Module-7_unit-3_NSNT 
                     Module 7 
                     Ball-milling 
                     Ball  milling  is  an  economic  and  facile  technique  to  produce  nanosized  materials.  It  is  a  top-down 
                     approach of nanoparticle synthesis which includes mechanical breakdown of large substances into smaller 
                     one. It is used in producing metallic as well as ceramic nanomaterials. In this module, the readers will 
                     learn the working principle and applications of Ball-mill technique in nanomaterials synthesis. 
                     The basic principle of a ball mill is very ancient. However, the machine itself could be produced only 
                     after the industrial revolution. Typically, ball mill is a grinder and is often employed blend materials by 
                     grinding/crushing them, for potential applications in mineral dressing processes, paints, pyrotechnics, 
                     ceramics as well as selective laser sintering. The working of a ball mill is based on impact and attrition: 
                     the impact, caused by the balls dropping from top of the shell, breaks down the particles, thereby resulting 
                     in reduction in size. Ball mill includes a hollow cylindrical shell which is rotated about its own axis. The 
                     axis of the shell is either horizontal or slightly inclined, thereby making a small angle with the horizontal. 
                     The shell is partially filled with balls. The balls form the grinding medium of the ball mill. These balls are 
                     usually made up of steel, ceramic, flint pebbles, or hard rubber. The inner wall of the shell is generally 
                     contains a coating of abrasion resistant material, e.g., manganese steel or rubber. Rubber lined (or coated) 
                     ball mills cause less wear to the products. The mill is nearly equal in length and diameter. 
                     In continuously operated mills, precursor material is fed from left side at an angle of 60°, and the end 
                     product is removed from right side at 30°. When the shell is rotated, the balls lift upwards on the rising 
                     side of the mill. After reaching near the top of the shell, the balls fall down, causing an impact on the 
                     particles trapped between these balls and the shell surface. This impact reduced the size of these particles. 
                     Large and medium sized mills are rotated mechanically on their axes. Small mills usually come with a 
                     cylindrical capped vessel sitting on two drive shafts which uses pulley and belt system to transfer rotary 
                     motion. Ball-milling finds wide applications in pyrotechnics and for producing black powder. However, 
                     some pyrotechnic applications such as flash powder do not use ball-mill due to their sensitivity to impact. 
                     Particle size can be reduced to as low as 5nm in high energy mills, though they are quite expensive. Such 
                     fine particles have enormous surface area and therefore the reaction rates are greatly enhanced. Further, 
                     ball  milling  is  widely  employed  in  mechanical  alloy  manufacturing  wherein  they  are  employed  for 
                     grinding and cold welding. The rationale behind cold welding is production of alloys from powders. 
                     There exists a critical speed at which grinding occurs. This speed is described as the minimum speed 
                     required to rotate the balls. 
                     The working of a ball mill can be understood as follows: A powder mix is positioned in ball mill and is 
                     subjected to high energy collision by the balls (Figure 1). The ball milling process can be summed up as: 
                          a.   It consists of stainless steel chamber and several small iron, silicon carbide, hardened steel, or 
                               tungsten carbide balls to rotate inside the mill. 
                          b.  Powder of material is put in the steel chamber. The powder is reduced to nanosize using ball mill. 
                               A magnet is positioned outside the chamber to apply pulling force on the material. This force 
                               raises milling energy as the milling chamber or container rotates the metallic balls. 
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                          c.   The ball and material - mass ratio is generally kept at 2:1. 
                          d.  These metallic balls impart very high energy to the powder resulting in crushing of the powder. 
                               The ball milling process generally takes 100 to 150 hrs to give uniformly crushed fine powder. 
                          e.   It is mechanical processing technique; consequently the structural as well as chemical changes are 
                               caused by the mechanical energy. 
                     The size of the nanopowders produced by this technique depends on the speed of rotation of the balls and 
                     the dimensions of 2 to 20 nm can be achieved. 
