MELTING OF NANOPARTICLES

SUBRATA CHAKRABORTY RINI THOMAS SANDEEPAN MAITY

MELTING STUDY OF ZINC NANOPARTICLE USINC DSC AND TEM

The melting temperature of nanoparticles can be 10K-100K lower than the bulk materials due to higher value of surface by volume ratio. According to Gibb’s- Thomson relation:-

4

1 slm mb

f s

ddT T

H

But there has also supper heating phenomenon in case of embedded nanoparticle. Now we are going to discus melting of Zn- nanoparticles. Zinc is widely used in galvanizing steel products to prevent surface corrosion and in a variety of alloys. ZnO nanoparticles are used for ultraviolet radiation absorption, as electro-optic and piezoelectric materials. As a part of our discussion we discus the melting behavior in two conditions (i) when the zinc particles are in isolated states and (ii)when they are confined by a ZnO shell of increasing thickness formed by the surface oxidation. WE will discus this topic using DSC and TEM. EXPERIMENTAL METHODS:- Nanocrystaline zinc sample was 99.9+% purred and the average particle size is 35-80nm. The container was opened inside a glovebox in inert atmosphere. The zinc is kept in several sealed glass vials. Purity of bulk zinc materials was greater than 99.9%. Two calorimeters were used: One was a Perkin-Elmer Pyris Diamond differential scanning calorimeter and the second a Thermal Analysis Q-100 assembly. The purge gas used for the first instrument was ultra high purity Ar and for the second equipment was ultra high purity N2. The instruments were calibrated against the melting point and enthalpy of melting of indium. For measuring the specific heat, Cp, of zinc nanoparticles over the 623 to 703 K range, the calorimeter was calibrated with sapphire of known Cp. In the calorimeter two aluminium DSC pans are differed by weight ~0.5mg. Base line obtained by averaging several heating scans at 20K/min of two

aluminium empty pans was substracted from the measured flow curve. Thus, errors from the weight difference between two DSC pans were eliminated. The experiment was done varying scan rate and each time separate sample vials are used and it was ensured that the oxidation of the nanoparticles to ZnO was minimum or negligible. The Cp values measured by the two methods agreed within ~1%.

Figure 1. Specific heat of nanoparticles and bulk zinc is plotted against the temperature. The specific heat data for bulk zinc from Grønvold and Stølen are plotted for comparison. For microstructural observations and chemical analysis, two transmission electron microscopes were used, namely JEOL2010F TEM/STEM and a Philips CM12 TEM. In this procedure, the nanoparticles were dispersed in toluene to reduce their reactivity and flammability in case the air contained moisture. A drop of this very dilute dispersion was then placed on a holey (containing holes) carbon film supported by a Cu grid and left in open air to allow the solvent to evaporate. The process also led to formation and/or thickening of the ZnO shell on the particles. RESULTS:- (i) Calorimetric Studies:- The figure shows that variation of Cp of zinc nanoparticle and bulk materials with respect to temperature. where the Cp data for bulk zinc from Grønvold and Stølen15 are included for comparison. Our data for bulk zinc are about 2.3% higher than the average Cp reported by them. Values of Cp measured here have ~1% error, and their data show variation by at least ~0.5%. The difference may arise partly from the difference between the respective samples and partly from the experimental procedures used. Figure 1 shows that Cp for nanoparticles is about 13% higher than that of bulk zinc and increases much more sensitively with increase in T than for bulk zinc. It has to be ensured during the

course of the experiment ZnO shell on the periphery Zn nanoparticle should not grows up. More than three sets of differential scanning calorimetric (DSC) scans were obtained varying different scan rate during heating and cooling between 298 and 723 K. Since the plots (figure 2-4) were featureless over most of the lower temperature range, only features in the interesting range of 623 to 723 K are relevant. (ii) Electron Microscope Studies of Nanoparticles:- The size distributon of zinc nanoparticle showed in figure 5A. from TEM images acquired by using Philips CM12 120 kV. The agglomerated state of the sample showed sharp facets of the nanoparticles pointing outward. In the TEM experiment, under the focused electron beam, the nanoparticles from the agglomerates spread out onto the holey carbon film and showed their triangular, hexagonal, and so forth, faceted shapes.

