BARC/2019/E/001 BARC/2019/E/001
PRODUCTION AND CHARACTERIZATION OF COPPER NANOPARTICLES BY EXPLODING WIRE METHOD by Rashmita Das, Basanta Kumar Das and Archana Sharma Pulse Power & Electromagnetics Division
2019 BARC/2019/E/001
GOVERNMENT OF INDIA DEPARTMENT OF ATOMIC ENERGY BARC/2019/E/001
PRODUCTION AND CHARACTERIZATION OF COPPER NANOPARTICLES BY EXPLODING WIRE METHOD by Rashmita Das*, Basanta Kumar Das and Archana Sharma [email protected]* Pulse Power & Electromagnetics Division Atchutapuram, Visakhapatnam-531011
BHABHA ATOMIC RESEARCH CENTRE MUMBAI, INDIA 2019 BARC/2019/E/001
BIBLIOGRAPHIC DESCRIPTION SHEET FOR TECHNICAL REPORT (as per IS : 9400 - 1980)
01 Security classification : Unclassified
02 Distribution : External
03 Report status : New
04 Series : BARC External
05 Report type : Technical Report
06 Report No. : BARC/2019/E/001
07 Part No. or Volume No. :
08 Contract No. :
10 Title and subtitle : Production and characterization of copper nanoparticles by exploding wire method
11 Collation : 45 p., 25 figs., 4 tabs., 3 ills.
13 Project No. :
20 Personal author(s) : Rashmita Das; Basanta Kumar Das; Archana Sharma
21 Affiliation of author(s) : Pulse Power and Electromagnetics Division, Bhabha Atomic Research Centre, Visakhapatnam
22 Corporate author(s): Bhabha Atomic Research Centre, Mumbai - 400 085
23 Originating unit : Pulse Power and Electromagnetics Division, Bhabha Atomic Research Centre, Visakhapatnam
24 Sponsor(s) Name : Department of Atomic Energy
Type : Government
Contd... BARC/2019/E/001
30 Date of submission : March 2019
31 Publication/Issue date : April 2019
40 Publisher/Distributor : Head, Scientific Information Resource Division, Bhabha Atomic Research Centre, Mumbai
42 Form of distribution : Hard copy
50 Language of text : English
51 Language of summary : English
52 No. of references : 21 refs.
53 Gives data on : Abstract : Copper wire of diameter 0.19mm and 0.26 mm was exploded in atmosphere and 60 vacuum. Experimental system has capacitance of 7.1 micro farad and inductance 700 nH. Energy deposition was measured from voltage and current waveform. Exploded material in air at atmospheric pressure was characterized with XRD. XRD spectrum of the sample collected after explosion in air shows increase in oxidation of copper due to increase in overheating factor. The sample collected after explosion in different pressure values were characterized by XRD, optical microscope, TEM and AFM. Optical microscope image of collected exploded material at the vacuum of 2.3×102 mbar indicate the formation of micro particles. TEM and AFM image of exploded material at 1bar pressure of nitrogen indicate the formation of copper nanoparticles. XRD pattern of the same sample shows the presence of pure copper
70 Keywords/Descriptors : EXPLODING WIRES; COPPER; NANOPARTICLES; X-RAY DIFFRACTION; SPATIAL RESOLUTION; TRANSMISSION ELECTRON MICROSCOPY
71 INIS Subject Category.: S37
99 Supplementary elements : PRODUCTION AND CHARACTERIZATION OF COPPER NANOPARTICLES BY EXPLODING WIRE METHOD
Rashmita Das*, Basanta Kumar Das and Archana Sharma
Pulse Power & Electromagnetics Division, Bhabha Atomic Research Centre, Atchutapuram, Visakhapatnam-531011 [email protected]*
Abstract Copper wire of diameter 0.19 mm and 0.26 mm was exploded in atmosphere and vacuum. Experimental system has capacitance of 7.1 micro farad and inductance 700 nH. Energy deposition was measured from voltage and current waveform. Exploded material in air at atmospheric pressure was characterized with XRD. XRD spectrum of the sample collected after explosion in air shows increase in oxidation of copper due to increase in overheating factor. The sample collected after explosion in different pressure values were characterized by XRD, optical microscope, TEM and AFM. Optical microscope image of collected exploded material at the vacuum of 2.3×102 mbar indicate the formation of micro particles. TEM and AFM image of exploded material at 1bar pressure of nitrogen indicate the formation of copper nanoparticles. XRD pattern of the same sample shows the presence of pure copper.
CONTENT
1. INTRODUCTION……………………………………………………………...... 1 2. NANOPARTICLES…………………………………………………………… ...... 1 2.1. Production of Nanoparticles……………………………………...... 3 2.2. Size Control of Nanoparticles……………………………………… ...... 4 2.3. Applications of Metallic Nanoparticles……………………… ...... 5 3. THEORY……………………………………………………………………… ...... 6 3.1. Basic Solutions of LCR Circuit…………………………………...... 6 3.2. Principle of Electrically Explosion of Wire………………...... 8 3.3. Optimal Fuse Cross Sectional Area……………………………...... 9 3.4. Optimal Fuse Length…………………………………………………...... 10 4. CHARACTERIZATION……………………………………………………… ...... 10 4.1. Scanning Electron Microscope (SEM)…………………………...... 10 4.2. Transmission Electron Microscopy (TEM)…………………… ...... 11 4.3. X-Ray Diffraction (XRD)……………………………………………...... 11 4.4. Atomic Force Microscopy (AFM)…………………………………...... 13 5. EXPERIMENTAL DETAILS………………………………………………...... 14 5.1. Collection of Nanoparticles Sample………………………… ...... 14 5.2. Observation of Explosion in Air………………………………… ...... 15 5.3. Observation of Explosion in Vacuum…………………………...... 15 6. ISSUES RELATED TO NANOPARTICLES GENERATION BY EXPLODING WIRE METHOD……………………………………………………………………… . 16 7. RISK OF NANOPARTICLES TO HUMAN HEALTH AND ENVIRONMENT…… . 17 8. RESULTS AND DISCUSSION……………………………………………… ...... 18 REFERENCES ………………………………………………………………………...... 19 APPENDIX …………………………………………………………………………...... 21 List of Figures
Fig.No. Title Page Fig.1 Schematic of Transmission electron microscope 24 Fig.2 Schematic of X ray diffraction 25 Fig.3 Schematic of Atomic force microscope 25 Fig.4 Schematic diagram of the exploding wire circuit 26 Fig.5 Photograph of exploding wire set up for the nanoparticles 26 generation Fig.6. Sample collected by gravitational method 27 Fig.7 Sample collected by Hepa filter 27 Fig.8 Current and voltage waveform for the charging voltage 7.5 kV 28 Fig.9 Current and voltage waveform for the charging voltage 9.5 kV 28 Fig.10 Current and voltage waveform for the charging voltage 10.5 kV 29 Fig.11 Current and voltage waveform for the charging voltage 11.5 kV 29 Fig.12 Energy deposited into the wire with variation of overheating 30 factor Fig.13 X-ray diffraction pattern of the sample for overheating factor 1.3 30 Fig.14 X-ray diffraction pattern of the sample for overheating factor 2.5 31 Fig.15 X-ray diffraction pattern of the sample for overheating factor 2.9 31 Fig.16 X-ray diffraction pattern of the sample for overheating factor 3.8 32 Fig.17 Voltage and current waveform at 4.2× 10-2 mbar of nitrogen gas 32 Fig.18 Voltage and current waveform at 2.3× 102 mbar of nitrogen gas 33 Fig.19 Voltage and current waveform at 103 mbar of nitrogen gas 33 Fig.20 Energy deposited into the wire during wire explosion for various 34 pressures of nitrogen gas Fig.21 XRD spectrum for the exploded material 34 Fig.22 Optical microscope image of exploded material at 2.3× 102 mbar 35 of nitrogen gas pressure Fig.23 SEM image of nanoparticles 35 Fig.24 Transmission electron microscope image of nanoparticles 36 Fig.25 Atomic force image of nanoparticles 36