                     This  method  holds  the  advantage  of  low  temperature  synthesis  as  against  the  traditional  methods 
                     employing high temperature synthesis for producing materials. Apart from synthesis of materials, this is a 
                     method of altering the circumstances which give rise to occurrence of chemical reactions occurring by 
                     varying  the  reactivity  of  the  as-milled  solids  (by  mechanical  activation  —  growing  reaction  rates, 
                     dropping reaction temperature — of crushed powders) or by means of boosting chemical activities during 
                     the milling process (mechanochemistry). This is an approach of encouraging phase alterations in starting 
                     powders  or  reactants  whose  elements  have  similar  chemical  composition:  viz.  amorphization  or 
                     polymorphic changes of compounds, disarranging of ordered alloys, etc. 
                     The method was devised by Benjamin in the late 1960s when he produced fine uniformly dispersed 
                     particles of various oxides, such as Al O , Y O , etc. in Ni-base super alloys which could not be produced 
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                     by  conventional  powder  metallurgy.  This  method  has  revolutionized  traditional  synthesis  techniques 
                     which mandated elevated temperature processings. In addition to synthesizing materials, this technique 
                     can modify reaction conditions. As-milled solids readily undergo the chemical reactions owing to the 
                     process  called  mechanical  activation  where  the  reaction  rate  is  increased  by  lowering  the  reaction 
                     temperature  of  the  ground  powders.  Additionally,  chemical  reactions  can  also  occur  during  milling 
                     process, termed as mechanochemistry. High energy ball-mill can also induce phase transformations in 
                     precursors such as: amorphization or polymorphic transformations of compounds, disordering of ordered 
                     alloys, etc., however, all these have similar chemical compositions. 
                     Alloys can be prepared by using different equipments such as, attritor, planetary mill or horizontal ball 
                     mill. The working principle for all these techniques is same. During alloy formation using ball-mill, two 
                     processes  simultaneously  occur:  (a)  fracturing,  and  (b)  cold  welding  of  powders.  Thus,  it  becomes 
                     imperative to create a balance between these processes for successful alloying. 
                     In  mechanical  alloying,  planetary  ball  mill  is  commonly  employed  because  it  involves  very  small 
                     quantities  of  precursor  powders.  Thus,  this  system  is  most  popular  for  use  in  research  laboratories. 
                     Planetary mill includes one turn disc or table and around four bowls. The turn disc and the bowls rotate in 
                     opposite directions to each other. The rotation of bowl on its axis plus the rotation of turn disc, create 
                     centrifugal forces. These centrifugal forces operate on the powder mixture and the balls, resulting in 
                     fracturing and cold welding of the powder mixture. The milling balls can attain impact energies of upto 
                     40 times more than that caused by the gravitational acceleration, in normal directions. Thus, planetary ball 
                     milling can be employed for high speed/energy milling. Figure 1 shows the schematics of ball milling. 
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                                    Figure 1 Schematics showing nanoparticle synthesis via ball milling method. 
                      
                     In high-energy milling, the powder mixture is subjected to highly energetic impact. Microstructurally, 
                     mechanical alloying has four stages: 
                          a.   Initial Stage: Initially, compressive forces from collisions of balls flatten the powder particles. 
                               Micro-forging causes variations in the shapes of individual particles, or cluster of particles, owing 
                               to  repeated  impact  of  high  energy  (kinetic  energy)  milling  balls.  In  spite  of  this,  such 
                               deformations of the powder cause do not effectively change the mass. 
                          b.  Intermediate stage: In this stage, considerable variations occur in contrast to the first stage. 
                               Powders experience considerable cold welding. The fine blending of the constituents of powder 
                               reduces  the  diffusion  distance  to  few  microns.  The  dominating  processes  at  this  stage  are 
                               fracturing and cold welding. Though dissolution may occur to a certain extent, yet the alloyed 
                               powder does not have homogeneous chemical composition. 
                          c.   Final  stage:  This  stage  exhibits  marked  refinement  and  size  reduction.  Microstructurally, 
                               particles appear more homogenous at this stage. This stage marks the formation of true alloys. 
                          d.  Completion  stage:  The  structure  of  the  powder  particles  is  tremendously  deformed  and  is 
                               metastable. The lamellae are not resolvable via optical microscope. Alloying beyond this stage 
                               does not cause any improvement in the constituents’ distribution. True alloy is formed which has 
                               same composition as the precursor. 