Figure 2. (A) Plots of dH/dT of zinc nanoparticles and bulk Zn against the temperature during heating. The curves are numbered in the sequence in which they were obtained. (B) Plots of dH/dT of Zn nanodroplets and bulk Zn against the temperature during cooling. In this figure and Figures 2-5, Curve 1′ was obtained during cooling of the molten (nm size droplets) from 573 K after heating to 713 K in curve 1 in panel A, curve 2 ′ after the sample had been heated to 723 K in curve 2 in panel A, and so on. The samples were heated and cooled at 10 K/min.

Figure 3. (A) Plots of dH/dT of zinc nanoparticles and bulk Zn against the temperature during heating. The curves are numbered in the sequence in which they were obtained. (B) Plots of dH/dT of Zn nanodroplets and bulk Zn against the temperature during cooling. The samples were heated and cooled at 20 K/min.

Figure 4. (A) Plots of dH/dT of zinc nanoparticles and bulk Zn against the temperature during heating. The curves are numbered in the sequence in which they were obtained. (B) Plots of dH/dT of Zn nanodroplets and bulk Zn against the temperature during cooling. The samples were heated and cooled at 40 K/min.

Figure 5. (A) Size distribution of zinc nanoparticles. (B) The TEM bright field image of zinc nanoparticles at 298 K before heating to 20K above its Tm () 693.2 K) and (C) after heating to 20 K above its Tm. The irregular particle size was determined using ( a2 + b2 )0.5 where a and b are respectively the longest and shortest end to end distance measured for each particle. The particle size distribution is shown in figure 5A. DISCUSSION:- In Figures 2A-4A, Tm of nanoparticles for the first heating cycle is 690.9 K for 10 K/min, 691.4 K for 20 K/min, and 691.7 K for 40 K/min rate. These values gradually decrease respectively to 688.6 K, 689.8K, and 690.1 K in the fifth heating cycle after the samples had been cooled at the same rate as heated. In comparison, Tm of the bulk sample remains at 692.7 K for the three heating rates. The difference, ΔT, between the onset and end temperatures of the melting endotherm is 15 K for 10 K/min, 16.8 K for 20K/min, and 20.2 K for 40K/min heating, and it decreases respectively to 7.5, 14.3, and 17.8 K on the fifth heating cycle. According to Gibb’s- Thomson relation,

also written as,

Where melting temperature of bulk is 693.2K. The calculated value of melting temperature according to the above formula for 35-80nm size spherical particle (R=17.5 to 40 nm) is in the range of 598-650K using reported data of others parameter in above equation. Where as the experimental melting temperature is 687.2K, which is 37-89K

higher than the calculated value. This deviation due to the formation ZnO shell on Zn nanoparticle ( Zn nanoparticle embedded in ZnO matrix). As Zn nanocore expand more rapidly on heating compare to ZnO shell, thereby producing hydrostatic pressure on nanocore . That’s why melting point is much higher compare to calculation. The magnitude of the increase would depend upon the volume and the entropy change on melting, and a condition may be reached when Tm would cease to decrease on thermal cycling. We now consider the effect of thermal cycling on Tm. For the 10 K/min rate, it decreases from 690.9 to 689.6 K from first to the second cycle and finally to 688.6 K in the fifth cycle, for the 20 K/min rate from 691.4 to 689.7 K and thereafter remains constant, and for the 40 K/min rate from 691.7 to 690.3 K and finally to 690.1 K. The initial decrees of melting point due to reduction of nanocore size due to oxidation. With the passage of time oxidation process slows down due to slow diffusion oxygen through ZnO shell and after certain cycle this process has been stopped. As seen in Figures 2-4 for the first heating and cooling cycle at the 10 K/min rate, melting begins at 690.9 K and crystallization at 688.5 K. For the 20 K/min rate, it begins at 691.4 K and crystallization at 689.2 K, and for the 40 K/min rate, it begins at 691.7 K, and crystallization at 688.2 K. The 2.2 to 3.5 K difference between the melting and the crystallization temperatures shows that zinc nanodroplets supercool before crystallization begins. The changes of enthalpy is measured using DSC according to following formula