List of Tables Table Title Page No. Table.1 Experimental parameter for explosion in air 37 Table.2 Grain size variation with overheating factor 37 Table.3 Table of the values of the experimental parameters for 38 explosion in vacuum Table.4 Transition of the copper wire from solid through micro particle 38 dispersion and finally to nanoparticles
1. INTRODUCTION Printed electronics has the potential to enable rapid prototyping and low-cost production of functional circuits. Metal nano-ink is used in applications such as flexible radio frequency identification (RFID) tags, active-matrix LCDs, e-paper, flexible organic light emitting diodes (OLEDs) and wearable electronics [1]. Metal nano-ink consists of metal nanoparticles, an organic binder, a precursor and a solvent. The most important component of metal nano- ink is the metal nanoparticles. The low price, high conductivity and high oxidation resistance are also important issues in printed electronics. Cu nano particles are a promising replacement for Au and Ag nano particles because Cu is 100 times cheaper($7 kg−1) than Ag and possesses a conductivity (1.68 × 10−8 Ω m) between Au (2.44 × 10−8 Ω m) and −8 Ag (1.59 × 10 Ω m). Silver and gold are too expensive, in spite of their oxidation resistance. To synthesize metal nanoparticles, various methods have been used such as Polyol processes [2], Gas phase condensation [3], Thermal plasma [4] and Wire explosion [5-10]. In the polyol process and gas phase condensation methods, the particle size and shape can be elaborately controlled. However, many chemical reactions required and low throughput rate are shortcomings regarding mass production. The thermal plasma method also requires a complex process system with a high energy density, namely strong gradients between the medium carrying the deposition precursor and the surroundings. The wire explosion process has many advantages, such as the simplicity of the process, having a rapid production speed and having a high throughput rate, which are appropriate for mass production in industry. Furthermore, in the wire explosion process, it is easy to fabricate different nanoparticles together in the same solution simply by changing the metal wire used in the process. The process consumes only metal wire, electricity and generates little waste.
2. NANOPARTICLES Nanoparticles are less than 100nm size with one or more dimensions. Nano powders consist of primary particles arranged in a larger macroscopic structure. As such, the primary particle size is supposed to be the smallest size of individual particles. The primary particles, in turn, can be made of several crystals or may consist of a crystalline core with an amorphous shell. Primary particle size is not necessarily equal to the crystallite size determined by X-ray diffraction where only the crystalline part is detected.
The larger structure made up by concentration of the primary particles due to chemical bonding is defined as “agglomerate”. Aggregates are similar to agglomerates. However, the particles formed are held together by physical Van der Waals bonding and may be broken down to its individual units of primary particles when subjected to stronger forces or change in surface charges.
Agglomeration and aggregation are considered as secondary particle growth. Secondary particle growth leads to polydispersity in powders. So the powder no longer remains mono dispersed. It becomes polydispersed in nature. Consequently, it needs to be described by a size distribution instead of an average size. Further, different physical and chemical processes used during particle synthesis may also cause polydispersity.
1 Size constrains often produce new behavior in nano material. Depending on the carrier confinement, material at nanometer dimensions is defined in three categories. a. Quantum Wells: carrier confinement is in one dimension. b. Quantum Wire: carrier confinement is in two dimensions. c. Quantum Dots: Carrier confinement is in three dimensions.
Degree (d) of electron confinement determine the material dimension (D)
Material dimension (D) = 3- degree of electron confinement (d)
Nanoparticles are of three categories
1. Ultra fine particles: These are produced unintentionally in a process as a byproduct and typically originated from combustion and food cooking.
2. Traditional nanoparticles: These are produced in larger quantities for already existing application in the market e.g carbon black, fused silica, titanium dioxide generally produced from decades in chemical industries and polymer industries
3. Novel nanoparticles: These are engineered particles according shape and size to meet some special function in nano range.
There are two types for the production of nanoparticles, bottom up method and top down method. The “bottom-up” approach is a nano-architectural phenomenon of self assembly of materials from cluster-to-cluster, molecule-to-molecule or atom-to-atom on top of a base substrate. Vapor phase deposited method, plasma assisted deposited method, Liquid phase method, colloidal method, Sol-gel method, electro deposition and electrical explosion method are some bottom up method. The Top down method refers to a set of fabrication technologies starting with a block bulk material which share the same material with the base substrate. The most common type of top down methods is ball milling, lithographic process and machining by focused ion beam.
Two of the major factors why nanoparticles have different properties (optical, electrical, magnetic, chemical and mechanical) than bulk material are because in this size-range quantum effects start to predominate and the surface area to volume ratio is increased . The increase in the surface-area-to-volume ratio is a gradual progression as the particles get smaller which leads to that atoms on the outside of the particle will increasingly begin to dominate the ones inside the particle. This changes the individual properties of the particle and how it interacts with other materials in the surroundings. The increase of relative surface area makes them very interesting for the industry, as high surface area is a critical factor in for instance efficient catalysis and in structures like electrodes. This can improve the performance of products like batteries, but also reduce resource usage in catalytically
2 processes and hence decrease the amount of waste. The large surface area also increases the mixing with other materials in the surrounding and is especially beneficial in intermixed materials like composites. Once particles become small enough they start to exhibit quantum mechanical behavior. Classical mechanics can explain the relation between theory and observation for large objects. However, only quantum mechanics can explain the behavior of objects as small as electrons. Quantum mechanics describes the matter and radiation taking quantization into account.
