Inflight Microwave Drying Process of Micro-droplets for High Resolution 3D printing

A new Additive-Manufacturing (AM) or 3D printing concept is proposed to improve the printing resolution for metal additive manufacturing in the frame of the SFA-AM project, Powder Focusing for Beam-Induced Laser 3D Printing. The project aims to transport small powder particles (largest diameter < 10 um) at a high throughput to a spot smaller than 20 µm. To realize the objective, a metal ion included liquid is suggested as a source medium similar to the inkjet process, and the droplet is employed as a container for the metal ion transport. The key challenges of achieving this goal are 1) droplet size control and 2) efficient microwave heating that can dry the liquid portion of the dynamic droplet at a high-speed (3 m/s). The most demanding part is the development of a miniaturized, very concentrated field density microwave resonator where the droplet is passing through. This follows from the extremely short interaction time between the microwave and the ejected droplets (research: 0.5 ms) which results from the requirement for a small system size for its integration. The primary medium examined for the experiment is water due to 1) a high dielectric loss proportional to the drying efficiency and 2) avoiding explosion risks at the laser sintering stage compared to organic solutions. Various droplet generation conditions were investigated to determine the optimum conditions for the custom droplet generator, in particular, two key parameters were focused on: frequency and flow rate. The droplets obtained were analyzed with respect to size, gap distance, and linearity affecting the uniformity of the AM process. Moreover, Navier-Stokes-based fluid dynamics simulations enabled understanding of droplets behaviors, especially break-up formation. Eventually, the optimal condition for producing the smallest droplets with a diameter of 100 µm is determined to be 15 kHz and 0.5 ml/min. Such droplets loaded with metallic particles could result in smaller than 20 um of remaining metal powders aggregate after the drying process.A new TEM (Transverse ElectroMagnetic) resonator has been developed for drying and sensing applications. A full-wave electromagnetic simulation software, CST Microwave Studio, supported the working mode of the device. Firstly, it is used for the dielectric characterizations of different water-ethanol mixtures contained in a micro capillary. The sensing relies on electric field perturbation due to the inserted sample, and is measured based on the change of both resonance frequency and Q-factor. These sample-dependent variations were analyzed to estimate the complex permittivity using different methods such as the Perturbation Method (PM), Least-Square Model (LSM), and Log-Linear Model (LLM). This work demonstrates that the proposed microwave module is capable of sensing nano-liter volume ranges even though the samples only partially influence the sensing area (error < 13 % in permittivity). In microwave heating, the developed resonator achieved a temperature increase of 28 K in the microwave exposal time of 0.5 ms when the microwave power of 45 W was applied. Finally, numerical models have been proposed to estimate the maximum temperature of the initial droplet before cooling starts attributable to heat and mass transfer. This model well corresponds to the microwave electromagnetic simulation resulting in similar temperature deviations. It helps to determine the net heating efficiency of the microwave device.

Polarization response of Perovskite films for microwave tunable applications

Konstantin Astafiev

Ferroelectric thin films are widely studied nowadays as potential replacements for semiconductors in modern tunable microwave devices such as tunable filters, phase-shifters, frequency mixers, power dividers, etc. Recent progress in the deposition of complex oxide films shows promise of significant improvements in the quality of ferroelectric thin films from the point of view of dielectric loss and electrical tunability. However, there still exist certain problems which limit a wide penetration of ferroelectric thin films into the market. The most crucial issue which needs further investigation is the difference in polarization response between ferroelectric thin films and bulk materials. In comparison to bulk materials, thin films show lower dielectric constants, and consequently a smaller electrical tunability, and much higher dielectric losses at microwave frequencies. Another important question which has rarely been investigated is the problem of the reliability of tunable devices based on ferroelectric films. Therefore, a systematic, experimental investigation to understand the physics behind these problems is a task of primary importance. The present work is devoted to the detailed theoretical and experimental investigation of the polarization response in SrTiO3 and KTaO3 thin films at radio and microwave frequencies over a wide range of temperatures and applied bias fields. Specifically, the following has been performed: The polarization response of SrTiO3 and KTaO3 ferroelectric thin films has been investigated at radio frequencies over a wide temperature range using a planar capacitor structure. The sensitivity of the dielectric properties to the deposition technique, conditions of films growth and type of the substrate used, and hence to the microstructure of the films, is clearly demonstrated. While SrTiO3 is paraelectric in bulk form, the appearance of a ferroelectric phase in SrTiO3 thin films on LaAlO3 substrate has been experimentally documented, with a transition temperature lying in the range of 90-120K. It is theoretically shown in terms of Landau theory, that the behavior of the SrTiO3 thin films can not be attributed to the effect of the film/substrate mechanical coupling. The ferroelectricity of SrTiO3 thin films on LaAlO3 substrates has been investigated in detail. The hysteresis loops have been monitored in a wide temperature range using thin film planar capacitors, the driving field being predominantly in the plane of the film. The microwave dielectric loss in perovskite ferroelectric materials has been theoretically investigated. A model of the quasi-Debye loss mechanism in SrTiO3 and Ba0.6Sr0.4TiO3 materials at microwave frequencies has been developed. Experimental investigations of the dielectric loss at microwave frequencies in SrTiO3 and KTaO3 thin films have been performed using a half-wave length microstrip resonator with a resonant frequency of around 8GHz. The field and temperature dependences of the loss tangent for the SrTiO3 and KTaO3 thin-film based planar capacitors have been measured. The impact of electrical stressing on the microwave performance of SrTiO3 films deposited onto different substrates (MgO and LaAlO3) has been investigated. The most important characteristics of tunable microwave components, including tunability and loss tangent, have been monitored. The influence of the addition of a low-loss linear dielectric material to a tunable ferroelectric material has been investigated in terms of electrostatic considerations. Calculations of the dielectric loss and dielectric non-linearity of ferroelectric-dielectric composites have been performed using three different models. On the basis of the results obtained, a figure of merit for the composite material has been evaluated.

