The influence of process parameters on precursor evaporation for alumina nanopowder synthesis in an inductively coupled rf thermal plasma

The process parameters of an inductively coupled thermal plasma used for nanopowder synthesis are experimentally investigated using various plasma diagnostics and in situ powder monitoring methods. An enthalpy probe technique is applied to characterize the plasma properties under particle-free conditions. The nanoparticle synthesis from microscale alumina precursors is monitored in situ by optical emission spectroscopy and laser light extinction measurements to investigate the powder evaporation. The synthesized powders are collected in a sampling unit and characterized ex situ by particle size analysis as well as by electron microscopy. At low flow rates of the torch central gas, higher plasma enthalpy, a laminar powder flow and increased evaporation of the precursor have been observed. A precursor- and an energy-deficient regime related to the precursor feed rate and plasma enthalpy are found from the emission line intensities of aluminium metal vapour. The number fraction of plasma-treated precursors, which is an important process parameter, is calculated from the precursor number density obtained from laser extinction measurements.

Process monitoring of alumina nanoparticle synthesis by inductively coupled RF thermal plasma

Jong-Won Shin

Since the first inductively coupled plasma (ICP) torch was developed in the 60's, RF thermal plasmas have found a variety of applications in material processing such as crystal growth, thermal spray coatings, spheroidization and vaporization of refractory materials. In recent decades, inductively coupled thermal plasmas have been used for the synthesis of high purity nanoparticles, thank to their remarkable advantages such as high energy density, variable operating pressures and low product contamination. The formation of nano-sized particles in thermal plasmas particularly using solid refractory precursor is a complex process. Hereby the injected precursor powders are heated by the high temperature of the thermal plasma. The heated powders are vaporized and decomposed into vapor species. During the following cooling, the vapor species are condensed to nano-sized particles. In this synthesis process, characterized by evaporation-condensation, the properties of the synthesized particles such as particle size, size distribution, phases and morphology are affected by various process parameters. Gas pressure, flow rates of the different torch gases, size of the precursor powders and powder loading in the plasma are considered as the influencing process parameters. In order to optimize and control the process for tailor-made nanoparticle synthesis, it is necessary to understand the effects of the process parameters on plasma properties and on the particle-plasma interactions resulting in final properties of the synthesized powders. In the present work, firstly, the enthalpy probe technique, a well-established plasma diagnostic tool for high pressure thermal plasma, has been applied to characterize the inductively coupled Ar-H2 thermal RF plasma used for alumina nanoparticle synthesis at different operating conditions. The plasma enthalpy has been determined, and the plasma temperature under a given gas pressure could subsequently be calculated assuming local thermodynamic equilibrium (LTE), once the gas composition has been determined by mass spectrometry. The gas velocity of the plasma jet could also be obtained by using the Bernoulli equation and stagnation pressure. Secondly, the influence of flow rates of central gas and precursor feed rates on the Al2O3 precursor evaporation has been investigated by process monitoring. Optical emission spectroscopy (OES) and laser light extinction (LE) measurements have been carried out to monitor in-situ the injection of the alumina precursors and to obtain information on the ongoing precursor evaporation. The emission line intensities of the aluminum vapor were used to determine the dependence of process parameters on precursor evaporation. Furthermore, the laser light extinction was used to calculate the number density of non plasma-treated powders, from which the number fraction of evaporated powders could be deduced. The size and morphology of the synthesized particles have been characterized ex-situ. Finally, the influence of quenching conditions on the condensation and formation of the nano-sized alumina particle has been studied by process analysis of the gas phase reactions of alumina. Optical emission spectroscopic was performed at different axial positions to monitor the gas phase reactions of the vaporized particles. Axial profiles of the emission line intensities of the main vapor species of alumina, Al(g) and AlO(g), were compared with the results of the enthalpy probe measurements and the thermal decomposition reactions of alumina found in literature. The results allow explanation of the alumina reactions in the thermal plasma as well as gives indications for the optimal axial position of the quenching gas injection. In addition to the determination of the optimal quenching position, the influence of the flow rates of quenching gas on the nanoparticle formation has been investigated. The synthesis of the submicron alumina using vapor-condensation principle predominantly leads to various thermodynamically metastable crystal structures called transition alumina. Therefore, the occurrence of different transition aluminas is discussed with regards to the flow rates of the quenching gas. The results presented in this thesis show that the synthesis process of the nanoparticle can be described on the basis of experimental results and basic information on the powder materials. The explanation of the influence of process parameters on powder evaporation and nanoparticle formation shows the usefulness of the in-situ plasma process monitoring, and the large potential for optimizing and controlling the nanopowder synthesis process in an inductively coupled RF thermal plasma.

