Constantin Vahlas

Nanoenergetic Materials for MEMS: A Review

New energetic materials (EMs) are the key to great advances in microscale energy-demanding systems as actuation part, igniter, propulsion unit, and power. Nanoscale EMs (nEMs) particularly offer the promise of much higher energy densities, faster rate of energy release, greater stability, and more security (sensitivity to unwanted initiation). nEMs could therefore give response to microenergetics challenges. This paper provides a comprehensive review of current research activities in nEMs for microenergetics application. While thermodynamic calculations of flame temperature and reaction enthalpies are tools to choose desirable EMs, they are not sufficient for the choice of good material for microscale application where thermal losses are very penalizing. A strategy to select nEM is therefore proposed based on an analysis of the material diffusivity and heat of reaction. Finally, after a description of the different nEMs synthesis approaches, some guidelines for future investigations are provided.

Parametric study for the growth of carbon nanotubes by catalytic chemical vapor deposition in a fluidized bed reactor

Multiwalled carbon nanotubes have been produced from H2–C2H4 mixtures on Fe–SiO2 catalysts by a fluidized bed catalytic chemical vapor deposition process. Various parameters such as the catalyst preparation, the residence time, the run duration, the temperature, the H2:C2H4 ratio, the amount of metal deposited on the support have been examined. The influence of these parameters on the deposited carbon yield is reported, together with observations of the produced material. This process allows an homogeneously distributed deposition of nanotubes (10–20 nm diameter), that remain anchored to the support.

MOCVD‐Processed Ni Films from Nickelocene. Part I: Growth Rate and Morphology

An experimental investigation of the relation between operating conditions and growth rate and morphology is presented for nickel films processed from nickelocene by MOCVD. It is shown that growth rates up to 1 μm h−1 can be obtained under the investigated conditions. Deposits present a nodular morphology, the nodules being composed of small (∼20 nm) crystallites. Deposition is favored at temperatures between 160 °C and 180 °C, and at pressures higher than 100 torr, with a high hydrogen partial pressure. This parametric window is delimited by the decrease of the dissociative adsorption of hydrogen on the surface and by the increase of the desorption of nickelocene.

Fluidization, Spouting, and Metal–Organic CVD of Platinum Group Metals on Powders

In the first part of this paper, processes used for the fluidization of particles in a CVD reactor are reviewed, then the concept of the spouted bed is introduced, and the possibility of using it in association with metal-organic (MO)CVD is discussed. In the second part, the particulars of the MOCVD of platinum group metals are recalled. Finally, the MOCVD of these metals in fluidized bed and spouted bed reactors is illustrated with results obtained during two research projects, one of which concerns the preparation of finely dispersed rhodium, platinum, and palladium-based catalysts by fluidized bed MOCVD, the other, the doping of NiCrAlY powders (a raw material for thermal barriers) with ruthenium by spouted bed MOCVD. Results on the morphology and purity of the deposits are presented, and the applicability of such techniques, either for industrial uses or for initial screening of dopants (nature and level), are discussed.

Thermal degradation mechanisms of Nicalon fibre:a thermodynamic simulation

Thermodynamic calculations for the thermal degradation of the Nicalon fibre in inert gas flow at atmospheric pressure have been performed, based on minimization of the Gibbs energy of the Si-C-O-H chemical system. The calculations are based on a critically selected thermodynamic database of the participating compounds. The results are presented by means of diagrams illustrating the quantities of condensed and gaseous species obtained as a function of treatment temperature. These are compared with recently reported TEM studies of as-received and heat-treated material, which illustrate the sequential morphologies of its structure and nanotexture as a function of treatment temperature. The main step of the observed degradation mechanism is successfully simulated in terms of the temperature, the oxygen content and the weight loss of the material. An endogenous oxidation mechanism is proposed for degradation of the fibre.

