OTHER METHODS FOR NANO MATERIALS

3.2. OTHER METHODS FOR NANO MATERIALS

A preliminary account has been given in the previous sections on how the nano particles of alumina can be prepared by various novel routes. Although these processes have their shortcomings to

a certain extent, as mentioned above, they also have many plus points, which need to be elaborated. A brief account of each of these processes is given here in this section to elucidate various points of interest about these methods of preparation of one of the most important ceramic materials like alumina, which has several important hi-tech application including the latest field of electronics. Although there

119 are various methods of preparation of nano particles of alumina, only some of the ‘novel’ techniques

NANO PARTICLES OF ALUMINA AND ZIRCONIA

will be explored here, which are arranged below not in any particular manner of importance, without any reflection on the other methods in terms of their importance in the nano world that are not described here.

3.2.1. Novel Techniques for Synthesis of Nano Particles

1. Gas Phase Condensation

The synthesis of nano-phase materials can be done by using 'Gas-Phase Condensation' method,

i. e. by evaporating a metal in an inert atmosphere and allowing it to condense on the surface of a cold finger, which is kept at liquid nitrogen temperature, i.e. 77°K. In order to eventually produce ceramic powders, the final metal clusters are oxidized and compacted in situ for the sintering process. The ultrafine metal particles are also produced by evaporating at a temperature-regulated oven, containing a reduced atmosphere of an inert gas, which is known as ‘‘inert gas phase condensation’’ [9]. Some details on these processes are given in the chapter 8.

2. DC Arc Plasma Method

The nano-crystalline alumina powders have been synthesized by ‘DC Arc Plasma’ by applying 300 Volts/15 Amp DC supply at atmospheric conditions by using aluminium electrodes, at an electrode separation of 3-5 mm in a closed chamber. In the beginning, the cathode and anode tips are positioned in order to facilitate ignition of the ‘arc’ by simply touching the anode to the cathode tip. The open circuit voltage (300 V) is applied to the electrode with a current limited to 7 A. The powder thus produced from the ‘arc’ is deposited onto the inner walls of the chamber. After sufficient cooling of the interior, this powder is gently scrapped only from the roof of the chamber and collected for the analysis.

This powder was characterized to be small nano particles by XRD and TEM techniques [14]. The distribution of such nano particles followed a log-normal behaviour. An interesting study was con- ducted by X-ray photoemission spectroscopy, which revealed that there is a presence of unreacted Al0 in the core of the nano particle, which is surrounded by Al 3+ ions, i.e. a layer of alumina in the particulate structure [14].

3. Sonohydrolysis of Alkoxide Precursor

The nao particles of γ-Al 2 O 3 with an average size of 5 nm were synthesized by the ‘Sono- Hydrolysis’ of aluminium tri-isopropoxide under the influence of power ultra-sound, i.e. 100 W/cm 2 , and in the presence of formic or oxalic acids as peptizers, which were followed by calcinations. The ultrasound-driven ‘cavitation process’ is shown to affect the agglomeration of inter-particle hydroxyls. The oxalate anions are found to be strongly adsorbed on the surface of the precursor nano particles, and thus have a retarding effect on the ultrasound-driven condensation process of inter-particle hydroxyls. The formic acid shows a lesser degree of adsorption on the surface of the precursor nano particles. The ultrasound-driven agglomeration of the primary particles and also the role of the organic modifiers on the microstructural properties of the precursor and the target alumina phases have been studied in details [15].