2.1. Production of Nanoparticles Two approaches have been known in the preparation of ultrafine particles from ancient times. The first is the breakdown (top-down) method by which an external force is applied to a solid that leads to its break-up into smaller particles. The second is the build-up (bottom-up) method that produces nanoparticles starting from atoms of gas or liquids based on atomic transformations or molecular condensations. The top-down method is the method of breaking up a solid substance; it can be sub-divided into dry and wet grinding. A characteristic of particles in grain refining processes is that their surface energy increases, which causes the aggregation of particles to increase also. In the dry grinding method the solid substance is ground as a result of a shock, a compression, or by friction, using such popular methods as a jet mill, a hammer mill, a shearing mill, a roller mill, a shock shearing mill, a ball mill, and a tumbling mill. Since condensation of small particles also takes place simultaneously with pulverization, it is difficult to obtain particle sizes of less than 3 μm by grain refining. On the other hand, wet grinding of a solid substrate is carried out using a tumbling ball mill, or a vibratory ball mill, a planetary ball mill, a centrifugal fluid mill, an agitating beads mill, a flow conduit beads mill, an annular gap beads mill, or a wet jet mill. Compared with the dry method, the wet process is suitable for preventing the condensation of the nanoparticles so formed, and thus it is possible to obtain highly dispersed nanoparticles. Other than the above, the mechanochemical method and the mechanical alloying method are also known top-down methods. The bottom-up approach is roughly divided into gaseous phase methods and liquid phase methods. For the former, the chemical vapor deposition method (CVD) involves a chemical reaction, whereas the physical vapor deposition method (PVD) uses cooling of the evaporated material. Although the gaseous phase methods minimize the occurrence of organic impurities in the particles compared to the liquid phase methods, they necessitate the use of complicated vacuum equipment whose disadvantages are the high costs involved and low productivity. The CVD procedure can produce ultrafine particles of less than 1 μm by the chemical reaction occurring in the gaseous phase. The manufacture of nanoparticles of 10 to 100 nm is possible by careful control of the reaction. Performing the high temperature chemical reaction in the CVD method requires heat sources such as a chemical flame, a plasma process, a laser, or an electric furnace. In the PVD method, the solid material or liquid material is evaporated and the resulting vapor is then cooled rapidly, yielding the desired nanoparticles. To achieve evaporation of the materials one can use an arc discharge method. The simple thermal decomposition method has been particularly fruitful in the production of metal oxide or other types of particles and has been used extensively as a preferred synthetic method in the industrial world. For many years, liquid phase methods have been the major
3 preparation methods of nanoparticles; they can be sub-divided into liquid/liquid methods, and sedimentation methods. Chemical reduction of metal ions is a typical example of a liquid/ liquid method, whose principal advantage is the facile fabrication of particles of various shapes, such as nanorods, nanowires, nanoprisms, nanoplates, and hollow nanoparticles. With the chemical reduction method it is possible to fine-tune the form (shape) and size of the nanoparticles by changing the reducing agent, the dispersing agent, the reaction time and the temperature. The chemical reduction method carries out chemical reduction of the metal ions to their 0 oxidation states (i.e., M n+ → M 0); the process uses non-complicated equipment or instruments, and can yield large quantities of nanoparticles at a low cost in a short time. Of particular interest in this regard is the use of microwave radiation as the heat source that can produce high quality nanoparticles in a short time period. Besides the chemical reduction method which adds a reducing agent (direct reduction method), other reduction methods are known, such as photoreduction using gamma rays, ultrasonic waves, and liquid plasma which can be used to prepare nanoparticles. These methods that do not use a chemical reducing substance have the attractive feature that no extraneous impurities are added to the nanoparticles. Other than these methods, spray drying, spray pyrolysis, solvo thermal synthesis, and the supercritical method are also known. The general technique in the sedimentation method is a sol–gel process, which has been used extensively for the fabrication of metal oxide nanoparticles. Introduction to Nanoparticles procedure transforms a solution of a metal alkoxide into a sol by hydrolysis, followed by polycondensation to a gel. Several books are available that provide details of the sol–gel process. The wet process (liquid phase method) guarantees a high dispersivity of nanoparticles compared to the dry method. However, if the resulting nanoparticles are dried, aggregation of the particles soon follows. In this case, re-dispersion can be carried out according to the process used in the solid phase method. The synthesis of nanoparticles requires the use of a device or process that fulfills the following conditions: • control of particle size, size distribution, shape, crystal structure and composition distribution • improvement of the purity of nanoparticles (lower impurities) • control of aggregation • stabilization of physical properties, structures and reactants • higher reproducibility • higher mass production, scale-up and lower costs
2.2. Size Control of Nanoparticles The physical and chemical properties of nanomaterials depend not only on their composition but also on the particle size and shape. Accordingly, a high quality synthesis protocol must first of all provide control over particle size and shape. For example, if the diameter of an Au nanosphere is made to increase, the surface plasmon resonance will be gradually shifted from 530 nm to the longer wavelength side. Thus, if nanoparticles differ in size, their optical characteristics will also change significantly. In optical applications of nanoparticles, simplification of the size distribution of the particles becomes a very important factor. Therefore, it is important to fabricate nanoparticles with a single target size in mind. Generally, in order to prepare monodispersed nanoparticles, it is imperative that the nanoparticles grow very slowly after the rapid generation of the seed particles [21]. If the size of the nanoparticles decreases (i.e., increase in specific surface area), then the increase in the surface energy of such nanoparticles will facilitate their aggregation. Consequently, after
4 their growth to the desired optimal size, it will be necessary to stabilize the particulate surface by addition of a dispersing agent. However, where the concentration of nanoparticles is unusually high, the decentralized stabilization will fall, because the protective action of the organic substrate (citrate) is no longer strong enough to prevent aggregation. It is important to realize that the physical properties of a nanoparticles can change with the aggregation ratio, even though the colloidal solution may contain nanoparticles of identical size.
Methods to separate out particles of a given target size from a colloidal solution which contains nanoparticles of various sizes are known. They are (i) separation by precipitation, (ii) centrifugal separation, (iii) gel filtration column, and (iv) gel electrophoresis. As a feature of each screening method, the precipitation separation is suitable for a large distribution of colloid nanoparticles in the solution. The centrifugal separation and the gel filtration column are well suited for solutions of colloidal nanoparticles with a narrow size distribution. Gel electrophoresis is a suitable method to separate nanoparticles taking advantage of the difference in charge density of the particles, and is suitable for separating particles with a small cluster size. In fact, a combination of these various methods might prove advantageous. However, a problem with sorting the various sized nanoparticles using these methods is that only a fraction of the nanoparticles of a given size may be collected, and then only in small quantities.
2.3. Applications of Metallic Nanoparticles The various characteristics of different nanoparticles relative to bulk metals are summarized below.
Optical function: The surface absorption plasmon of Au and Ag can express various colors by changing the size of the particle, the form or shape of the particle, and the rate of condensation. A new paint that has the durability of an inorganic pigment and the vivid color of an organic substrate can be made. Nanoparticles smaller than the wavelength of light can be used to make high penetration conductivity materials (there is little absorption, dispersion, and reflection).