EPFL2006

Preparation and properties of KNbO3-based piezoelectric ceramics

Evelyn Hollenstein

During the last several years, new lead-free piezoelectric materials have been developed to replace the lead-based materials, such as PZT. Presently, the family of lead-free ceramics showing the most promising piezoelectric properties is based on potassium sodium niobate (KNN). The KNN ceramics have been investigated since 1960s, but many problems arise, especially during the synthesis. Potassium and sodium based powders are moisture sensitive leading often to disintegration of samples after sintering and to moisture dependent properties. Another drawback is the poor densification during sintering. Elaborated procedures (hot pressing, special handling of powders) are needed to produce high quality KNN ceramics in a reproducible way, and their properties are inferior to those of PZT. Li, Ta and Sb modified KNN, however, were reported in 2004 to exhibit properties comparable to those of PZT. These modified compositions promise to be a new generation of environmentally safer piezoelectric materials. The goals of this thesis are to prepare selected compositions within this family and examine their properties relevant for applications in medical transducer. The emphasis of the work is on piezoelectric properties and their stability with respect to the temperature, humidity, and preparation conditions. The unmodified, and lithium (K, Na, Li)NbO3 and the lithium with tantalum modified KNN ceramics, (K, Na, Li)(Nb, Ta)O3 have been produced by the conventional solid state synthesis. The conventional processing steps have been adapted with a goal to obtain reproducible high quality samples without using complex techniques such as hot pressing or special powder handling. In particular, the particle grain size and particle size distribution have been controlled for all the steps; this control starts with the initial powders. To reduce the particle sizes the most efficient milling method has been found to be attrition milling. Another important point is the compositional homogeneity. To improve this homogeneity, a second calcination step has been added to the process. Finally, the sintering step is sensitive, the sintering temperature range in these compositions is as narrow as 5 °C and in some cases, the dwell time is reduced to minimum (several minutes) to avoid grain growth. The densities of the so-obtained ceramics are higher than 95%, but some compositional inhomogeneities have been observed in electron microscopy. The electromechanical properties at room temperature are promising for example d33 = 240 [pm/V], kt = 51%, kp = 45% with ε = 919 [-] and tanδ = 2.6% for ceramics modified with 7 at% lithium, and d33 = 310 [pm/V], kt = 46%, kp = 46% with ε = 829 [-] and tanδ = 2.4% for ceramics modified with 3 at% lithium and 20 at% tantalum. These properties are especially enhanced for compositions with the orthorhombic to tetragonal phase transition close to room temperature. Dielectric and piezoelectric (resonance, converse and direct) measurements as a function of temperature have demonstrated that the orthorhombic to tetragonal phase transition is strongly dependent on temperature unlike in PZT, where it is controlled primarily by the composition. The thermal behaviour of ceramics is thus influenced by the presence of the phase transition in the analysed temperature range. The thermal stability has been analysed in the perspective of medical transducers, which are subjected to sterilisation cycles up to 140 °C. A depolarisation after the first cycle has been observed for all the compositions, the smallest being for the lithium and tantalum modified KNN ceramics. After the second cycle, the properties stabilise, being the best for the compositions (K0.465Na0.465Li0.07)NbO3 and (K0.485Na0.485Li0.03)(Nb0.80Ta0.20)O3. As the enhanced piezoelectric properties of these materials are related to the presence of a phase transition, the determination of the contributions of the domain walls in either phase is then of practical importance. The intrinsic and extrinsic contributions of the piezoelectric response can be separated by analysing the piezoelectric response as a function of the applied stress (direct measurements). It has then been shown that the intrinsic contribution is always higher than the irreversible domain wall motion contribution. This latter contribution is nevertheless higher in the orthorhombic phase than in the tetragonal phase. Finally, as the potassium niobate based ceramics have shown in the past their moisture sensitivity, aging measurements have been done in different atmospheres. Surprisingly, the properties of the modified KNN ceramics studied are not found to be dependent on the moisture and only small aging has been noticed. After 100 days, the (K0.485Na0.485Li0.03)(Nb0.80Ta0.20)O3 ceramics show a decrease of the electromechanical properties below 10% and the coupling coefficient are almost stable with time. With these electromechanical properties and their time/thermal stabilities the modified KNN ceramics are promising substitutes to lead-based materials, in particular the lithium and tantalum modified ceramics.