EPFL2004

Piezoelectric thin films for bulk acoustic wave resonator applications

Roman Lanz

Bandpass filters for microwave frequencies realized with thin film bulk acoustic wave resonators (FBAR) are a promising alternative to current dielectric or surface acoustic wave filters for use in mobile telecommunication applications. With equivalent performance, FBAR filters are significantly smaller than dielectric filters and allow for a larger power operation than SAW filters. In addition, FBARs offer the possibility of on-chip integration, which will result in substantial volume and cost reduction. The first passive FBAR devices are now appearing on the market. They mainly cover needs in miniaturized RF-filters for the new bands around 2 GHz. A FBAR is essentially a thin piezoelectric plate sandwiched between two electrodes and acoustically isolated from the environment for energy trapping purposes. If the isolation is effectuated by an acoustic Bragg reflector, one speaks of solidly mounted resonators (SMR). Piezoelectric aluminum nitride (AlN) thin films are predominantly used in the emerging FBAR technology because AlN exhibits a sufficient electromechanical coupling coefficient kt2 , low acoustic losses at microwave frequencies, a low temperature coefficient of frequency, and its chemical composition is compatible with CMOS requirements. This thesis has two research directions. In the first part, FBAR structures based on AlN thin films were investigated for applications at X-band frequencies (7.2-8.5 GHz), i.e. operating at much higher frequencies than the ones used for present products. The goal was to identify property limitations related to such high frequencies, and to demonstrate to industry high performing SMR filters at 8 GHz. In the second part, a new material for FBAR devices was studied. The motivation is that AlN allows for a restricted filter bandwidth only, limited by its coupling factor of maximal 7%. Monocrystalline KNbO3 appears as an ideal alternative with its high coupling factor kt2 of 47%, and relatively large sound velocity of 8125m/s for longitudinal waves along the [101] direction. Piezoelectricity of KNbO3 films grown on electrodes has never been characterized. Single crystal results indicate that the optimal film texture would be (101). In this thesis, the growth of KNbO3 films on Pt electrodes was studied with the goal to achieve this texture uniformly, and to characterize piezoelectric properties. X-band FBAR's were first studied with numerical simulations based on a one-dimensional theory of the thickness-extensional bulk acoustic wave (BAW). Thickness, acoustic properties and electrical conductivity of the electrodes have a large impact on the resonator characteristics. There are conflicting requirements with respect to optimum acoustic and electrical properties of the electrode materials. An optimum thickness was calculated for 8GHz FBARs that use Pt bottom and Al top electrodes. The characteristics of ladder filters have been calculated based on the impedances of single resonators. The adjustable filter parameters, i.e. the areas of series and shunt resonators, frequency de-tuning between series and parallel resonators, and number of π-sections were screened for a process window offering maximum filter bandwidth with lowest ripple and low insertion loss for a given out-of-band rejection. An important result of the numerical simulations was that the bandwidth of ladder filters can be doubled by de-tuning the series and parallel resonators by more (1.3 times) than the difference of resonance and anti-resonance frequency. This also leads to a flatter passband while keeping the