Catalyst function: Reaction efficiencies can be enhanced since the specific surface area of such nanoparticles is large compared with existing particles; to the extent that the surface terrace is regular at the atomic level, a hyperactive catalyst with high selectivity can be made: for example, Au nanoparticles.
Thermal function: When the particle diameter is small (less than 10 nm), the melting point is also lower than a bulk metal. Electronic wiring can be made with nanoparticles that have a low boiling point, for example, a polymer.
Electrical function: Since superconductivity transition temperature rises so that particle diameter is small (less than 1 nm), it can be used to make high temperature superconductivity material.
5 Mechanical function: Since the mechanical characteristics improve, mechanical strength can be sharply raised by mixing the nanoparticles with metals or ceramics.
Magnetic function: The attractive force of a magnetic metal increases on reduction of the particle diameter, such that soft-magnetic materials can be made in the form of an alloy of nanoparticles. Moreover, a permanent magnet can be made
3. THEORY 3.1. Basic Solutions of LCR Circuit For a simplified discharge circuit a DC supply charges the capacitor in series. Spark gap is open while charging the capacitor. When a discharge is required, a signal is applied to spark gap excitation circuit, this convert the circuit as closed path permitting the capacitor bank to discharge through load. The relationship of voltages, current and time are derived from the basic equation which describes the instantaneous voltage in any closed circuit.
Eq.(1)
Where V is capacitor voltage, I is instantaneous current, t is time, C is capacitance, L is inductance and R is resistance.
Three modes of operation can occur in a simple series circuit, depending on the relative magnitudes of resistance, inductance and capacitance in the discharge loop. The derivation of the equations which follow can be found in electrical engineering texts and handbooks.
a. The Oscillatory Condition ( 2 )
For the oscillatory discharge which persists several cycles, it can be assumed that the discharge current and capacitor voltage vary sinusoidally. From this assumption, the following equations can be derived.
Frequency: , where f is frequency ( hertz), L is the inductance ( henrys) C is the capacitance ( farads) , T is period of an oscillation ( seconds).
Peak current: 1.57 , where is maximum current ( amperes) during time dt, dV is capacitor voltage( volts) during time dt, dt is time for capacitor voltage to change from max positive value to max negative (second) Inductance
6 Percentage reversal = 100, where is maximum voltage occurring at time ( volt) and is maximum voltage occurring one half cycle later. b. The Critically Damped Condition ) When the critically damped condition prevails, the discharge current does not reverse direction as in the oscillatory case. Instead a maximum current is reached at an earlier time. The current then diminishes rapidly at a rate determined by the ration of the circuit constants. In many applications, a critically damped discharge is desired to avoid current reversal and to complete the discharge in minimum time. The following relations are useful.
Peak current: 0.736 where is maximum current (amperes), V is voltage on capacitor prior to discharge (volts), R is resistance ( ohms) Time to reach peak current: where is time (seconds), L is the inductance (henrys), C is capacitance ( farads)
Inductance : , Resistance : , Capacitance :
When a critically damped discharge is required, the problem is to achieve the proper circuit constants to insure the critical relationship.
c. The Over-Damped Condition ( 2 ) The over damped discharge reaches its peak current earlier than the other conditions, but maximum current amplitude is lower. Duration of currents near the highest level and significant capacitor voltage persists longer than the critically damped condition. The following fundamental relations are useful.
Instantaneous current: , where V is the initial voltage on capacitor (volts), R is resistance ( ohms), L is inductance ( henrys), C is capacitance ( farads).
N= Time of current maximum:
The extreme over-damped condition occurs when resistance is far greater than the critical damping resistance.
7 3.2. Principle of Electrically Explosion of Wire
The principle of electrically explosion of wire is simple. Current through the wire will deposit energy in the form of heat into the material. If the rate of energy deposition is such that adiabatic heating occurs then the material will heat up, melt and vaporize. During vaporization there is sharp change of the resistance of wire due to change in wire density. The transition between fluid to vapor occurs so rapidly that it is called electrically explosion. Further classification of mechanism is of four categories [11].
a. Melting : The energy necessary to vaporize the material is more than the available energy. This is called pseudo explosion. This results into molten metal.
b. Slow explosion: in this process the rate of energy deposition is so slow that it cannot deposit total energy to vaporize the material before the rise of instabilities in liquid. Thus in slow explosion part the material vaporizes early and the resistance of the wire increases before total energy required for vaporizing whole wire deposits into it.
c. Fast explosion: Time taken to deposit sufficient energy into wire is shorter that the creation of instabilities to develop. The wire remains undisturbed till the explosion takes place.
d. Explosive ablation : The rate of energy deposition is so fast that , it creates non uniform distribution of the energy in the conductor due to electrical and thermal skin effect. Large amount is deposited into outer layer of the conductor. Due to which outer layer vaporize faster than the bulk one. This is called onion peel off where layer by layer vaporizes.
To understand further understanding of wire material, it is better to divide the stages of the wire into four categories heating, melting, vaporization and expansion separately. The initial conducting stage of the material is governed by geometry and material constant of the wire and easy to interpret. Wire parameters such as cross sectional area, length, material, medium , ambient pressure, temperature, thermal conductivity of the medium, and current will have an impact on time of explosion, energy consumption, commutated energy, peak voltage, voltage hold off, and rate of resistance rise. Heating and melting of wire due to joules heating describe initial stage of the exploding wire. During heating resistivity of the material depends upon introduced energy into wire. The three factor which has profound impact of the initial stage is cross section. Length and material.