CVD of Pure Copper Films from Amidinate Precursor

Copper(I) amidinate [Cu(i-Pr-Me-AMD)]2 was investigated to produce copper films in conventional low pressure chemical vapor deposition (CVD) using hydrogen as reducing gas-reagent. Copper films were deposited on steel, silicon, and SiO2/Si substrates in the temperature range 200–350°C at a total pressure of 1333 Pa. The growth rate on steel follows the surface reaction between atomic hydrogen and the entire precursor molecule up to 240°C. A significant increase of the growth rate at temperatures higher than 300°C was attributed to thermal decomposition of the precursor molecule. It is shown that [Cu(i-Pr-Me-AMD)]2 meets the specifications for the metal organic chemical vapor deposition of Cu-based alloy coatings containing oxophilic elements such as aluminum.

Selection of metalorganic precursors for MOCVD of metallurgical coatings: application to Cr-based coatings

Metalorganic chemical vapor deposition (MOCVD) is expanding for low temperature deposition of metallurgical coatings. The abundance of metalorganic compounds that can be supplied by chemists makes the design and the selection of suitably tailored metalorganic precursor a fundamental key to develop successfully an MOCVD process for the production of a desired thin film material. The stringent requirements for metalorganic precursors for deposition of metallurgical coatings are critically reviewed in this paper and discussed using typical examples on the growth of Cr-based coatings. Particular emphasis is given to the control of the metalloid incorporation in the coatings and on the employment of single-source precursors instead of separate precursors for MOCVD of multi-element material.

Reaction and Transport Interplay in Al MOCVD Investigated Through Experiments and Computational Fluid Dynamic Analysis

An improved reactive transport model of a metallorganic chemical vapor deposition process for the growth of aluminum films from dimethylethylamine alane is developed. The computational fluid dynamics model is built under PHOENICS software for the simulation of the coupled fluid flow, heat transfer, and chemistry. The growth mechanism of aluminum films is based on wellestablished, in the literature, reaction order and activation energy of gas-phase and surface reactions. The improvement of the model against a simplified model is established. The interplay of reaction and transport is elucidated. In particular, the important effects of the gas-phase reaction and of the showerhead system are revealed; accounting for gas-phase along with surface reactions for the flow details in the showerhead and for the three-dimensional geometry induced by the distribution of the holes in the showerhead yields substantial enhancement of the predictive capability of the model. The satisfactory agreement between model predictions and growth-rate measurements allows one to understand and improve the process. The model is further used to investigate the effect of key operating parameters on the characteristics of the aluminum films. Simulation results are suggestive of modifications in the operating parameters that could enhance the growth rate and its spatial uniformity.

Precursors and operating conditions for the metal-organic chemical vapor deposition of nickel films

The different precursors that have been tested or that are actually in use for the processing of thin films of nickel by the chemical vapor deposition technique are reviewed. Applications of thin films of nickel are presented. Deposition conditions and characteristics of the films obtained from different precursors are detailed. Ni(CO)4, NiCp2, Ni(MeCp)2, Ni(hfa)2, Ni(dmg)2, and Ni(den)2 appear as the most promising ones. The final choice of a precursor for the MOCVD of Ni films depends on requirements concerning the process (e.g. safety issues, deposition temperature, growth rate) and the purity of the deposited films.

LPCVD WSi2 Films Using Tungsten Chlorides and Silane

This paper makes a systematic study of blanket and selective low pressure chemical vapor deposition (LPCVD) WSi 2 films from tungsten chlorides, silane, hydrogen, and argon on silicon as well as on patterned oxidized silicon substrates. Experiments were performed by varying the initial gaseous WCl 4 to SiH 4 ratio (Rx) or the deposition temperature (T d ). Initially, yield and CVD-phase diagrams of the W-Si-Cl-H-Ar chemical system were drawn, based on thermodynamic simulations. The deposition of pure WSi 2 phase and mixed solid phases involving W, W 5 Si 3 , WSi 2 , and Si was predicted to occur in relation to process parameters