ripples below ±0.2dB. Solidly mounted resonators and filters were fabricated using an acoustic multilayer reflector consisting of AlN and SiO2 λ/4 layers. All films were sputter deposited in a high vacuum sputter cluster system with 4 process chambers. The films were patterned using standard photolithography and dry etching processes. The SMR exhibited a strong and spurious-free resonance at 8GHz with a high quality factor of 360 and electromechanical coupling coefficient of 6.0%. The temperature coefficient of frequency was -18ppm/K, and the voltage coefficient of frequency was -72ppm/V. Passband ladder filters with T- and π-topology consisting of 3 to 14 SMR were successfully demonstrated with a center frequency of 8GHz. These filters were optimized for maximum bandwidth and exhibited an insertion loss of 5.5dB, a rejection of 32dB, a 0.2dB bandwidth of 99MHz (1.3%), and a 3dB bandwidth of 224MHz (2.9%). There was good correspondence between measured and simulated filter and resonator characteristics. For perfect agreement, parasitic elements needed to be taken into account. These were a series resistance of 5Ω and a parallel conductance of 2mS in case of single resonators. The series resistance can be explained with resistive losses in the electrodes, whereas the parallel conduction was due to conduction along the surface. For π-filters, an additional series inductance of 100pH was needed to obtain a satisfactory fit. This inductance increased the out-of band rejection and insertion loss. Besides the group delay variation, all industrial specifications were met. KNbO3 was in-situ sputter deposited at 500 to 600°C using a rf magnetron source. A dedicated sputter chamber with load-lock and oxygen resistant substrate heater was built for this purpose. The high volatility of potassium oxide requires a potassium enrichment of the target. Targets with several excess concentrations (in the form of K2CO3) were studied. Stoichiometric KNbO3 films were obtained with targets containing 25 and 40% excess K. Zero and 10% excess yielded K deficient films, whereas 100% and 200% excess K led to highly unstable targets with K accumulation on the target surface, resulting in K rich second phases. The potassium-to-niobium ratio in the films depends strongly on sputter pressure and substrate temperature. Dense films, nucleated with cubic {100} texture, were obtained on platinized silicon substrates with a 10nm thick IrO2 seed layer at substrate temperatures of 520°C. At lower temperatures the films were amorphous, and at higher temperatures the films were composed of individual and facetted KNbO3 grains. The cubic high-temperature {100} texture results in a mixed (101)/(010) texture in the orthorhombic room temperature phase. The measured relative permitivity of 420 indicates that both orientations are equally present. Micro-Raman confirms the orthorhombic line splitting. Piezoelectrical and ferroelectrical activity were verified by means of a piezoelectric sensitive atomic force microscope. A very large piezoelectric activity was observed on some of the grains, and the polarization could be switched on most of the grains. However, the average d33,f = e33/c33, as measured by means of laser interferometry, showed a modest value of 24pm/V. The effective coupling factor is derived as kt2=2.8%, which is small relative to the theoretical value of 47%. The high dielectric constant and the absence of piezoelectric activity along the [010] direction are responsible for the reduction of the kt2 factor. Film roughness, complexity of deposition process and open poling issue make KNbO3 integration into BAW devices a difficult task.

