THERMODYNAMIC CONTRIBUTION IN THE STUDY OF A FREE COPPER AND GADOLINIUM ATOMIC MIXTURE BEAM PRODUCED BY ELECTRONIC BOMBARDMENT

The evaporation of metals in vacuum by electron beam heating is suitab le for large scale applications such as production of thin films for o ptics or electronics, physical vapour deposition (PVD) or titanium ref ining. Although, this technique is commonly used, the physics of an at omic beam produced by electron beam heating has been little studied. I n this paper, we have undertaken the study of the expansion of a binar y metal vapour of copper-gadolinium. The use of liquid alloys needs th e knowledge of accurate thermodynamic properties. Moreover, to study a n alloy evaporation behaviour, that is to say to know the vapour compo sition for a given source temperature, the activity values are essenti al. Since the gadolinium vapour pressure is much less than that of cop per (at T = 1443 K, P-(Cu)degrees = 0.302 Pa and P-(Gd)degrees = 0.001 Pa), we have only measured the activity of Cu for a gadolinium conten t X-Gd ranging from 0.276 to 0.844 (atomic composition), and for a tem perature range from 1282 to 1644 K using a multiple effusive cell coup led to a quadrupole mass spectrometer. For each composition, the exper imental activities referred to the pure liquid Cu are fitted and expre ssed as In(a(Cu)) = (A +/- delta A) + (B +/- delta B)/T. From these ex perimental results, the gadolinium activity in each alloy can then be calculated through an integration of the Gibbs-Duhem relation :Sigma x (i) dln a(i) = 0. To compute the gadolinium activity we use a Margules ' polynomial development for the Cu partial molar excess Gibbs energy : Delta G(Cu)(ex) = RTln(a(Cu)/x(Cu)) = A(T)(1 - x(Cu))(2) + B(T)(1 - x(Cu))(3). An integration of the Gibbs-Duhem equation yields :Delta G( Gd)(ex) = RTln(a(Gd)/x(Gd)) = (A(T) + (3/2)B(T))(1 - x(Gd))(2)-B(T)(1- x(Gd))(3). The coefficients A and B are linear versus temperature. Fro m these measurements, we have determined the liquidus curve in Cu-rich region, the eutectic composition at T = 1133 K and the free enthalpie s of formation of CuGd between 948 and 1103 K and of Cu2Gd between 104 3 and 1133 K. If can be noticed that they fairly agree as well with pr evious calculations (activities and Gibbs energies of formation of int ermetallic compounds), than with experimental results (Delta H-m) at l ower temperature which allow us to extrapolate the activities at high temperatures for the electronic bombardment study. The atomic vapour i s produced by an 11 kV electron beam with 15 kW maximum power. With ou r experimental set up, we can measure the atomic beam characteristics such as the net mass flow rate by the weight loss of the ingot during the evaporation, angular flux distribution of the atoms in binary beam vapour by deposition pastilles and chemical analysis and radial veloc ity for each compound present in the vapour in vertical axis by time-o f-flight method. These experimental data are compared with those compu ted with a test-particle Monte-Carlo (PTMC) code including an inelasti c collision algorithm that accounts for electronic-translational energ y transfers. It has been shown that the translational velocities of so me atoms evaporated by electron beam impact are larger than those pred icted by an adiabatic expansion. It is now well known that these monoa tomic atoms have several low energy metastable levels. At the evaporat ion surface temperature, these thermally populated levels are an inter nal electronic energy reservoir. When inelastic atomic collisions occu r, part of this internal energy is converted into translational energy and increases the atomic velocity. In our evaporation surface tempera ture, only the Gd metastable levels are populated. The first metastabl e state of copper is located at 11202.565 cm(-1) and is not populated at the temperature reached at the evaporating surface, conversely Gd a toms have many low lying metastable levels (215 cm(-1), 533 cm(-1)) wh ich are populated. We studied experimentally the vapour characteristic s for several ingot compositions (29 at %, 50 at %, 70 at %, 80 at %) in a 1100 W-3300 W gun power range. From the measurements of the atomi c beam vapour characteristics, we put in evidence some essential pheno mena related to expansion of metal alloy produced by electron beam bom bardment The first one is the narrowing of the angular flux distributi on of Gd with the Cu present and conversely a broadening of the Cu dis tribution with increasing Gd concentration. The second aerodynamic phe nomenon is the increase in the velocity of the heavier element (Gd) at oms on addition of the lighter(Cu), a phenomenon which increases with the concentration of the latter The simulations are in good agreement with the experimental values.