Nano materials
Properties #
High chemical reactivity and catalytic property due to high surface area to volume ratio.
Change in thermal properties compared to bulk. Ex: melting point of nano materials is very much different compared to bulk media.
Electronic properties: the band gap of nanomaterials increase with decrease in size. Thus, semi conductors can be tuned by playing with the size.
Optical properties: due to change in size, the energy difference between the orbitals also change resulting in a very different excitation wavelength of the nano material.
Magnetic : due to small size of the nanomaterials the arrangement of surface atoms or molecules becomes highly aligned in one or two directions and this gives rise to properties like super paramagnetism, ferro and ferri magnetism.
Synthesis of nanomaterials #
All the synthetic materials can be classified into two categories
Top down: the top down methods include volmil method, electric sputtering method, laser ablation, photo chemical etching techniques
Bottom up
- In the bottom up method techniques like pyrolysis, sol-gel, canonization, evaporation, electro spinning, emulsion templating are used.
For preparation of nanotube, nanowires, nanorods techniques like anodization, evaporation and electro spinning are used.
For nanoporous materials, emulsion templating and photochemical etching methods are used
For nano particles, ballmill, electric sputtering and pyrolysis are used.
Sol-gel #
Nano scale solid materials from small molecules. These molecules are generally hydroxide or alcoxides of metals such as silicon, titanium, gold. The process involves conversion of the monomeric hydroxide or alcoxides into polymeric colloidal solution.
The colloidal solution as a precursor for the formation of an integrated network or gel like structure which is formed due to the removal of liquid through evaporation or heating. Evaporation leads top formation of a compact structure leading to formation of aerogel if the interconnected network is heated rapidly or treated with supercritical fluid for removal of the liquid material then a porous structure is formed which is known as aerogel (has large pores which are formed after heating).
The whole process of sol-gel process can be divided into 6 steps:
Formation of stable compounds through hydrolysis and condensation.
Gelation which is formation of an oxide or alcoxide of bridged network through poly condensation.
Aging of gel which is also known as synerasis: in this step the bridged network collapses with removal of the liquid material.
Drying of gel: after the formation of aerogel/xerogel it is very important to remove water and other volatile compounds from the mixture. Hence it is heated to temperatures around 200 Celsius. This step is very important because the rate of heating, the critical point where removal starts and the temperature decreases highly influences the size of the nanomaterials. The next two steps are not performed generally.
Dehydration: removal of surface bound metal hydroxide and metal alcoxide grows by heating up to 800 deg celsius.
Densification and decomposition : the material after dehydration is heated to temperatures above 800 deg celsius to remove the remaining organic materials. It also collapses the pores of the gel.
Advantages of sol-gel #
It is very economic and usually low temperature maintained process
The thickness of the protein can be varied from a few nanometers to 500 nanometers.
Any geometry or complex shape can be achieved very easily in the gel.
High purity and the composition is also controllable. If any spent is to be added the its amount can be also controlled very easily.
Disadvantages of sol-gel #
These materials made through sol-gel process are not at all industry friendly because of weak bonding, low wear and tear and high permeability.
Coatings
Develops cracks very easily.
Organic materials trapped inside the pores have different thermal expansion properties compared to the nanoparticles and thus can have problems during temperature variation.