INERT GAS CONDENSATION.pptx

inert gas condesation

1. INERT GAS CONDENSATION A NANOPARTICLES PREPARATION METHOD……………… PRESENTED BY D.V.ANANDA RAO M150405ME M.Tech MATERIALS SCIENCE AND TECHNOLOGY

2. CONTENTS INTRODUCTION CLASSIFICATION PROCESS PROCESS PARAMETERS APPLICATIONS ADVANTAGES LIMITATIONS

3. INTRODUCTION Nanoscience refers to the science of very minute particles having their dimensions of the order of 10 -9 m Making of materials in NANO range is called NANO fabrication To synthesize nanostructured materials two approaches are there B ottom-up approach Top-down approach

4. Top-down approach: Under this process of fabrication, bulk materials are broken into nano -sized particles Eg . Ball millling plasma arching Bottom-Up approach : Refers to the building up of a material from the bottom i.e., atom by atom molecule by molecule cluster by cluster Eg . Inert Gas Condensation vapour deposition methods

5. Inert gas condensation Inert-gas condensation (IGC) is a bottom-up approach to synthesizing nanostructured materials, which involves two basic steps The first step is the evaporation of the material The second step involves a rapid controlled condensation to produce the required particle size In this unit, the chamber is evacuated to a pressure of about 2 × 10 −6 Torr by an oil diffusion pump. A crucible containing the metal to be evaporated is slowly heated

6. EVAPORATION TECHNIQUES Thermal Evaporation Laser Vaporization Sputtering Electrical Arc Discharge Plasma Heating

7. The evaporated metal atoms collide with the inert-gas atoms inside the chamber, lose their kinetic energy, and condense in the form of small, discrete crystals of loose powder, on the nitrogen cooled cold finger unit

8. Thermophoresis Nitrogen-filled collection device (cold finger),carry the condensed fine powders from the crucible region to the collector device, where they are collected via thermophoresis (as a result of a temperature gradient within the flow, which induces the particles to travel in the direction of decreasing temperature ) The phenomenon is observed at the scale of few nanometers

9. INFLUENCE OF PROCESS VARIABLES ON PARTICLE SIZE Parameter (increasing) Average particle size Inert-gas pressure Increases Inert-gas temperature Decreases Inert-gas molecular weight Increases Inert-gas flow rate Decreases Crucible temperature Increases Size Increases Evaporation rate Increases

10. Inert-gas pressure The size of the particles varies with changes in the gas pressure. It was suggested that it is the partial vapor pressure of the precursor, rather than the overall system pressure, that determines the powder particle size The total pressure, however, serves to regulate the diffusion of vapor from the growth source Diffusion increases when the total pressure is lowered due to a wide dispersion of the particles, thus restricting the growth by coagulation

11. Inert-Gas Temperature A rise in the inert-gas temperature leads to a decrease in the temperature gradient near the crucible and the nucleation zone moves away from the crucible to a region of lower vapor density, resulting in smaller clus - ters When the inert gas is very cold, most of the clusters are nucleated in a region of high vapor density, leading to rapid growth and large ultimate sizes since the temperature gradient close to the crucible is steepest

12. Inert-Gas Type An inert gas is generally used because the frequent collisions of the metal vapor atoms with the gas atoms decrease the diffusion rate of atoms away from the source region These collisions cool the metal atoms; consequently the diffusion rate is reduced and this allows achievement of supersaturation. The inert gases ( Ar , He, Ne, and Xe ) limit diffusion by shortening the mean free path Heavier gas atoms are most effective in limiting the mean free path and confining the metal vapor

13. Inert-Gas Flow Rate Increasing the inert-gas flow rate reduces the length of time that the clusters spend in the growth region of high vapor density and consequently leads to a decrease in the particle size They have less time to grow and so are smaller. It also increases the effectiveness of cooling, so that the temperature gradient is larger near the crucible and more clusters are nucleated

14. Chamber Size and Distance IGC operation in a large chamber leads to the deposition of nanostructured particles on a large area of the cold finger. The deposition of particles on a large area of the cold finger is preferable for easy collection of particles. Sufficient free space is required not only for convection in the IGC chamber but also for the collection of nanophase particles if the condensation chamber is very large, then growth may stop when the vapor is exhausted even before the exit is reached

15. Evaporation Rate Evaporation rate is the mass evaporated per unit area in unit time. The production rate is determined mostly by the evaporation rate. High evaporation rates have resulted in larger particles. The evaporation rate ( W g ) in a gas atmosphere is given by =( -P)  

16. APPLICATIONS Suitable to produce metal nanoparticles Controlled sintering after particle formation used to prepare composite nanoparticles ( pbS / Al;Si / ln;Ge / ln;Al / ln;Al / pb ) The types of nanostructured materials prepared by the IGC method include metals (e.g., Cu, Fe, Ni, Pd , and W), ionic compounds (e.g., Fe , Ca , , and Ti ), and also covalent substances (e.g., Si)  

17. ADVANTAGES R eactive condensation is possible, usually by adding to the inert gas in order to produce nanosized ceramic particles A wide range of materials including metals, alloys, inter- metallic compounds, ceramics, semiconductors, and com- posites can be synthesized by this technique. It is possible to produce virtually any material that can be vaporized. IGC is a very flexible technique in terms of the range of cluster sizes that can be made. It is possible to control the size and size distribution of the clusters/nanoparticles over a large range by altering process parameters such as tem- perature and pressure. Thus, particles of well-defined or predetermined size can be synthesized with enhanced properties.  

18. LIMITATIONS C hamber to be kept under high vacuum during deposition, substantial pumping of a large flow of an inert gas is nec - essary . The pumps make up most of the cost and bulk of the apparatus Agglomeration of particles is a problem in consolidated nanopowders . The van der Waals forces caused by a tem- porally varying charge distribution in each individual nanopowder particle can cause rapid agglomeration into branched bodies. These entities are difficult to break up on compaction and sintering, and thus lead to inter agglomerate voids and residual porosity in the sample.

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