8 3.3. Optimal Fuse Cross Sectional Area
For a wire of length ‘l’ and diameter ‘d’, power dissipated into the wire due to flow of current I(t) is proportional to .l tP )( tI )(. 2 Eq.(2) s
ρ is the resistivity of the material , ‘s’ is the cross section of the wire. If e is the internal energy per unit mass of the material then at any point of time power dissipated into wire can be written as mtP .)( de Eq.(3) dt where m is the mass of the wire and resistivity is a function of deposited energy, so above equation can be rearranged to
.l 2 mtI .)(. de s dt
l 1 )(. 2 dttI de .ms e)(
tI )( 2 1 .dt de .s 2 e)(
is the density of the material. Ideally the wire has to be completely transition from solid through liquid to vaporization. The energy to vaporize material equal to ev should match with tb . Up to the point of vaporization the above equation can be written as 1 tb ev 1 )( 2dttI ade Eq.(4) s2 e)( 0 e0 a is called the action integral of the material and as the right hand side of the integral is solely dependent on material properties and hence considered constant for any material. This is derived taking the heating to be slow adiabatic. Slow here means heating faster than the thermal time constant of the medium but slow compared to time constant of the material to result the uniform heating of wire and negligible heat transfer of the medium. However for exploding wire experiments, a needs to be multiplied by a factor k1, where 1 9 3.4. Optimal Fuse Length The energy dissipated into wire material is given by Eq.(5) Where and are mass density and maximum deposited energy per unit mass in normal condition. The energy required to drive the material from room temperature to vaporization is given by Eq.(6) and are heat capacities of solid and liquid . and are heat of melting and vaporization. , and are initial temperature, temperature at the time of melting and temperature at the time of boiling respectively. Analysis of the data shows that only 62% energy necessary to vaporize the material is deposited into the wire. The length of the fuse can be calculated as Eq.(7) . . For most of the material used in exploding wire, vaporization energy is 80% of total energy and rest of the energy is required to reach to the point of vaporization. 4. CHARACTERIZATION At the end of the 20th century, most efforts were dedicated to develop synthesis and characterization techniques to produce and probe nanoparticles of smaller and smaller sizes. In order to able to tailor the properties of nanoparticles, careful investigation of the particles is crucial. If characterization is performed directly after adjustment of process parameters for the nanoparticles production process, it is easy to determine whether that specific adjustment improved or impaired the properties of the particles. In addition thorough characterization with regard to intended application is important. In this report we are emphasizing the following characterization techniques. 4.1. Scanning Electron Microscope (SEM) The electron column of the SEM consists of an electron gun and two or more electromagnetic lenses operating in vacuum. The electron gun generates free electrons and accelerates these electrons to energies in the range of 1-40 keV in the SEM [13]. The purpose of the electron lenses is to create a small, focused electron probe on the specimen. Most SEMs can generate an electron beam at the specimen surface with spot size less than 10 nm in diameter while still carrying sufficient current to form acceptable image. The spatial resolution of the SEM depends on the size of the electron spot, which in turn depends on both the wavelength of the electrons and the electron-optical system which produces the scanning beam. The resolution is also limited by the size of the interaction volume or the extent to which the material interacts with the electron beam. The spot size and the interaction volume are both large 10 compared to the distances between atoms, so the resolution of the SEM is not high enough to image individual atoms, as is possible in the shorter wavelength (i.e. higher energy TEM). 4.2. Transmission Electron Microscopy (TEM) Microscopy is useful when investigating small structures. The resolution of conventional visible light microscopy is limited. In electron microscopes, the resolution depends upon the wavelength of electron. In a TEM, a monochromatic beam of electrons is accelerated through a potential of 40 to 100 kilovolts (kV) [14]. Schematic of TEM is shown in (Fig.1). Since the De Broglie wavelength of electron is much shorter (in the order of pm) than the wavelength of visible light, much smaller feature can be resolved with electron microscope. The resolution of electron microscope is not only limited by wave length of electron but also by the quality of electromagnetic lenses used. TEM consists of illumination system, the objective lens and the imaging system. The role of the illumination system is to create electrons and transfer them to the sample, consists of electron gun and condenser lenses. An electron gun comprises an electron source to create electrons and an assembly that accelerate and direct electrons to rest of the illuminating systems. Once the electrons have been created by electron gun, the condenser lenses focus the electron beam onto specimen. Since electrons are used for the illumination instead of light, electromagnetic lenses are used instead of glass lenses to direct the electron beam. Electromagnetic lenses in the TEM are fixed. The strength of electromagnetic lenses is changed by changing the current through the coil in order to change focus, intensity of illumination and magnification of electron beam. 4.3. X-Ray Diffraction (XRD) X-ray diffraction is widely used technique for studying structure of the crystal, in which monochromatic photons are scattered by the electrons surrounding the atoms in a crystal [15]. In order for the photons to interact with crystal, their wavelength must be of the same order of magnitude as the spacing of planes in the crystal, hence X-rays are used. Since the atom in the single crystal is periodically placed, all lattice points in the crystal are said to emit a scattered wave. These scattered waves will interfere with each other according to Bragg’s Law. Depending upon the phase difference, the scattered wave can interfere constructively or destructively producing interference patterns with intense spots, from which information is obtained about symmetry and spacing of the scattering centers. Normally XRD is a bulk characterization technique. Schematic of XRD is shown in (Fig.2). But when the grazing incident angle is used, the penetration depth is reduced and the technique becomes more surface sensitive. The study of small (disordered) particles with XRD differs slightly than the larger crystal. There are three type of X-ray diffraction possible. 1. Laue method: In this method wavelength of X-ray is variable but angle is fixed. Again Laues diffraction method has two sub categories. a) Transmission Laue method: In this method photographic film is placed behind the crystal. Some of the diffracted beam partially transmitted through crystal. Diffraction beam produce spot on the film. These spot lie in ellipse or hyperbola. b) Back reflected Laue method: In this method diffraction beam produce spot on the film 11 placed between X-ray source and crystal. These spot arranged into hyperbola. 2. Rotating crystal method: In this method wave length of X-ray is fixed and angle is variable in part. A single crystal is mounted with one of it’s axis or some important crystallographic direction normal to monochromatic X-ray beam. Film will be kept around the crystal axis of film coincide with axis of rotation of crystal. 3. Powder diffraction method: In this method wavelength of X-ray is fixed and angle is variable. Each powder particle is a tiny crystal oriented in all possible direction w.r.t incident beam. In Debye Scherrer method, a narrow strip of film is curved into a short cylinder. Crystal is placed on the axis of cylinder. Diffraction beam comes out as a cone and cut cylindrical strip in line. From diffraction pattern, lattice parameter can be determined by following equation (Eq.8) [15]. Where h, k, l are the Miller indices. a = λ ( h2 + k2 + l2)1/2 / 2Sinθ Eq.