Journal: Bulletin of Materials Science PDF
Published: 6-Nov-14 Volume: 32 Issue: 5 Page Number: 471–479
DOI: 10.1007/s12034-009-0070-4 ISSN: 0250-4707 (Print) 0973-7669 (Online)
Authors: Vikram V. Dabhade, Ram Mohan R. Tallapragada, Mahendra Kumar Trivedi
Citation: Dabhade, V.V., Tallapragada, R.M.R. & Trivedi, M.K. Bull Mater Sci (2009) 32: 471. https://doi.org/10.1007/s12034-009-0070-4
Next to atoms and molecules the powders are the smallest state of matter available in high purities and large quantities. The effect of any external energy on the shape, morphology and structure can thus be studied with relative ease. The present investigation deals with the effect of a non-contact external energy on the powders of antimony and bismuth. The characteristics of powders treated by external energy are com-pared with the as received powders (control). The average particle sizes, d50 and d99, the sizes below which 99% of the particles are present showed significant increase and decrease indicating that the energy had caused deformation and fracture as if the powders have been subjected to high energy milling.
To be able to understand the reasons for these changes the powders are characterized by techniques such as X-ray diffraction (XRD), surface area determination (BET), thermal analytical techniques such as DTA–DTG, DSC–TGA and SDTA and scanning electron microscopy (SEM).
The treated powder samples exhibited remarkable changes in the powder characteristics at all structural levels starting from polycrystalline particles, through single crystal to atoms. The external energy had changed the lattice parameters of the unit cell which in turn changed the crystallite size and density. The lat-tice parameters are then used to compute the weight and effective nuclear charge of the atom which showed significant variation. It is speculated that the external energy is acting on the nucleus through some reversible weak interaction of larger cross section causing changes in the proton to neutron ratios. Thus the effect is felt by all the atoms, and hence the unit cell, single crystal grain and grain boundaries. The stresses generated in turn may have caused deformation or fracture of the weak interfaces such as the crystallite and grain boundaries.
Antimony; bismuth; external energy; powder.
Apart from atoms and molecules the next smallest mate-rials available are powders. These could be single crystal-line or polycrystalline, the particle size of which is in the micrometric or nano metric range (< 100 nm) (Suryanara-yana 1995, 1999). Particles exhibit fine microstructures and can contain such a high density of defects (point de-fects, dislocations, sub (crystallite) boundaries, grain boundaries, inter phase boundaries, etc.) that the spacing between neighbouring defects in nano size powders can even approach the inter atomic distance (Gleiter 1992). As the grain size becomes smaller and smaller, a larger and larger fraction of atoms resides on the single crystal grain boundaries (at a 6 nm grain size, nearly half the atoms reside on the grain boundaries), thus the behaviour of nano sized powders is often dominated by events at the grain boundaries (Mayo 1996).
Due to the extremely small size of the grains and a large fraction of the atoms located at the grain bounda-ries, materials made from these powders possess proper-ties like higher strength/hardness, enhanced diffusivity, improved ductility/toughness, reduced elastic modulus, increased specific heat, higher electrical resistivity, higher thermal expansion coefficient, lower thermal con-ductivity, and superior magnetic properties much improved over those exhibited by conventional grain sized (> 10 μm) polycrystalline materials (Suryanarayana 1995).