Where Tref is the 640K and 700K for heating and cooling respectively for each scan rate. For the bulk zinc, ΔHm is 7.16, 7.07, and 7.05 kJ/mol for 10, 20, and 40 K/min heating rates and ΔHcryst is 7.05, 7.06, and 7.00 kJ/mol. These ΔHm and ΔHcryst are within the experimental and analytical errors of ~1.2%, and they agree with the known ΔHm of 7.07 kJ/mol. For the nanoparticles, ΔHm is 5.46 ( 0.06 kJ/mol. In the fifth thermal cycle, ΔHm decreases to 2.87 for 10 K/min, to 4.49 for 20 K/min, and to 5.13 kJ/mol for 40 K/min rate. In contrast, ΔHcryst varies with the cooling rate, and it is 4.6, 4.4, and 5.0 ~0.06 kJ/mol, respectively, for 10, 20, and 40 K/min cooling rate. In the fifth thermal cycle it decreases to 2.2 kJ/mol for 10 K/min, to 3.8 kJ/mol for 20 K/min, and to 4.85 kJ/mol for 40 K/min rate. Thus, ΔHm and ΔHcryst decrease to about half of their values when thermally cycled at 10 K/min, but only by 2-6% on thermal cycling at 40 K/min. This indicates that the decrease is due to reduction of the zinc core’s mass as it is consumed to form ZnO, and the reduction was more for slower rate of thermal cycling than for the faster rate. Thus, the decrease in ΔHm and ΔHcryst would be an artifact of our use of the initial mass of nanoparticles as the mass of zinc for calculating these quantities. Without correcting for the amount of zinc in nanoparticles, it is not possible to ascertain how much of the decrease in ΔHm and ΔHcryst is due to the size effect.

Figure 6. (A) Enthalpy of Zn nanometer size particles and melt in units of J mol-1 K-1 plotted against the temperature. The sample was heated at 10 K/min, and the curves numbered are in the sequence in which they were obtained. Also plotted is the enthalpy of bulk Zn in J mol-1 K-1. (B) The enthalpy of Zn nm-size droplets during crystallization on cooling is plotted against the temperature. Due to higher value of Cp for nanoparticle compare to bulk according to figure1; CpdT for nanoparticle is higher than the bulk materials. Since dCp/dT seen in Figure 1 is also higher, this means that not only its enthalpy is higher but also the slope of the enthalpy in the T plane is higher. If the corresponding dCp/ dT for the melt did not greatly change, then the ΔHm of nanoparticles would be lower than that of the bulk. This is a general feature of melting of all nanoparticles. CONCLUSSION:- The melting point of Zn nanoparticle is lower than the bulk materials due higher number surface atom compare to the bulk. But the observed melting point of the nanoparticle during the experimental period is 37-89K higher than the calculated value (according to the Gibb’s-Thomson relation). This effect due to the oxidation of Zn nanoparticle during the experimental cycling. As Zn nanoparticle is embedded in ZnO matrix supper heating phenomenon played simultaneously with the effect of particle size. The oxidation process is very slow is confirmed observing scan rate effect in the artifact of enthalpy difference between crystallization and melting. The enthalpy of melting for nanoparticle is lesser than that of bulk materials. REFERENCE:- Lina Gunawan and G.P.Johari J. Phys. Chem. C 2008, 112, 20159–20166,

Size-dependent melting of nanoparticles-Thermodynamic model Introduction: Thermodynamic model first published in 1909, used to understand the size-dependent melting of nanoparticles. Pawlow deduced an expression for the size-dependent melting temperature of small particles based on the thermodynamic model which was then modified and applied to different nanostructures such as nanowires,prism-shaped nanoparticles, etc. The model has also been modified to understand the melting of supported nanoparticles and superheating of embedded nanoparticles. Though there are several theoretical models , thermodynamic model has lots of merits compared to other models such as 1.Different variations in the results even for the same material can be easily explained using different melting hypotheses. 2. The model can be modified to understand the size-dependent melting of nonspherical nanoparticles such as prism-shaped, decahedral, pyramidal etc. along with nanowires and thin films. 3. The model is consistent with molecular dynamic (MD) simulations. Thermodynamic model and different hypotheses According to the model, there are different melting processes .They are

1. Homogenous melting hypothesis 2. Liquid skin melting 3. Liquid nucleation and growth

Homogenous melting hypothesis:- In this process, the entire solid is in equilibrium with entire melted particles which corresponds to homogeneous melting hypothesis (HMH). There is no surface melting for such a case.

The melting temperature Tcm of nanoparticles can be expressed as

--1

γ ‘s are surface energies of solid–vapour and liquid–vapour interfaces of the material, ΔHf -bulk latent heat of fusion, D - diameter of nanoparticle, ρs and ρl - densities of solid and liquid respectively TCM - the bulk melting temperature. Liquid skin melting (LSM) :- LSM considers the formation of a liquid layer over the solid core at low temperature that remains unchanged till the particle transforms completely to liquid at the melting temperature as shown below.