(8) Grain size of the Powder can be determined using Scherrer’s Formula(Eq.9)[16] D = kλ / B Cosθ Eq.(9) Where B = Full width half maximum of diffracted peak, k= Scherrer’s constant = 1.5, λ = 1.54056 Angstroms X-ray diffraction intensity depends upon the crystal structure of the material. This is characterised by structur efactor and form factor. Structure factor : Resultant wave scatterd by all the atoms of the unit cell is called as structure factor. It describes how th eatomic arrangement given by uvw for each atom affect the scattered beam. u,v.w arethe unit vector in reciprocal lattice space. Atomic scattering factor. Form factor (f) : It gives effeiciency of scattering of an atom in a given directon. Possible reflection are describedby reciprocal lattice point. Reciprocal lattice vectore of one plane is given by G= hu+kv+lw Relative intensity of various reflection depends upon on the number , position and electronic distribution of the atoms in unit cell. If unit cell contains N atoms 1,2....N with cordinate (u1, v1, w1), (u2,v2,w2)...... (uN, vN, wN) and atomic scattering factor f1, f2....fN then 12 Structure factor F =∑ Eq.(10) 1. For simple cubic case there is one atom per unit cell at (000). In this case F=f i.e independent of hkl as intensity is same for all direction. 2. For face centred cubic lattice, there are four atoms per unit cells positioned at (0,0,0), (1/2, ½, 0), (0.1/2,1/2) ( ½,0,1/2). For this case equation (10) reduced to F = 1 ⋯. = 4f ( If h , k and l unmixed i.e all odd or all even) = 0 ( if h.k and l are mixed ) 3. For body centered cubic lattice there are two atoms per unit cell positioned at ( 0,0,0) and ( 1/2., ½, ½) respectively. For this case equation (10) reduced to F = 1 = 2f ( If ( h+k+l) is even = 0 if (h+k+l) is odd 4.4. Atomic Force Microscopy(AFM) The AFM can be used to investigate any surface even very poor or non conducting. The instrument measures forces on a sample by scanning the surfaces by scanning the sample with the tip attached to a flexible cantilever [17]. Schematic of AFM is shown in (Fig.3). The resolution obtained by AFM is determined in large part by the size of probe tip used for imaging. The way the AFM operates is similar to the principle of record player in that tips move up and down in response to surface features. An optical read out or piezoelectric crystal, translates the motion of cantilever into an electrical signal. The outcome is a three dimensional image of the surface structure displayed on screen. Maximum resolution is typically on the atomic scale in the lateral and vertical direction. The three main class of interaction are contact mode, non contact mode and tapping mode. Contact mode known as repulsive mode is the common method used in AFM. The tip and sample remain in close contact when the scanning proceeds. One of the problems remaining in contact with the sample is caused by excessive tracking forces applied by the probe to sample which can damage the image and distort the image data. An effort to avoid this problem is to use non contact mode. In non contact mode the cantilever is oscillated in attractive regime, meaning the tip is quite close to the sample but not touching it. Attractive Van der Waal forces acting between the tip and sample are detected, the tip scans above the surface and topographic image has been created. The tapping mode can be used to obtain high resolution topographic imaging of the sample that are easily damaged, poorly immobilized or difficult to image by other AFM techniques. Tapping mode improves AFM functionality by placing the tip in contact with surface to provide high resolution and lifting 13 the tip off the surface to avoid dragging the surface across the surface. The advantage of the tapping mode is, it prevents the tip from sticking to the surface and causing damage during scanning. 5. EXPERIMENTAL DETAILS The fundamental circuit for Exploding Wire is shown in (Fig.4).Capacitor C was charged from high voltage power supply and discharged into wire. If the purpose of the research is to study the wire explosion itself, several avenues of approach are available. Electrical measurement may be made of current, voltage and power. Since the wire itself has very low resistance and inductance, the circuit parameters are extremely important in any study of wire explosion. Designer takes special precaution to make circuit inductance as low as possible to allow maximum current through the wire. Circuit parameters is written in Appendix –A. The exploding wire experiment was carried out in a cylindrical SS chamber with the provision to install the wire straight and various ports have been provided for the injection of the gas, pumping port for connection to the vacuum pump and port for vacuum measurement. Selection of wire diameter is written in Appendix-B. Material parameter is written in Appendix- C. Photograph of the experimental setup is shown in (Fig.5). 5.1. Collection of Nanoparticles Sample There are usually three methods for nanoparticles collection. a. Gravitational Method In this method one myler sheet is placed on the bottom of chamber. After explosion of the wire, the powder is allowed to settle down due to gravity for 20 minutes on the bottom of the chamber. Then the particles are collected by scratching the myler sheet. Image of the collection of copper nanoparticles is shown in (Fig.6). b. Pumping Method through Baffle After explosion, the chamber is pumped by one rotary vacuum pump through baffle. Baffel is made up of SS in the size of standard 25KF. For pumping fine orifices are made on the baffle such a way that both the sides of baffle are optically opaque. So through these orifices, the chamber is pumped by rotary pump where as nanoparticles settle down on the surface of the baffle. c. Hepa Filter Hepa filter is the micron porous size air filter. In this process of collection of nanoparticles, one mesh is placed in the transition flange of pumping port. The hepa filter is placed on the mesh. Then after explosion the explosion material was sucked by vacuum pump through filter. So powder travel to the filter and stick on it. Then sample is collected scratching the filter paper. Image of the collection of copper nanoparticles is shown in (Fig.7). 14 5.2. Observation of Explosion in Air The experiments were carried out by imparting different levels of electrical energy to the exploding conductor. Typical experimental parameter is given in (Table.1). The voltage developed across the wire was measured by voltage probe and current through the wire was measured by current transformer. Voltage and current waveform of different charging voltage are shown in (Fig.8-11).Re-striking occurs when higher voltage is applied. Energy deposited in the wire for respective charging voltages are shown in (Fig.12). The powder samples were characterized by x- ray diffraction (XRD) to determine the grain size variation and evolution of thermal oxidation of copper at higher temperature. XRD pattern of different phases of copper and copper oxide nanoparticles with the variation of overheating factor(K), which is defined as the ratio of energy deposited into the wire to sublimation energy are shown in (Fig.13-16). Sublimation energy required for the wire in our case is 113 Joule. The grain size was measured by Scherrer’s equation. Variation of grain size with overheating factor is shown in (Table.2).Here the explosion was carried out in atmospheric air. It is observed that for the overheating factor 1.3, core of nanoparticles is copper with very little oxidation of Cu2O on the surface. But with increase in overheating factor, surface oxidation of copper increases in the same ambient condition. 