach a close-packed state when heated. Sintering between some of the primary particles was observed at 900℃. At 1,000℃ the primary particle size increased due to growth by sintering. Thus, it was assumed that the aggregate restructured at 900℃ to reach a close-packed state(Df=3) composed of primary particles without complete coalescence. This assumption was used to obtain the value of Df.

To determine Df, the change in mobility diameter of four different initial aggregates sizes was measured as a function of restructuring temperature at the same residence time(5.10sec) in the restructuring zone. The mobility diameter of the aggregates started to decrease at 700℃ and continued to 1,000℃(Figure 2).  The slope of a double logarithmic plot of dp vs dcl gave 3/Df. With increasing temperature, the aggregates became more compact and Df increased from 2.33 to 3.00(Figure 3). The number of primary particles in the smallest aggregate was estimated to be about 300.

The value of the activation energy obtained from the titania aggregate is almost an order of magnitude higher than the value obtained from aggregates of metal nanoparticles. The high value of the activation energy of titania aggregate is probably due to the stronger bond strength of titania aggregates compared with metal aggregates. The value of the frequency parameter (Ao), which strongly depends on the restructuring temperature, of titania aggregates is also two or three orders of magnitude higher than those of the silver and copper aggregates. The higher restructuring temperature required for the titania aggregates compared with metal aggregates is clear from the present results.

REFERENCES

[1] Siegel, R. W., Annu. Rev. Mater. Sci., 21, 559(1991).

[2] Gurav, A., Kodas, T., Pluym, T., and Xiong, Y.,  Aerosol Sci. Technol.,

19, 411(1993).

[3] Weber, A. P., and Friedlander, S. K., J. Aerosol Sci., 28, 179(1997).

[4] Friedlander, S. K., Jang, H. D., and Ryu, K. H., accepted in J. of Appl.

Phys. Lett(1997).

[5] Seto, T., Shimada, M., and Okuyama, K., Aerosol Sci. Technol. 23,

183(1995).