This predicts a faster variation with respect to the inverse of the particle size The melting temperature shows non-linear dependence with the size.

--2 Liquid nucleation and growth (LNG):

• A liquid layer nucleates and grows with temperature. • Surface melting is the main mechanism here.

--3

Comparison of melting temperature according to HMM, LNG and LSM [See Ref 1]

It may be noted from Eqs that the melting temperature varies differently for different melting processes .Both HMM and LNG predict a linear variation of melting temperature with the inverse of size .

The coefficients of eqs (1)–(3) are related as γsl ~ (γsv−γlv). Based on equations (1)–(3), the melting temperature of nanoparticles can be represented by

where z = 2 for HM and LSM z = 3 for LNG. The δ value is positive only for LSM and zero for the other two cases since the liquid skin layer is absent . Not only different materials, but different , facets of the same material also exhibit different melting processes. Almost all theoretical models assume spherical geometry for nanoparticles. But, supported as well as free nanoparticles are not necessarily spherical.For cylindrical geometry, according to the thermodynamic model based on HMM and LNG, the melting temperature of a particle with diameter D and height H are respectively,

Eg:- nanowires ,nanorods, pancake-like nanoparticles For a prism-shaped nano-particle,

where D is the edge length and H is the height of solid prism-shaped nanoparticles Superheating of nanoparticles:- Super heating is the phenomenon of occurance of melting at a higher value than the bulk melting point. Superheating has been observed for Pb and In nanocrystals embedded in aluminium matrix, for Bi in Zn and Ge in SiO2 etc. . Recently, super-heating has been reported for nanoparticles embedded in some matrices while the same nanoparticles when embedded in some other matrices, show lower melting temperature. The enhancement or depression of the melting temperature of the embedded nanoparticles depends on the epitaxy between the nanoparticles and the embedding matrix, mainly attributed by the suppression of vibrational motion of the surface atoms by the interface epitaxy. The melting starts from the centre and proceeds towards the surface in such a case., while the melting starts from the surface and proceeds towards the centre for free nanoparticles. Thermodynamic model can be modified to explain the superheating of nanoparticles in embedded condition . It is written as

The interfacial energy difference can be expressed by the following equation

where θ is the angle of contact developed at the triple point of solid, liquid and matrix phases. if θ < 90◦ - MP of the embedded particles increase and θ > 90◦ - depression of melting point happens A definite epitaxial relationship between the matrix and the particle suppresses the vibrational motion. The epitaxial relationship suggests that γMS is smaller than γML However, superheating observed for Ge in SiO2 indicates that the epitaxial relation between the material and the matrix is not the essential criterion of superheating. Superheating has also been reported for free nanoparticles and is explained to be due to the different phases of the materials when reduced to nanodimension.

Size-dependent melting behavior of Zn nanowire Arrays Most studies on the melting properties of nanomaterials are focused on the nanoparticles, and only a few relate to the nanowires. As one of the most important one-dimensional (1D) nanostructures, metal nanowires have attracted a great deal of research interest in recent years, because of their importance in fundamental low-dimensional physics research as well as in technological application. Discussing the Applied physics letter of Xue Wei Wang et al(see Ref 3), where the melting behavior of Zn nanowires embedded in the holes of porous anodic alumina membrane with different diameters was studied by using the differential scanning calorimetry. The melting temperature of Zn nanowire arrays shows the strong dependence on nanowire sizes. In the respective letter, they employed the direct current electrodeposition method to prepare the Zn nanowire arrays in the holes of the porous anodic alumina membrane (PAAM) with the diameter from 22 to 225 nm, respectively. In the electrodeposition process, Zn nucleates at the bottom of the pores and grows along the pores to the top. X-ray diffraction and transmission electron microscopy (TEM) were used to study the crystalline structure and morphology of nanowires. For X-ray diffraction (XRD) measurements, the overfilled nanowires on the surface of the PAAM template were mechanically polished away. It is found that they posses HCP structure with preferred orientation along the [001] direction . For TEM observation, the PAAM was completely dissolved with alkaline solution. Figure1 shows a typical TEM image of Zn nanowires. It is clear that the nanowires have a high-aspect ratio and the diameter is uniform.