5.3. Observation of Explosion in Vacuum Typical experimental parameter is given in (Table.3). For each set of experiment the pressure was varied to investigate the actual size distribution of the exploded material. The pressure in the experimental chamber was varied from 4.2×10-2 mbar to 1 bar with nitrogen. The current was measured using a current transformer (CT), while the voltage was measured using a voltage divider. Typical current and voltage waveforms are shown in (Fig.17-19). The energy deposited into the wire was calculated from voltage and current curves and is shown in (Fig.20).Vaporization energy required for the wire of length 12cm and diameter of 0.26mm is 207J. XRD spectrum of the exploded material is shown in (Fig.21). Energy deposited into the wire at 4.2 ×10-2mbar amounts to 120 Joule which is not sufficient even to melt the wire and it was dissipated for the breakdown of the surrounding gas medium. When the pressure was increased to 2.3 ×102mbar, the energy deposited into the wire was 150 Joule and the wire exploded producing micro size particles. Ringing behavior of current waveform shown in (Fig.17&18) is due to lesser amount of energy that is deposited into the wire which keeps the path between the two electrodes close and behaves as a shorted LC system. The particle size was studied using optical microscope from the sample collected at 2.3 ×102mbar nitrogen pressure. The image of the optical microscope is shown in (Fig.22). For pressure of 1 bar, energy deposited was highest i.e, 250 Joule. As observed in (Fig.19) most of the deposited energy dissipated for current interruption. Under these experimental conditions the explosion of wire occurred and the wire disintegrated into nanoparticles. Transition of the copper wire from solid through micro particle dispersion and finally to nanoparticles is shown in (Table.4). The images of the nanoparticles observed by SEM, TEM and AFM are shown in (Fig.23-25). 15 6. ISSUES RELATED TO NANOPARTICLES GENERATION BY EXPLODING WIRE METHOD a. Size Many fundamental properties are size dependant. On the nano scale, the total free energy is a sum of free energy of the free energy of the bulk and surface of the nanoparticles [18]. Surface free energy contributes largely with the decrease of size. Size affects the particle motion. In aqueous environments and as aerosol, there is a tendency of nanoparticles to form aggregates that are much larger than the primary size particles initially produced. So, integral properties need to be specified by number concentration, surface concentration and mass concentration. b. Shape Recently, there are number of research papers emphasizing the shape dependant properties variation on the nano scale [19]. For spherical particles, the measured diameter from different instruments can be related because no corrections need to be made for shapes, but for non spherical particles or agglomerates and aggregates that are irregularly shaped, volume equivalent diameter need to be formulated c. Surface contamination Surface contamination is the most important part of the processing the materials which has the direct impact on the functionalization. Vacuum plays the important role in every stage of processing like production, storage, characterization. So, in situ characterization techniques become important to avoid the surface contamination in larger extent and to avoid the loss of chemical activity of the material. d. Excessive heat production problems There are many processes where the generation of nanoparticles involves generation of heat as in exothermic reaction as well high temperature in laser ablation, exploding wire method, and plasma assisted nanoparticles generations process etc. [20]. The rapid cooling process and sample collection process becomes crucial to get the desired shape size of the nanoparticles and of desired range. e. Yield of the product In all the above process, though the aim is to produce samples of mono dispersed, single- sized particles, more often than not there is a range of nanoparticles sizes each slightly different than the other than that produced. At this time it is often difficult to make the sample of nanoparticles containing exactly same number of atoms as like any molecular formula. f. Characterization Techniques To fully understand the properties of functional nano structures, it is necessary to know the portion of all the atoms in the nanostructures.It is difficult to quantify surface properties such as surface coverage and the number of functionalized groups at the interface of nanoparticles. 16 Because of these difficulties, the development of suitable characterization techniques remains an important priority. g. Environmental Effect Nanoparticles are also difficult to handle in production process. Their impact on health and the environment has also become a serious concern. The process to recapture nanoparticles when they are released into the environment is difficult. 7. RISK OF NANOPARTICLES TO HUMAN HEALTH AND ENVIRONMENT Nanoparticles cause more inflammation when inhaled and deposited in the lung than larger particles of the same material, which is believed to be caused by the increased surface area that follows a decrease in particle size. This theory is well established. Inhaled nanoparticles are able to translocate from the point of intake to secondary organs in the body. The effects nanoparticles cause in the secondary organs are not known. However, nanoparticles cause different levels of interaction with the biological system and have different mobility based on the size, shape and chemical composition. Therefore it is not possible to address the hazard sand risks of nanoparticles in a general way, as each type nanoparticle need to have its own toxicity understood.The occupational health authorities are aware and updated on the potential risks of nanoparticles and see the need to assess nanoparticles on an individual basis. They are prepared to amend the existing regulations, but they are aware that today, this is not possible as it is not known whether the exposure levels should be set based on mass, volume, particle numbers or surface area However, the risks of nanoparticles have not a high priority among the authorities today, as there are no clear indications whether the nanoparticles constitute a risk or not to human health. The European Commission believes that the benefits of nanotechnology will outweigh the potential risks, but stresses that today, there are no identified risks of nanoparticles, only identified hazards. The companies are using engineering control in production which protects the products from contamination as well as the workers from the products. When the workers are handling the nanoparticles they apply the personal protective equipment normal for a chemical or dusty working environment like masks and gloves. However, today nanoparticles are often treated as dust in terms of precautionary measures, but the question is whether the precautionary measures applied by the producers need to take the uncertainties into account and address nanoparticles as in a worst case-scenario. There are limited precautionary measures taken to protect public health and the environment from exposure to nanoparticles. Precautionary measures for these purposes seem today to have a very low priority. The European Commission has a duplex role as they are both promoting nanotechnology, but at the same time has a legal obligation to protect the citizens and the environment. The EC is not taking any precautions of today and it is likely that questions of precautionary measures 17 will be left fully to the individual member states. However, no direct precautionary measures have been taken by the occupational health authorities, but some countries like the UK are more active than other countries and have already established working groups and contact networks covering both industry and scientists on this issue. 