Experimental study for the properties of Fe 3 O 4 dusty plasma using the air in vacuum chamber system

In this work, we carried out an experimental study of thedustyplasma by taking the dust material Fe3O4 with radius of the any grain0.1μm - 0.5μm. In experiment we use air in the vacuum chambersystem under different low pressure (0.1-1) Torr. The resultsillustrated that the present of dust particles in the air plasma did noteffect on Paschen minimum which is 0.5 without dust and with Fe3O4dusty grains.The effect of Fe3O4 dust particles on plasma parameters can benotice in direct current system in glow discharge region. The plasmaparameters which were studied in this work represent plasmapotential, floating potential,electron saturation current, temperatureof the electron, the density of the (electron, ion), and Debye length.The measurements of parameters are taken by four cylindrical probeswhich diagnosed at distance 4cm from the diameter of the cathode inPaschen minimum at pressure 0.5 Torr. The diameter profiles ofplasma parameters in the present of dust are non- uniform. Plasmapotential and the floating potential of probe becomes more negativelywhen the dust immersed into plasma region. The electron densityincreases in the present of dust particle which lead to decrease theelectron temperature and Debye length. The behavior of thoseparameters shows the discharge current and discharge voltageincreases without dust while the discharge current and voltagedecreases when Fe3O4 dust particles embedded.


Introduction
Typical plasmas consisting of electrons and ions or similar particles with large-mass difference essentially cause temporal as well as spatial variations of collective plasma phenomena [1]. However, the space-time parity can be maintained in pair-ion plasmas with equal mass or slightly different masses [2]. Plasmas not only found in naturally occurring plasmas, but are also used for technological applications. In many industries, e.g., integrated-circuit fabrication, since the deposited is strongly damaged by a highenergy electron, a plasma source having no energetic electrons is required. For this purpose, a radio frequency plasma source has also been developed [3,4]. Dust plasma is characterized by charged grains immersed in plasma of electrons and ions. The grains could acquire charges of the order of 10 3 to 10 4 elementary charges and often become strongly coupled, which gives rise to novel phenomena such as the crystallization, phase transitions, and waves [5]. A variety of linear and nonlinear collective are known to occur in dust-contaminated plasma dusty plasma [6], and relative theoretical research has received new impulse, since roughly a decade ago, thanks to laboratory and space dusty plasma observations. An issue of particular importance in dusty plasma research is the formation of strongly coupled dusty plasma crystals by highly charged dust grains, typically in the sheath region above a horizontal negatively biased electrode in experiments [7]. A dusty plasma experiment with non-magnetic grains are presented for the dependence of the number density and lattice parameters of the dust layer on the ratio of the magnetic dipole-dipole force to electrostatic force between the grains and the orientation of the grain magnetic moment with respect to the layer [8]. Thermodynamic properties of a Yukawa system consisting of dust particles in plasmaare studiedin presence of anexternal magnetic field. It is assumed that dust particles interact with each other by modified potential in presence of magnetic field. Accordingly, a modified expression for internal energy has been obtained [9]. Hypocycloid and epicycloids motions of irregular grain (pine pollen) are observed for the first time in unmagnetized dust plasma. Hypocycloid motions occur both inside and outside the glass ring which confines the grain [5].
Magnetite, Fe 3 O 4 , with an inverse cubic spin structure (a = 0,8396 nm) exhibits electrical conductivity (ρ = 10 mΩ.cm at T = 300 K) due to a hopping of spin polarized electrons between ferrimagnetically ordered Fe 2 + and Fe 3 + ion states [10]. The nanoparticle powder of magnetite has already been synthesized for use as ferrofluid. Also due to their stability and nontoxic property Fe 3 O 4 nanoparticles have been investigated for various medical applications. As an example, if magnetite nanoparticle is coated with proper organic compounds, the resulting material can be dispersed in water and is biocompatible. Due to the interaction of these magnetic nanoparticles with external field they may be guided by appropriate magnetic field to a specific point in body and used as contrast agent in magnetic resonance imaging or for targeted drug delivery. Such applications require the nanoparticles to be super paramagnetic. Moreover, the physical properties and, in particular, magnetic properties depend on the size of particles and their distribution [2].