Fig1 shows the TEM image of Zn Nanowire with diameter 45nm[See Ref 3]

For the study of melting behavior, Differential Scanning Calorimetry (DSC) experiments were carried out and the heat flow was recorded at the scanning rate of 10 °C/min. The size-dependent endothermic peak of the nanowires is observed(fig 2). It is clear that the onset point of the endothermic peak shifts to low temperature with the decrease of the diameter.

Fig 2. DSC trace of Zn nanowire arrays with diameters of 25 nm (curve a), 45 nm (curve b), 65 nm (curve c), 90 nm (curve d) 145 nm (curve e), and 225 nm (curve f). [See Ref 3] To analyse the size dependence of the melting temperature for Zn nanowires, melting temperature Tm versus the reciprocal diameter (1/D) of Zn nanowires is plotted and found that the Tm(D) obviously deviated from the linear relation with the diameter though they were a roughly linear relation when the diameter was lower than 65 nm.

Fig 3 shows melting Temp Tm of Zn nanowire arrays as a fn of the reciprocal of diameters [See Ref 3]

It has been shown that the variation of the melting temperature of Zn nanowires is nonlinear and the bulk melting temperature of nanowires or clusters cannot be extrapolated from the data in the intermediate size range and the extrapolated bulk melting temperature for them is remarkably lower than the experimental value for bulk. According to the thermodynamic model, the melting temperature of a nanowire is given by

---* It can be noted from above eq that Tm(D) should show a linear dependence on 1/D in case of spherical nanoparticles. According to the report of Lai et al. the heat of fusion ∆Hf depends on the diameter of the nanowire D by

--** where ∆H0 is the heat of fusion for bulk materials and t0 is the given critical thickness of liquid layer covering the solid core at the melting temperature Tm. The exponent n is 3 for spherical nanoparticles and 2 for nanowires. It can be seen from the Eq. ** that with the increase of diameter D, the value of t0 /D obviously decreases, which results in the increase of the heat of fusion ∆Hf. Therefore, according to this relation , plot of Tm(D) vs 1/D should be curvilinear, and this is in agreement with the result of Fig3. Moreover, if we linearly extrapolate the melting temperature from the small diameter to the bulk Zn, it is about 414.3 °C as line b shown in fig. ie lower than the experimental value of 419.58 °C. However, if we extrapolate the curve from the large diameter to the bulk Zn, we will find that the melting point is near the experiment value for the bulk as curve ,which means that the ∆Hf change with the diameters should be considered. Reference:

• PRAMANA IAS Vol. 72, No. 4 pp. 617–628 ,April 2009 -K K NANDA , Size-dependent melting of nanoparticles:Hundred years of thermodynamic model .

• M Attarian Shandiz J. Phys.: Condens. Matter 20 (2008) 325237 (9pp) Effective coordination number model for the size dependency of physical properties of nanocrystals .

• Xue Wei Wang, Guang Tao Fei,a Kang Zheng, Zhen Jin, and Li De Zhang, APPLIED PHYSICS LETTERS 88, 173114 2006, Size-dependent melting behavior of Zn nanowire arrays.

• G. K. Goswami and K. K. Nanda APPLIED PHYSICS LETTERS 91, 196101 2007 Comment on ”Size-dependent melting behavior of Zn nanowire arrays” .

• Xue Wei Wang, Guang Tao Fei,a Biao Wang, Min Wang, Kang Zheng,Zhen Jin, and Li De Zhang, APPLIED PHYSICS LETTERS 91, 196102 (2007), Response to “Comment on ‘Size-dependent melting behavior of Zn nanowire arrays”.

• A Safaei1 , M Attarian Shandiz1 , S Sanjabi1,3 and Z H Barber J. Phys.: Condens. Matter 19 (2007) 216216 (9pp) Modelling the size effect on the melting temperature of nanoparticles, nanowires and nanofilms .

• K. K. Nanda, S. N. Sahu, and S. N. Behera ,PHYSICAL REVIEW A 66, 013208 2002 , Liquid-drop model for the size-dependent melting of low-dimensional systems.

