Vasile Tiron

On the HiPIMS benefits of multi-pulse operating mode

High power impulse magnetron sputtering (HiPIMS) technology is particularly interesting for its high ionization efficiency of the sputtered target elements. However, one of its major drawbacks is the typically lower deposition rate, compared to direct current magnetron sputtering (DCMS), mainly due to the metal ion back-attraction. Using HiPIMS with very short pulses (less than 5 µs), it is possible to partially overcome the deposition rate limitation. In this contribution our focus is the optimization of the HiPIMS process with respect to DCMS, based on the physical understanding of the plasma state when using sequences of pulses, instead of general single high power pulse. Sequences of consecutive very short high-power pulses significantly increase the deposition rate compared to standard HiPIMS (s-HiPIMS), as proven by quartz crystal microbalance measurements and by scanning electron microscopy cross section images of the deposited films. Tailoring the sequences of multi-pulses (m-HiPIMS), experimental results undoubtedly show that tungsten deposition rate is at least 50% higher than in the s-HiPIMS, for the same average power. This finding is explained via tuneable diode laser absorption spectroscopy measurements of the sputtered W atoms correlated with electrostatic probe ion detection and time evolution of the plasma potential measured at the substrate position. Moreover, poly-crystalline thin film structure changes to a mainly preferential orientation (2 0 0) and film roughness is drastically reduced as provided by x-ray diffraction and atomic force microscopy, respectively.

Dielectric elastomers with dual piezo-electrostatic response optimized through chemical design for electromechanical transducers

A new class of elastomers that simultaneously shows sensing, actuation and energy conversion functionalities is synthesized to meet the current requirements for electroactive materials. These new materials consist of a silicone network (polydimethylsiloxane-α,ω-diol crosslinked through chain ends) semi-interpenetrated with different percentages (2, 5, and 10 wt%) of polyimide derivatives stepwise modified by different strategies to improve the compatibility with the silicone core network. By addressing the right chemical pathway, the resulting semi-interpenetrated structures (S-IPNs) show noticeable dielectric permittivity, eps′ (up to 11), and breakdown strength, Ebd (up to 88 μm V−1), improvements as compared with the starting polymers (silicone with eps′ = 2.9 and Ebd = 38 μm V−1 and our best polyimide with eps′ = 6.2 and Ebd = 23 μm V−1). The S-IPNs with 10 wt% polyimide are able to gain energy up to 132 mJ cm−3 at 100% strain and up to 164 mJ cm−3 at maximum strain to develop large actuation strain (up to 8.7%) and show very good piezo-response (up to 44 pm V−1), making them highly suitable for cutting-edge electromechanical applications. For a better evaluation, S-IPNs are compared with one of the commercially available dielectric elastomers, most often used for this purpose.

Structural and Magnetic Properties of FeCuNbSiB Thin Films Deposited by HiPIMS

Using high-power impulse magnetron sputtering (HiPIMS) technique, Fe73.5Cu1Nb3Si15.5B7 thin films with thickness between 40 and 700 nm were deposited. The influence of deposition conditions (deposition time and argon pressure) and of the annealing temperature on the structure and coercive magnetic field of the magnetic films was analyzed. In the as-deposited state, the coercive magnetic field presents a minimum at about 10 mtorr argon pressure. The X-ray diffraction studies show that in the as-deposited state, the samples are amorphous, while after annealing at temperatures between 400°C to 525°C, α-Fe(Si) grains start to nucleate, the grain size varying from 2 to 18 nm. The Curie temperature of as-deposited amorphous phase and the onset of the primary crystallization are 355°C and 460°C, respectively. The lowest coercive magnetic field was obtained after annealing at 475°C.

Fast Imaging Investigation on Pulsed Magnetron Discharge

A time-resolved fast imaging technique was applied for the investigation of both gas (Ar) and sputtered material (Al) excited species in a high-power short-pulse magnetron discharge.

All-silicone elastic composites with counter-intuitive piezoelectric response, designed for electromechanical applications

New all-silicone composite materials were prepared using polar silicone particles dispersed within a high molecular weight polydimethylsiloxane (PDMS) matrix. Polar silicones with either CN or Cl groups attached to the polysiloxane chain in similar proportions were prepared by appropriate post-polymerization reactions and submicron particles were obtained and stabilized with a hydrophobic commercial surfactant, Pluronic L81, while the polar polymers were cross-linked within the particles in most cases. The mechanical and dielectric properties of the all-silicone composites were measured and discussed as a function of the nature and number of polar groups in the filler particles and in correlation with morphological aspects. Soft elastomeric materials with a Young’s modulus of 0.12–0.5 MPa, dielectric permittivity up to 4.7 at 104 Hz and dielectric strength up to 63 V μm−1 were obtained, and promising electromechanical performance resulted from theoretical calculations. Dielectric relaxation spectrometry at varying temperatures revealed dynamic relaxations in agreement with DSC data, and high dielectric relaxation strength values (Δe) were calculated. We also report for the first time piezoelectric behaviour of all-polymer composites containing amorphous components, measured using Piezoelectric Force Microscopy (PFM) at ambient temperature (above Tg) without poling. Average longitudinal piezoelectric coefficients (d33) of 24 and 13 pm V−1 were found for composites with cyano- and chloro-silicones, respectively. The variation of d33 with stretching was also followed and explained in correlation with morphological aspects. The promising properties of the all-silicone composites create the premises for possible applications as stretchable electronics, including tactile sensors, acoustic transducers and wearable devices.