EPFL2004

Conductivity, dielectric and piezoelectric properties of SrBi4Ti4O15

Strontium Bismuth Titanate is a very promising material for high temperature piezoelectric applications, its elevated ferroelectric phase transition (530°C), linear piezoelectric properties under low field and relatively low room temperature conductivity (compared to others Bismuth Titanates) make it very attractive for precision sensors. However, under severe conditions (low frequency, high field, high temperature or low oxygen partial pressure) some of those advantages disappear. Piezoelectric response is dominated by charge drift in general becomes unstable. Above all, at high temperature and low oxygen partial pressure, a large conductivity increase reduces the piezoelectric efficiency of the material, in this work, electrical conductivity, piezoelectric properties and dielectric permittivity of SrBIT ceramic have been investigated in conditions of high temperature, low oxygen partial pressure, low to moderate driving field and frequency. As a result of this research, a better understanding of SrBIT properties was achieved. Thanks to a careful study of SrBIT processing as a bulk ceramic, a reproducible route was established. Many basic mechanisms leading to both SrBIT crystallization and sintering have been identified. It was demonstrated that a detailed knowledge of the exact processing conditions is required in order to achieve high quality material. DC conductivity measurements were carried out as a function of temperature, oxygen partial pressure and dopant concentration. It was found that the apparent activation energy for conduction for undoped SrBIT was 1 eV between 140 and 220°C and 1.5 eV between 450 and 700°C. Decrease of the activation energy in the lower temperature range has been discussed considering grain boundary conductivity, as a transition from electronic to ionic conduction or as consequence of small polarons conduction. It was shown that lower activation energy resulted from Manganese doping (0.5 eV), this was interpreted as either growing influence of grain boundary conductivity as dopant concentration increases or as shallow hole trap formation. DC conductivity measurements and acceptor/donor doping experiments demonstrated p-type conductivity in the low temperature range (up to 220°C) as donor doping decreases conductivity, while oxygen partial pressure controlled measurements indicated n-type conductivity at higher temperature (above 700°C). An electrical impedance analysis was performed with several equivalent circuits. The aim of these models was to simulate the impedance of SrBIT. The best approximation was found with a distributed element of Havriliak-Negami type. However, as the physical justification for this circuit was not clear, the investigation of the grain, grain boundary and electrode impedances was performed with multiple discrete parallel RC elements. With temperature, grain size and oxygen activity variations, the identification of three separate contributions as grain, grain boundary and electrode was realized. The anisotropy of conductivity and permittivity was demonstrated with textured material and both DC and AC analysis. With the master curves built for the electrical modulus, it was found that the impedance probably related to Bismuth oxide layers produces an additional high frequency are. From this finding and by comparing characteristic relaxation frequencies of undoped, 2 mol.% Mn and 4 mol.% Nb SrBIT, it was determined that conductivity is higher in the ab plane direction than in the c direction within both perovskite units and Bismuth oxide layers. With conductivity measurements under controlled oxygen partial pressure, it was found that an acceptor-based (intrinsic or extrinsic) model could be used to describe the electrical conductivity under controlled oxygen partial pressure of both undoped and 2 mol.% Mn doped SrBIT. However, as neither a pO2-1/6 region (intrinsic oxygen vacancies compensated by electrons) for undoped SrBIT nor a pO2-1/4 region (oxygen vacancies compensated by singly ionized acceptors) for Mn doped SrBIT were seen, it was concluded that the acceptorcontrolled model is not sufficient for a complete description of SrBIT. For this reason and in order to include the low oxygen partial pressure behavior of undoped SrBIT, a donor-based (intrinsic) model was also considered. The source of intrinsic donors would be in that case Bi3+ cations sitting on Sr2+ sites in the perovskite sublattice. Considering Bismuth vacancies as the negative compensating species, both pO2-1/4 and pO2-independent regions could be predicted with the model. However, even if the donor-controlled model seems to better match conductivity measurements in the full PO2) range, rejecting the acceptor-based model would be an error. It is actually not demonstrated that in SrBIT the concentration of exchanged Bismuth cations is always (all temperature, pressure) larger than the natural acceptor impurity concentration. It is very likely that the cation exchange is dependent of the oxygen partial pressure. It is also not proved as suggested in the literature that direct compensation between exchanged Strontium and Bismuth exists, reducing the net donor-excess. With the acceptor-controlled model, the mass-action constants for reduction and for ionization of intrinsic carriers across the band gap were determined. The band gap of SrBIT was estimated to be 3.5 eV. The ionic conductivity of SrBIT was determined at high temperature with measurements under controlled oxygen partial pressure. It was found that the electrical conductivity of SrBIT is probably mixed (electronic and ionic) as the estimation of the transference number provided quite large values (t=0.8 at 800°C). From electronic and ionic conductivity data, mixed conduction can actually be predicted in a large temperature range (above room temperature). The piezoelectric measurements using direct effect demonstrated that it is actually possible to unlock piezoelectrically active ferroelectric domain walls and create non-linear piezoelectric properties in undoped SrBIT. This occurs above a threshold elastic field, which is thermally activated. With a piezoelectric composite, it was demonstrated that the electromechanical coupling between two different phases creates a piezoelectric relaxation. This one could be positive or negative depending on the respective properties of each composite's component. It was shown experimentally that a small temperature change is sufficient to transform a positive relaxation into a negative one. While these experiments did not provide a detailed microstructural explanation for the piezoelectric relaxation observed in 2 mol.% Mn doped SrBIT, they gave a first insight into an original phenomenological approach. Microstructure and piezoelectric properties were related with the calculation of a piezoelectric relaxation composite made of two textured samples. This demonstrated that a piezoelectric relaxation may occur just because of anisotropy. Microstructural reproduction of this coupling is actually an important feature of Aurivillius phases.

EPFL2000

Piezoelectric thin films for bulk acoustic wave resonator applications

Roman Lanz

EPFL2004

Polarization response of Perovskite films for microwave tunable applications

Konstantin Astafiev

EPFL2004

Conductivity, dielectric and piezoelectric properties of SrBi4Ti4O15

EPFL2000