8. RESULTS AND DISCUSSION In the exploding wire experiment, nanoparticles form by Brownian coagulation of the explosion material. For the sample exploded in air, it is observed that for the overheating factor 1.3, core of nanoparticles is copper with very little oxidation of Cu2O on the surface. But with the increase in overheating factor, surface oxidation of copper increases in the same ambient condition. Grain size measured shows decreasing trend with increase in overheating factor. This indicates for higher temperature of vapor molecules and small size of the particles, rate of coagulation is very small. A change of the wire surrounding medium in the explosion process presents a simple and flexible technique to modify the properties of the nanoparticles. Effect of nitrogen pressure has a significant role for the production of pure copper nanoparticles using electro explosion wire technique of copper wire. At a nitrogen pressure of one atmosphere, the energy deposited into the wire is maximum. But the energy deposited decreases with decrease in the gas pressure as the energy goes into the breakdown of the gas. Electrical glow discharge creates the shortest path between the electrodes and is seen as ringing in the current waveform. At sub atmospheric pressures, for a particular combination of electrode gap and applied voltage, electrical break down of the gas occurs and is attributed to the electrical glow discharge regime. Purity of the copper nanoparticles produced in presence of nitrogen is due to the unstable bonding between copper and nitrogen. Mechanism of the nanoparticles formation can be explained as follows. Due to joule’s heating the temperature of the conductor rises followed by melting and heating of the wire which is in liquid phase. Before transition of the gas phase liquid goes through boiling point and superheating. The vapor particles formed during vaporization process get cooled with the collision with the atoms which are at room temperature. Generation of nanoparticles occur through nucleation. There are two type of nucleation; homogeneous nucleation and heterogeneous nucleation. In the exploding wire system, the molecules in the medium act as site for condensation. So it is considered as heterogeneous nucleation. After nucleation of the particles coagulation of the particles takes place due to collision of the particles through Brownian motion. Mass of the wire is 57 mg. We collected the sample through gravitational method, which is measured to be 50 mg. 18 REFERENCES [1] “Copper nanoparticles applications for ink”,https://www.nanoshel.com/copper- nanoparticle-applications/, (access date 15.09.2018) [2] Dong, H., Chena, Y.C. and Feldmann, C. “Polyol synthesis of nanoparticles: Status and options regarding metals, oxides, chalcogenides, and non-metal elements”, Green Chemistry,17(8):4107-4132, 2015. [3] Grammatikopoulos, P. et al. “Nanoparticle design by gas-phase synthesis”, Advances in Physics: X, 1(1):81-100, 2016. [4] Macwan, D.P. et al. “Thermal plasma synthesis of nanotitania and its characterization”, Journal of Saudi Chemical Society, 18(3):234-244, 2014. [5] Lerner, Marat I. et al. “Synthesis of Al nanoparticles and Al/AlN composite nanoparticles by electrical explosion of aluminum wires in argon and nitrogen”, Powder Technology, 295(July):307-314, 2016. 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Sugimoto (Series: Surfactant Science, v.92), New York : Marcel Dekker, 2000. 20 Appendix-A Circuit Parameters The relationship of voltages, current and time are derived from the basic equation which describes the instantaneous voltage in any closed circuit. In our case time period obtained by shorting the circuit is 14 microsecond . Inductance is given by ~ 700nH. Resistance of the circuit ln =0.07 Ohm Voltage reversal k = 100~ 80% is maximum voltage occurring at time ( volts) is maximum voltage occurring one half cycle later. For our case 2 =0.6. So 2 This implies the circuit parameters match with the condition for the LCR UNDER DAMPED CIRCUIT. Inductance of wire : =0.15μH 21 Appendix-B Diameter of wire Current profile in damped sinusoidal oscillation is given by ω is angular frequency of sinusoidal waveform. However for exploding wire experiments, action needs to be multiplied by a factor k1, where 1 For T is time period of damped oscillation, For 9kV = 23kA For a current generator of 23kA and time period of 14 micro second, and taking k1to be 2 diameter of the wire is nearly 0.28mm. 22 Appendix-C Material parameters 1. Crystal structure : FCC 2. Heat of vaporization : 3630 J/gm 3. Heat of sublimation : 3730 J/gm 4. Copper molecular weight : 63.55 g/mole 5. 1st ionization energy of wire : 745.5KJ/mole 6. Density of copper : 8.96 gm/cm3 For dimension of copper wire used: length 12 cm and diameter 0.26mm 1. Heat of vaporization: 207J 2. Heat of sublimation: 212J 3. 1st Ionization energy: 670 J 23 Fig.1. Schematic of Transmission electron microscope 24 Fig.2. Schematic of X ray diffraction Fig.3. Schematic of Atomic force microscope 25 Fig.4. Schematic diagrram of the exploding wire circuit Fig.5. Photograph of exploding wire set up for the nanoparticles generation 26 Fig.6. Sample collected by gravitational method Fig.7. Sample collected by Hepa filter 27 20000 15000 Current 10000 5000 Voltage 0 Voltage (V), Current (A) Current (V), Voltage -5000 -10000 -10.0µ -5.0µ 0.0 5.0µ 10.0µ 15.0µ 20.0µ 25.0µ 30.0µ Time(Sec) Fig.8. Current and voltage waveform for the charging voltage 7.5 kV 25000 20000 15000 Current 10000 Voltage 5000 0 Voltage (V), Current (A) Voltage -5000 -10000 -20.0µ -10.0µ 0.0 10.0µ 20.0µ 30.0µ 40.0µ 50.0µ 60.0µ Time(Sec) Fig.9. Current and voltage waveform for the charging voltage 9.5 kV 28 25000 20000 Current 15000 Voltage 10000 5000 0 -5000 Voltage (V), Current (A) -10000 -15000 -20000 -5.0µ 0.0 5.0µ 10.0µ 15.0µ 20.0µ 25.0µ 30.0µ Time(Sec) Fig.10. Current and voltage waveform for the charging voltage 10.5 kV 25000 20000 15000 Current 10000 Voltage 5000 0 -5000 Voltage(V), Current(A) -10000 -15000 -20000 -10.0µ 0.0 10.0µ 20.0µ 30.0µ 40.0µ 50.0µ 60.0µ 70.0µ Time(Sec) Fig.11. Current and voltage waveform for the charging voltage 11.5 kV 29 Fig.12. Energy deposited into the wire with variation of overheating factor Fig.13. X-ray diffraction pattern of the sample for overheating factor 1.3 30 Fig.14. X-ray diffraction pattern of the sample for overheating factor 2.5 Fig.15. X-ray diffraction pattern of the sample for overheating factor 2.9 31 Fig.16. X-ray diffraction pattern of the sample for overheating factor 3.8 40000 40000 -2 4.2 x 10 mbar Current 20000 20000 0 0 Current (A) Current Voltage (V) Voltage -20000 -20000 0.0 40.0µ 80.0µ Time (Sec) Fig.17. Voltage and current waveform at 4.2× 10-2 mbar of nitrogen gas 32 2 2.3 x 10 mbar 20000 20000 Current 0 0 Current(A) Voltage (V) Voltage -20000 -20000 0.0 40.0µ 80.0µ Time(Sec) Fig.18. Voltage and current waveform at 2.3× 102 mbar of nitrogen gas 3 10 mbar 20000 20000 Current 10000 10000 Voltage Current(A) 0 0 Voltage(V) 0.0 40.0µ 80.0µ Time(Sec) Fig.19. Voltage and current waveform at 103 mbar of nitrogen gas 33 Fig.20. Energy deposited into the wire during wire explosion for various pressures of nitrogen gaas Fig.21. XRD spectrum for the exploded material 34 Fig.22. Optical microscope image of exploded material at 2.3× 102 mbar of nitrogen gas pressure Fig.23. SEM image of nanoparticles 35 Fig.24. Transmission electron microscope image of nanoparticles Fig.25. Atomic force image of nanoparticles 36 Table.1. Experimental parameter for explosion in air Sl no: Parameter Value 1 Capacitance 7.1 µF 2 Inductance 700 nH 3 Charging voltage 7.5kV to 11.5 kV 4 CT (1:1000) V/A 5 HV probe (1:1000) 6 Surrounding gas medium Air 7 Pressure 1 bar 8 Diameter of the wire 0.19 mm 9 Length of the wire 12 cm Table.2. Grain size variation with overheating factor Sample Over heating Grain size (nm) No factor XRD 1 1.3 59.68 2 2.5 51.13 3 2.9 45.9 4 3.8 39.61 37 Table.3. Table of the values of the experimental parameters for explosion in vacuum Sl no: Parameter Value 1 Capacitance 7.1 µF 2 Inductance 700 nH 3 Charging voltage 9 kV 4 CT (1:1000) V/A 5 HV probe (1:1000) 6 Surrounding gas Nitrogen medium 7 Pressure 1 bar 8 Diameter of the wire 0.26 mm 9 Length of the wire 12 cm Table.4. Transition of the copper wire from solid through micro particle dispersion and finally to nanoparticles. Pressure Physical state of the copper Deposited Energy (mbar) wire (Joule) 4.2 ×10-2 Not broken 120 2.3×102 Micro particles 150 1×103 Exploded and nanoparticles 250 38