4. Ultra-Sonic Flame Pyrolysis

The nano-crystalline α-Al 2 O 3 was synthesized in an ‘Ultra-sonic Flame Pyrolysis’ (UFP) set-up showing the formation of the nano particles in the flame. The aluminium nitrate can be ‘ultra-sonically’ dissolved in methanol-water mixture, which was pyrolysed in an oxy-propane flame in order to yield

NANO MATERIALS

nano-crystalline α-Al 2 O 3 , which was confirmed by XRD. This technique, known as ‘Flame Aerosol Technique’ (FAT), is an emerging nano-crystalline synthesis method, which utilizes the ‘power of the flame’, i.e. the extreme spatial and temporal temperature gradients for bulk processing of nano materi- als. In a normal ‘flame spray pyrolysis’ (FSP), the liquid precursors are burnt. But, the latest variant, i.e. UFP, utilizes an ultra-sonic ‘nebulizer’ to atomize liquid into a spray having micron and sub-micron size droplets [16].

These droplets are then fed into an oxidizing stable pre-mixed burner that is used to produce an oxy-propane flame, which is 30 mm long in pre-mixed hot zone surrounded by a ‘diffusion’ flame of length of 150 mm. As the liquid traverses the flame, it is stripped off the organic and solvent compo- nents. The metal-oxide clusters generated in the hot zone ultimately coalesce into oxide nano-sized particles onto a copper substrate, which is kept below the nebulizer tip, and whose distance with the burner tip is adjustable. The nano particles thus produced are scrapped and then collected for the analy- sis. It is important to control the following variables : (a) ‘flame parameters’, (b) ‘droplet residence time’, and (c) ‘capture distance’. This is necessary in order to obtain uniform sized nano particles. This process has also been used to produce bulk quantities of nano-crystalline titania, zirconia and alumina [16].

5. Chemical Route

This particular subject is quite extensive, and there is a tremendous amount of literature on the precursor materials for alumina, and hence only some of them will be mentioned here. Ramanathan et al [17] have prepared α-Al 2 O 3 powder at 1400°C by using urea and AlCl 3 as precursor materials. Borsella et al [18] synthesized α-Al 2 O 3 fine powder from the gas-phase precursors at 1200°-1400°C. Ding et al [19] have synthesized α-Al 2 O 3 at 1250°C via the reaction 2AlCl 3 + 3CaO → Al 2 O 3 + 3CaCl 2 . Yu et al [20] synthesized ultrafine particles of α-Al 2 O 3 at 1200°C. In a comprehensive study, Sun et al [21] have prepared α-Al 2 O 3 fine powder by using ammonium aluminium carbonate hydroxide (AACH) of 5 nm as precursor materials at only 1050°C without any seed, and then down to 850°C by using 5% of α- Al 2 O 3 (100 nm) as seed crystals, and the final materials had particles with 30 nm-150 nm in size. Fanelli and Burlew [22] have reported synthesis of a fine particle of transition alumina by the treatment of aluminium sec-butoxide in sec-butanol. However, due to the need for high temperatures and pres-

sures as pre-requisites for the chemical reaction process, the direct formation of α-Al 2 O 3 via a hydro- thermal method has not been commercially exploited.

Finally, Pati et al [23] have reported a novel chemical route for the preparation of nano-crystal- line single-phase α-Al 2 O 3 with an average particle size around 25 nm. In this case, sucrose and PVA are used as the fuels and the medium to keep the metal ions , i.e. Al 3+ ions, in homogeneous solution. This leaves sufficient flexibility for the system to distribute the metal ions uniformly throughout the liquid medium before setting to a ‘solid mass’. In order to ensure this, the amount of sucrose in the precursor solution has been optimized (at 4 mole). The route involves ‘dehydration’ of the solution of the metal ions with sucrose and PVA, which is followed by ‘decomposition’ of the polymer matrix. After drying of the carbonaceous mass, it is ground to a fine powder forming the precursor, which when heat-treated at maximum 1150°C results in nano-sized alumina powder. The particle size is found by these workers to decrease with increasing sucrose content, which is an interesting observation in the alumina precur- sors [23].

The above sums up some of the details of the so-called ‘other methods or techniques’ of prepara- tion of nano-crystalline alumina powder from different precursor materials. The following section will give details of preparation of nano particles by high energy attrition milling route.

NANO PARTICLES OF ALUMINA AND ZIRCONIA