• X. W. Wang, G. T. Fei, K. Zheng, Z. Jin, and L. D. Zhang, Appl. Phys.Lett.88, 173114 2006. S. L. Lai, J. Y. Guo, V. Petrova, G. Ramanatch, and L. H. Allen, Phys.Rev. Lett.77, 99 1996

Hysteresis in the melting kinetics of Bi nanoparticles Introduction: In this study Bi nanocrystals embedded in amorphous Ge on Si substrate were prepared by pulsed laser deposition. The kinetics of the melting was studied using Raman Spectroscopy as this spectrum of solid is stongly dependent on temperature, and XRD also. Raman scattering measurement: Raman spectroscopy measurement were performed in vacuum using Jobin-Yvon U1000 double monochromator. The 514.5-nm line of an Argon laser (Spectra Physics)was used as the excitation source at a power level of 50 mW.The diameter of the laser spot at the surface of the sample was of the order of 100 mm. Due to low raman signal each spectrum was the result of the addition of several scan.

Fig1: Raman spectra of Bi nanocrystals embedded in Ge as a fuction of temperature In the Fig1intensity change of stokes Raman spectra with the variation temperature has been presented. The recorded temperatures are 440, 520 and 600K. At the low temperature observed modes are Eg ( 70 cm-1) and A1g ( 97 cm-1) of the Bi. As expected with the increase of the temperature the peaks are shifted to the lower wavenumbers because of anharmonic effect and as the melting point reached the peaks

are replaced by broad bands characterizing the liquid state. When the temperature decreases in the inverse process occurs.

Fig2: Raman frequencies of the A1g and Eg peaks as a fuction of temperatures.

In the fig2 the frequencies of the peaks are plotted as function of the temperature during heating and cooling cycle As temperature increases the frequencies of the two peaks Eg and A1g are converged in the one band (presented by gray circles). From the fig2 it can be inferred that it an hysteresis behavior of melting and solidification cycle. From this result one can assume that the difference between melting and solidification temperature is atleast of the order of 50K. XRD measurement: XRD- patterns were obtained using Siemens D500 diffractometer coupled to a Cu anode (K-alpha=1.5405Å). The solid liquid phase transition was monitored by XRD. Due to the small intensity of the signal long accumulation and short angular range were needed. In range the selected two peaks characterizing the Bi nanocrystals are 22.80( 003) and 27.70 (012). In the fig3 the XRD patterns are presented for increasing temperatures from ambient temperature to 620K and then for decreasing temperature down to ambient. As the temperature increases the structure changes at 520K, the peak at 22.80 (003) markedly sharpens and increased in intensity. At 590K practically all the features of the XRD patterns disappeared indicating Bi has almost melted. Nevertheless, some very low intensity bands at the above mentioned peak positions are still observable. But 620K the XRD pattern has been completely disappeared. Upon cooling at 520K the peak

Fig.3. X-Ray diffraction patterns as a function of temperature. reappeared indicating the crystallization. Finally at room temperature the staring XRD pattern has been recovered,though less intense. Conclusion: The solid liquid transition of Bi nanocrystals embedded in the amorphous Ge have been investigated by Raman spectroscopy and XRD. The observed melting behavior is similar in both technique. A melting –solidification temperature hysteresis of the order of the 50K observed. References: 1.E.H –Poniatowski, V.H. Lara, Thin Solid Films 453-454 ( 2004) 467-470, Hysteresis in the melting kinetics of Bi nanoparticle

Preparation and field emission properties of carbon nanotubes cold cathode using melting Ag

Nano-particles as binder Introduction: Carbon nanotubes are seems to be most efficient electron emitter due to high aspect ratio, high mechanical strength and chemical stability. Recently people have prepared the carbon nanotubes paste with organic or inorganic binder, but the low conductivity of the binder can lead to problems like out-gassing and arcing during the field emission. The present paper describe the preparation and field emission of the CNTs paste with Ag nanoparticle as binder. Being silver as very good electrical conductor it can be a suitable binder to make the CNTs paste. Though the melting point of the bulk Ag is 960.50C, but the Ag nanoparticles(below 100 nm) can be melted below 2000C. So it is possible to attach the CNTs with the Ag nanoparticles at a lower sintering temperature. Preparation: The preparation involved the multiwalled carbon nanotubes and Ag nanoparticles was ultrasonically dispersed in 1:2 mass ratio in the ethanol solution for 2 hr. The suspension was filtered and the wet powder was mixed with terpineol and other low boiling organic materials. This paste was then dispersed in the Si surface which was pre cleaned ultrasonically in acetone and ethanol and then annealed for 30 minutes to remove the organic materials and melt Ag nanoparticles and the CNTs fixed with Ag nanoparticles Characterization: Characterization of the prepared CNTs paste was done the high resolution transmission electron microscopy and Scanning electron microscopy. Fig1 is the HRTEM images of the melted Ag nanoparticles which form a thick film upon melting at 1500C for 30 min. The image in the inset is the HRTEM image of Ag nanoparticles of the size 10 nm. It is cleared from the HRTEM image that melting of Ag nanoparticles (10 nm) happens at 1500C.