Ion Energy Distribution in Thermionic Vacuum Arc Plasma

The purpose of this paper is to investigate the ion energy distribution in thermionic-vacuum-arc plasma. Using emissive probe technique, the spatial plasma potential distribution has been measured. In some conditions, the anode plasma potential is constant and close to the anode potential. Also, a sharp potential drop characterizes the potential distribution between the anode plasma and the vessel walls. Moreover, we have analyzed the equivalent electric circuit of the plasma during arc discharge operation. The total anode discharge current is the sum of the thermoelectric current flowing from the filament and the current flowing from the anode to the vessel walls. The current intensity, which flows to the vessel, is directly proportional with the arc voltage, and it is a measure of the plasma ionization degree. In some experimental conditions, the value of this current reaches up to 35% of the total anode discharge current. In order to obtain the ion energy distribution, a magnetic deflection ion energy analyzer was used. It was found that ions have quasi-monoenergetic energy, and its value is strongly related to the plasma potential distribution. The optical emission spectroscopy measurements confirm these results denoting that ions are quasi-monoenergetic and highly energetic (up to 1 keV).

Carbon and Tungsten Sputtering in a Helium Magnetron Discharge

This study reports on carbon and tungsten deposition on a heated silicon substrate under He+ bombardment in a magnetron sputtering device. The discharge was operated at constant pressure of 1.33 Pa for two discharge current intensities (200 mA and 600 mA) and target power density up to 40 Wcm-2. The deposited films were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffractometry (XRD). The topography and cross section reveals the influence of the target power density on the surface roughness, grains size and thickness of the deposited films.

Fabrication of 2D TiO2 Nanopatterns by Plasma Colloidal Lithography

The present study investigates the capability of plasma colloidal lithography technique consisting on three fabrication steps, colloidal mask deposition, thin film deposition with colloidal masks and colloidal mask lift off, to fabricate 2D patterns of TiO2 on silicon substrate. For the plasma assisted thin film deposition step, we used reactive high power impulse magnetron sputtering, a technique known to yield highly compact thin films with good control of composition and crystallinity. The fabricated 2D nanopatterns were investigated by atomic force microscopy and scanning force microscopy. Highly ordered 2D crystal array of TiO2 triangular islands with the maximum height of about 10 nm were obtained independently of the TiO2 thin film deposition time (thickness). The relatively low value of TiO2 pattern height is explained by the shadow effects of the mask, a large number of particles from the gas phase contributing on the film deposited on the mask, until complete filling of the void spaces between nanoparticles of the mask.

Plasma sputtering depositions with colloidal masks for fabrication of nanostructured surfaces with enhanced photocatalytic activity

Titanium oxide/silicon oxide (TiO2/SiO2) 2D patterns were obtained by magnetron sputtering depositions of Ti on close-packed and size-reduced colloidal masks on Si and quartz substrates, followed by mask lift-off and ending with thermal oxidation. The physical processes involved in growing 2D Ti patterns and their oxidation are analyzed. For the magnetron sputtering deposition, two regimes are considered: the low-pressure regime when the flux of sputtered atoms is anisotropic, and the high-pressure regime, when the flux of sputtered atoms is isotropic due to frequent collisions. Moreover, magnetron sputtering operation modes, such as dc sputtering and high power impulse sputtering, are compared. The changes in pattern size and morphology determined by the oxidation of the Ti patterns and Si substrate are analyzed. The hydrophilicity induced by UV-light irradiation and the visible-light photocatalytic activity towards the degradation of the methylene blue of the fabricated TiO2/SiO2 patterns were considerably higher when compared to the performances of uniform TiO2 films.

Pulsed Magnetron Sputtering: The Role of the Applied Power on W Coatings Properties

The interaction of low/high density transient plasma with tungsten (W) target was investigated via the sputtered material: in the gas phase, measuring the ion energy distribution function, and as deposited coatings, analyzing the compositional, structural, morphological, mechanical and tribological properties. The W target was sputtered in Ar atmosphere, at two different target power densities (instantaneous peak values of 0.2 and 3 kW/cm2), using a pulsed magnetron discharge. All obtained coatings were polycrystalline. A porous columnar structure was observed for the films deposited at low target power density, while denser-than-bulk films were obtained at high target power density. Comparing the high and low power modes, the W+ ion flux is seven times higher in the first case, while the deposition rate is three times higher in the second one.