EPFL2007

On the Powder Formation in the Plasma Enhanced Chemical Vapor Deposition Process for the Deposition of SiOx Barrier Coatings

Marina Ricci

Nowadays thin films play an important role in everyday life and in industries. Thin film technology has been developed primarily for the growing demand for development of smaller and smaller devices that require advanced materials and new processing techniques suitable for micro and nano technology. One of the methods of producing thin films is by means of plasma technology. This is a technique by which plasma, also known as 'the fourth state of matter', is used to generate ions, electrons and radicals which deposit at a surface where they will form a thin film. The source of these depositing species could be either gaseous, liquid or solid in origin. Most of the time a plasma is generated by means of an electrical discharge. Plasma technologies are used for various applications. One of these is Plasma Enhanced Chemical Vapor Deposition (PECVD). PECVD systems have widespread applications in the electronic sector, because of their exibility in depositing different films such as silicon oxide thin films (SiOx). These films, currently deposited by organosilicon compounds, such as hexamethyldisiloxane (HMDSO), have dielectric and good barrier properties. The aim of the work at CRPP consists in optimizing PECVD processes for the deposition of these films on polymers. In this study, the species in gaseous phase and the powder produced in oxygen-diluted HMDSO plasmas were experimentally characterized in a radiofrequency (RF) capacitively-coupled reactor at 13.56 MHz. The gaseous phase of these plasmas and the particle formation were studied by in-situ infrared absorption spectroscopy and optical emission spectroscopy. The study of the powder formation is important because the particle deposition imposes an upper limit on the deposition rate. Particle formation was firstly studied as a function of HMDSO/O2 ratio. Analysis of the time evolution of the infrared absorption spectra gives the development of the size, the number density, the structure and the particle composition. At high oxygen dilution the formation of large particles is observed. The analysis of HMDSO/O2 plasma at low O2 content suggests that at the beginning of the process very small particles are formed by polymerization; the same particles do not grow much and keep sizes around 50 nm. The admixture of inert or non-reactant gases, such as Ar or N2, into the plasma promotes polymerization of the monomer; a high value of one of these inert or non-reactant gases leads to small powder particle size, since only polymerization takes place in this case. Particle size around 50 nm is also found in this case. At the same time we see a decrease of the carbon content and a weak reduction of the stoichiometry in SiOx indicated by the shift of the SiOSi asymmetric stretching vibration. This reduction also influences the deposited layers, less oxidized and, thus, quite far from a stoichiometric character. Hexamethyldisilazane (HMDSN) and tetramethylsilane (TMS) are other reagents that have been identified as alternatives to HMDSO. The in-situ infrared absorption spectra show that HMDSO/O2 and HMDSN/O2 plasmas are quite similar, but in the second case a lower SiOSi stretching peak intensity, due to NH and SiN peaks, is seen. TMS/O2 and HMDSN/O2 plasma spectra show a low SiOSi bending and stretching peak intensity, due also to the oxygen atom absence in their chemical structure. Two separate chapters are respectively devoted to the silicon nitride thin films (SiNx) and plasma crystal formation. SiNx layers are deposited by three different organosilicon compounds, such as HMDSN, TMS and bis(dimethylamino)dimethylsilane (BDMADMS). No powder formation is visualized in this contest. The ex-situ infrared absorption spectra show the difficulty to produce stable SiNx films; they present a low SiN peak intensity and are quite hygroscopic. Furthermore, in HMDSO/O2 plasmas at high oxygen dilution and high pressure, later in time into the discharge, accretion leads to larger and larger particles which can finish up in particles forming a plasma crystal. The study of RF harmonics shows the presence of instabilities, clear indications of the passage from small particulate to plasma crystal structure.

EPFL2011