Both micro meter and nano meter sized powders are produced by methods like mechanical milling (Mohan et al 1999), inert gas condensation, spray conversion process, chemical processes (Amarchand et al 2000), and electro deposition (Suryanarayana 1995), etc. Often mechanical milling is used for synthesis of fine and nano sized pow-ders in bulk quantities using simple equipment and at room temperature (Suryanarayana 1999). During this process, the metal powder particles are subjected to severe mechanical deformation from collisions with the milling tools. Consequently, plastic deformation at high strain rates (~ 103–104 s–1) occurs within the particles and the average grain size can be reduced to a few nano-meters after extended milling (Benjamin 1976; Fecht 1996). Plastic deformation generally occurs by slip and twinning at low and moderate strain rates, while at high strain rates it occurs by the formation of shear bands, consisting of dense networks of dislocations. The plastic strain in the material increases due to increasing disloca-tion density in the early stages of ball milling. At a threshold dislocation density, even at moderately elevated temperatures, the material relaxes into sub grains sepa-rated by low-angle boundaries, leading to a decrease of atomic level strain. During subsequent milling the pro-cess of high deformation/sub grain formation is repeated resulting in the sub grains becoming finer and finer, and the relative orientation of the sub grains with respect to each other ultimately becoming completely random. Once the sub grains reach a critical level of refinement, further refinement becomes virtually impossible since the stresses required for dislocation movement are enormously high due to the Hall–Petch strengthening. Thus nano sized powders with a minimum grain size are produced (Suryanarayana 1999). Titanium powders of about 2 μ particle size when subjected to high energy attrition mill-ing in an argon atmosphere after 15 h of milling yielded an average particle size of 35 nm (Dabhade et al 2001). Thus it is now possible to produce large quantities of ultra fine and nano powders by high energy milling.
In the present study the effect of a non contact external energy on antimony and bismuth powders was investi-gated. These powders were chosen as they belong to the same group in the periodic table and have low melting points. Metals of low melting point generally exhibit low bond energy and were thought to be more significantly affected by the external energy. Powders were chosen as they are the finest form of these metals readily available in high purity levels. Powders also exhibit a high surface area and thus were expected to be more receptive to the external energy.
Antimony and bismuth powders (–325 mesh) of 99⋅5% purity were obtained from Alpha Aesar. Five sets of each powder were prepared, the first set which was untreated was designated as control while the other sets exposed to external energy referred to as treated samples. The con-trol and the treated samples were characterized by X-ray diffraction (XRD), laser particle size analysis, surface area determination (BET), differential thermal analysis (DTA–DTG), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), simultaneous differ-ential thermal analysis (SDTA) and scanning electron microscopy (SEM).
Average particle size and size distribution were obtained using SYMPATEC HELOS-BF laser particle size ana-lyzer with a detection range of 0⋅1–875 μm (micro meters). From the particle size distribution, d50, the average parti-cle size and d99 (maximum particle size below which 99% of particles are present) for the control (untreated or as received powders) are taken as standard and are com-pared with the results obtained on four separately treated powders. Surface area determination was carried out on a SMART SORB 90 BET surface area analyser with a measuring range of 0⋅2–1000 m2/g. X-ray diffraction was carried out using a powder Phillips, Holland PW 1710 XRD system. A copper anode with nickel filter was used. The wavelength of the radiation was 1⋅54056 Å (10–10 m or 10–8 cm). The data is obtained in the form of 2θ vs intensity chart as well as a detailed table containing 2θ°, d value Å, peak width 2θ°, peak intensity counts, relative intensity %, etc. The ‘d’ values are compared with stan-dard JCPDS data base and the Miller indices h, k and l for various 2θ° values are noted. The data are then analysed using PowderX software to obtain lattice parameters and unit cell volume. Differential thermal (DTA)–thermo-gravimetric (TGA) combined analyses were carried out from room temperature to 900°C at a heating rate of 10oC/min for antimony powders while for bismuth pow-ders it was carried out from room temperature to 400°C at a heating rate of 5°C/min. Scanning electron microscopy of control and treated powders was carried out using a JEOL JSM-6360 instrument.
The details of the experiments and the original data obtained prior to analysis are given in link: http://www.divinelife.us/Transcendental_Science/Transcendent-al_Science.html.
2.1 Method of data analysis
The percent change in particle size of various treated powders with respect to control powders were computed using the formula
In a similar manner the change in particle size of d99 (%) was computed. Percent change in BET surface was calcu-lated in a similar manner as indicated in (1).
The crystallite size was calculated using the formula
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