Fig1: HRTEM image of the Ag nanoparticle after sintering at 1500C for 30 min.The inset image in the HRTEM image of the Ag nanoparticles of 10 nm size. Fig2 is the scanning electron microscopy (SEM) image of the CNTs cold cathode. The carbon nanotubes are embedded on the matrix of the Ag nanoparticles.

Fig2: SEM image of the CNTs cold cathode as prepared

Fig3 is the HRTEM image of the CNTs cold cathode with the Ag nanoparticles as the binder. This HRTEM image clearly indicates that CNTs are fixed in the Ag nanoparticles matrix.

Fig3: HRTEM image of the CNTs cold cathode indicating the fixing of CNTs with Ag nanoparticles. Field Emission Properties: The measurement of the field emission from the CNTs paste cold cathode is shown in the schematic diagram given below. Steel anode was used to determine the current. The CNTs cathode area was 2 nm x 2 nm and the distance from anode to cathode was 100 µm. The pressure was 2 x 10-4 Pa for the measurement.

Fig4: Schematic representation of the experimental setup for the measurement of field emission. From the experiment the turn on field (at which the current density was 10 A/cm2) and the threshold field ( at which the current density was 10mA/cm2) were 2.1 and 3.9 V/µm. The field emission was obtained as 41 mA/cm2 by applying the field 4.7 V/µm. Table1: Results obtained from experiment Applied Field(V/µm) Current Density 2.1 10µA/cm2 3.9 10mA/cm2 4.7 41mA/cm2 Fig5 describes the current voltage behavior of the results obtained from the experiments. It is possible to explain the current voltage behavior from the Fowler- Nordheim equation, which is given below

)(2/331083.6

2)(61054.1)(

E

xEAxEI Exp

Where I(E) = Current, E= voltage, A= area of electron emission,Φ= work function, β= constant. So according to this equation the current density vs voltage should show an exponential behavior and lnI(E) vs E should show linear behavior and this is the characteristics of any field emiitter.

Fig5: Field Emission properties of the CNTs cold cathode. The inset shows the Fowler-Nordheim plot of the current emission. CONCLUTION: CNTs cold cathode using Ag nanoparticles as binder can be prepared at a lower temperature due the nano dimension of the Ag. The prepared cold cathode is turns out to be more efficient compared the CNTs cold cathode with organic or inorganic material as binder. References: 1. Y. Qin, Q. Zou ; Applied Surface Science 253 ( 2007) 4021-4024, Preparation and field emission properties carbon nanotube cold cathode using melting Ag nanoparticle as binder.

Structural stability of Icosahedral FePt nanoparticles Introduction: This paper describes the structural stability of the FePt nanoparticles in the presences of the various electron beam. With the electron beam flux 200 A/ cm2 the nanoparticles shows a typical melting and recryatallisation. Melting behavior of nanoparticles: To understand the melting behavior of the FePt nanoparticles in the presence of the electron beam HRTEM images has been taken by a video recorder and a CCD camera. In the Fig1 8 pictures are there at various time intervals. Description of the figures is given below. (a) Starting of the melting of the icosahedral nanoparticles after 30 min. (b) Melted nanoparticles (c) Recrystallisation to a truncated icosahedral structure. (d) Melting of the truncated structure after 30 min. (e) Melted nanoparticles.

(f) Recrystallisation to a unstable twin structure. (g) Again melting within 1 min. (h) Recrystallisation to a single crystal.

CONCLUTION: The structural stability was investigated using various beams of the electron flux and in the presence of the electron beam 200 A/cm2 , the nanoparticles shows a typical melting behavior. References: 1. R. Wang, H. Zhang, Nanoscale, 2009, 1,276-279, Structural stability of icosahedral FePt nanoparticle.

CONCLUTION: As a whole we can conclude that melting of bulk material is not size dependent property where as particle with very small dimension like nanoparticle the melting point is a function of the size. Substance in the nano dimension melts at low temperature compared to bulk due large surface to volume ratio resulting in less cohesive energy. The melting phenomenon of the nanoparticle can be monitored using HRTEM, DSC, XRD and Raman Spectroscopy.