Theory
The single Langmuir probe is a small conductor that can be introduced into the plasma to collect ion or electron current that flow into it in response to different voltage. The I-V graph is divided into three regions ion saturation, transition, and electron saturation region [11] ,as shown in Fig. 1.

Fig.1: I-Vcharacteristics of singleLangmuir probe[12].
It is assumed that the probe current does not disturb the plasma. The probe diameter is less than the electron mean free path; the electrons are in thermal equilibrium among themselves at a temperature T e with a Maxwellian kinetic energy distribution.
Region I: when V p (probe potential) is made negative bias, electrons are repelled and only ions collected .The ion current passes through an area A in the plasma were determined from the ion currents in the ion saturation regions using the orbital motion limit (OML) probe theory. The advantage of using OML theory is that the ion density can be determined without the knowledge of the electron temperature. Here, it is assumed that the plasma is isotropic, the electron temperature is much higher than the ion temperature (T e >>T i ). The probe sheath is thick and non-collision assuming a Maxwellian energy distribution is in the unperturbed plasma .The following formula for a cylindrical probe is used to determine the ion current in the OML regime [11].
where I i is the probe current (ion current), A p is the surface area of the probe, M i is the ionic mass (atomic mass unit, 1 amu =1.67x10 -27 kg for proton), n i is the ion density, V p is probe potential (plasma potential).
In the short, typical I-Characteristics obtained with this arrangement as shown in Fig. 1 The increasing of probe negative bias leads to repel all electrons and the current becomes pure ion current.
Region II : when the probe potential V p is made less negative , probe collects both ions and electrons (the high thermal energy ) as the potential ( probe bias) is changed further in the positive direction, the ion and electron currents collected just cancel . This probe-plasma potential, V ƒ is the floating potential. For thermalized plasma, this voltage is approximately 1/2 KT (expressed in eV). Increasing probe potential beyond V ƒ results in a steep rise in electron current.
This current varied exponentially with probe bias voltage .this current eventually saturates at the plasma space potential value (V p ) due to space charge limitation in current collection. In region, II the electron current is given [13].
wherev th is the thermal velocity. The electron temperature can therefore be calculated directly from the I-V characteristics of the probe. The slope yields the electron temperature [13]: Where T e in (K). e T Slope 1 

(5)
Where T e in (eV) Methods of calculating the electron density described as following: in region III for positively biased of probe, the probe collects all the electrons and repels all the ions the electrons current collected is nearly constant. From this current, which is called the electronsaturation current I s , the electron density can be calculated from the following relationship [14]: whereT e is the electron temperature and the n e is the electron number density. The floating potential can be calculated from the following equation, as the bias voltage at which I i + I e =0, [15]  

Experiment set up
The vacuum chamber of this device is made of Pyrex cylindrical tube. This tube has two open ends closed by two stainless steel flange.One of it was connected to pumping systems, while the other was used to immerse the nitrogen gas, inside the chamber which consists of two disc aluminum electrodes (one of them used as cathode while the other used as the anode) 8 cm in diameter and 2 cm thickness. The inter distance between them is 8cm. Fig. 2 illustrates the experimental device with a discharge circuit.
Nitrogen plasmas are produced in the plasma chamber at plasma range from 0.1 torr to 1 torr. Iron (II and III) oxide is the chemical compound (Fe 3 O 4 ) dust particles ~185µm in size are dropped from the top of chamber into plasma by a dust dropper (i.e. duster). This dust dropper consists of dust container having mesh and 3vdc motor. The electric motor works by remote control system. This motor gives two motions to the dust container which are rotation and vibration motions. The weight of the dust particles that immersed into the plasma is equal to 0.5 g. Fig. 3 shows the schematic diagram of vacuum chamber with electric circuit.
The glow discharge is formed between electrodes when a d.c. constant voltage of about 3 kV is applied between them. Therefore, the electrical breakdown is formed in nitrogen gas at relative pressures, of about ≈ 0.1-1torr.