                     Factors  affecting  the  milling  performance  include  mill  geometry,  ratio  of  angular  velocities  of  the 
                     planetary  to  the  system  wheel,  temperature,  grinding  atmosphere,  chemical  composition  of  powder 
                     mixtures, chemistry of grinding tools, etc. 
                     Surface and interfacial contamination is the main problem with nanoparticles produced by high-energy 
                     milling. This contamination may be caused by the milling tools (milling balls), ambient gases (traces of 
                     oxygen, nitrogen, and other rare gases present in the environment). Nonetheless, the milling speed and 
                     processing time can be optimized to alleviate the problem of contamination. Additionally, by coating the 
                     grinding tools with ductile materials, the chances of contamination can be reduced further. To minimize 
                     contamination  from  ambient  gases,  the  powder  mixture  is  loaded  in  inert  glove  box  and  then  the 
                     contained is properly sealed by an “O-ring”. Aside from these concerns, other drawbacks of ball-milling 
                     include  long  processing  times,  no  control  over  particle  morphology,  possibilities  of  particle 
                     agglomeration, residual strain in the crystallized phases. 
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                     Despite these limitations, high energy ball milling is extensively used due to simple design, working, and 
                     use to provide finely ground particles. Most popular application of ball milling is production of oxides of 
                     various metals for applications in sensor devices such as gas sensing. 
                     Ball mill is crucial to numerous industries as an equipment for producing extremely crushed materials, 
                     e.g. cement, refractory materials, fertilizers, glass ceramic, ore dressing of ferrous as well as non-ferrous 
                     metals, etc. Ball mill can be used to grind ores and other materials which can be both wet and dry. On the 
                     basis of removal of end products, ball mill can be of two types: (a) grate type, and (b) overfall type. 
                     Additionally, the grinding media can be of different types as well. The critical attributes of a grinding 
                     medium include size, density, hardness, and composition. We will discuss these properties in detail: 
                          a.   Size: The size of the grinding media directly influences the dimensions of the produced particles. 
                               As the size of milling balls is reduced, the size of final products also decreases. However, the 
                               grinding media cannot be made infinitely small. A limitation on the size of grinding media is 
                               imposed by the size of the largest pieces of material to be crushed. Thus, the milling balls must be 
                               considerably larger than the largest particle in the powder mixture required to be ground. 
                          b.  Density: The density of the grinding media should more than that of the material being crushed. 
                          c.   Hardness: The hardness of the grinding media should be such that it is durable enough to grind 
                               the material, and it should not be very hard. Very hard media can damage the cylindrical shell at a 
                               faster pace. 
                          d.  Composition: Different grinding applications have different requirements. The possible presence 
                               of grinding media in the finished product is also considered, and some applications make use of 
                               this grinding media in the end product. Other applications take into consideration the reactions 
                               between the grinding media with the material being crushed. These applications include: 
                                      If the color of produced material is important, the color as well as the material of the 
                                         grinding media must be considered. 
                                      If highly pure form of product is required, the grinding media must be selected such that 
                                         it  can be easily separated from the finished product (for example, steel dust produced 
                                         from stainless steel media can be magnetically separated from non-ferrous products). 
                                         Alternatively, the media of same material as the material being ground can be used. 
                                      In case of flammable products being ground, steel media can cause ignition and lead to 
                                         explosion. In such cases, either wet-grinding, or non-sparking media, e.g. ceramics or 
                                         lead balls can be used. 
                                      Certain grinding media (e.g., iron) can react with corrosive materials.  In such cases, 
                                         stainless steel, ceramic, and flint grinding media are often employed. 
                     To avoid oxidation and explosion, the reactor can be filled with an inert gas which does not react with the 
                     material being ground. 
                     Applications 
                     It is extensively employed to grind materials like coal, pigments, and feldspar for pottery. Both wet and 
                     dry grindings are possible, however wet grinding is usually done at low speeds. Ball-milling increases the 
                     solid-state  chemical  reactivity  in  multiple  components  systems.  Furthermore,  it  can  also  be  used  to 
                     produce amorphous materials. 
                     Advantages 
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