Results and discussion
The d.c. glow discharge is produced when a d.c. constant potential of about 3kV is applied between two electrodes and the distance of 8 cm was from the cathode and anode. The plasma discharge is formed, the space charge is established and then the electrode potential will drop. Figs.4 and 5 show the current and voltage of discharge respectively. It is clear from these figures, when the Fe 3 O 4 dust is embedded, the discharge current decreases while the voltage of discharge was increasing. This behavior can be explained as; the increasing of losses of electrons causes by classical diffusion and collected of electrons on the dust surface causes to decrease the discharge current and increase of voltage discharge [17]. The plasma characteristic (such as V p , V f , I es , n i , n e , T e , De  ) which werw found by using cylindrical Langmuir probes at pressure 0.5 torr at distance 4 cm from the cathode and anode (in half distance) were evaluated and investigated on the plasma region. Fig. 6 shows the photographs for the four probes A, B, C, D at distance ( 1, 3, 5, 7) cm respectively from the edge of the diameter of the electrode. Fig.6: Photograph of the probeA, B, C and D at distance (1, 3, 5

and 7) cm with dust (Fe 3 O 4 ).
The plasma potential with diameter position at different distance of the probe is illustrated in Fig. 7. The presence of Fe 3 O 4 dust inside cathode sheath causes the plasma potential goes toward negative values. This behavior can be explained as, when the Fe 3 O 4 dust embedded inside the plasma sheath, the electrons come first (because of their high mobility) after that the ions come later. Because of the polarity of chargeddust depends on the emission of electrons, so that, the density of the ions decreases more than electrons density. Therefore, the potential of plasma becomes negative. Moreover; Fig.8 illustrates the experimental data of electrons saturation current(I es ).
As mention above, the presence of Fe 3 O 4 dust in cathode sheath (cathode fall) collected the electrons according to that behavior, the saturation current of electron reduced. Fig. 9 shows the floating potential (V f ) with different distances of the probe.  According to the Eq. (4) the electron temperature was measured from the slope of ln I e -V curve of the probe characteristics. Fig.10 indicates the influence of Fe 3 O 4 dust on radial profile of T e . The figure shows that the pressure of dust particle reduces the electron temperature. This reduction may be caused by; the electrostatic potential of dust grain surface causes to repel the electrons from it. Thus, the electron energy, furthermore, according to the experimental data of I es from Fig. 8 and experimental data of T e from Fig. 10, with Eq. (7) the radial distribution of n e with and without dust particles are plotted in Fig. 11. It can explicated from this figure, the losses of electrons caused by classical diffusion and the collision of electrons with other plasma particle (ions and neutral atoms) and the electrostatic potential of dust cause to reduce of electron density. According to Eq. (2), the diameter profile of the ion density along the half distance 4 cm is shown in Fig.12. It is clear from this figure that, the ion density distribution is non-uniform with and without Fe 3 O 4 dust along the plasma region in both distances. In addition, in the cathode sheath, the present of dust particles has influence on the ion density greater than in the plasma bulk [17]. Fig.13 shows the Debye length De  as a function of the diameter position at different distances of the probe. It is noticed from the figure that shape note that, a small decrease change in the Debye length in the case of Fe 3 O 4 dust compared with the absence of dust.

Conclusions
According to the above discussion, the following points can be concluded:  The present of dust particle in the air plasma did not affect on Paschen minimum which is 0.5 torr without dust and with Fe 3 O 4 dusty grains.  The influence of dust particles on the plasma characteristics profile along the plasma region at pressure 0.5 torr was investigated. The current and voltage of discharges that present the experimentally observed decrease of discharge current and discharge voltage when the Fe 3 O 4 dust was embedded.  Plasma potential and the floating potential of probe become more negatively when the dust is immersed into plasma region. The electron density increases in the present of dust particle which leads to decrease the electron temperature and Debye length.