Oxide films volume change

The observation of viscous flow in SiOz films was first reported by EerNisse(23,24). Essentially, a compressive intrinsic stress was found to exist in Si02 films grown below 1000°C and this stress was relieved at higher temperatures. The densification of SiO films was reported(20,21) and a unified model that explains both the occurrence of stress in Si0 and the higher density was published(25). This model utilized the concept of viscous relaxation in a Maxwell solid. The main idea is depicted in Fig.7 where the molar volume change is seen to cause the stress and density increase which are both relieved via viscous flow at sufficiently high temperatures. These ideas were recently incorporated into a revised oxidation mode1(26). Part of this revision modifies the oxidation expression to include the stress driven viscous relaxation. From a consideration of SiC as a simple Maxwell solid the expression for F becomes ... 

Currently, the most promising artificial muscles are electrochemomechanical actuators. These devices are designed to transduce electrical energy into mechanical work through electrochemical reactions in conductive polymer films. The most common CPs currently under investigation for this application include PPy and PANI [61,95], utilizing the volume change upon oxidation/reduction accompanied by ions/solvents... 

The oxidation of the CP film promotes the inclusion of electrolyte into the film which causes swelling and anticlockwise bending. The reduction of the film induces the expulsion of electrolyte and therefore the device bends in the clockwise direction. In these systems, a conductive counter electrode is required to allow the current flow and generate electrochemical reaction that causes the volume change of the CPs. Valero et al. describe a PPy-dodecylbenzenesulfonate-qjerchlorate/tape bilayer artificial muscle with reversible movements through subsequent oxidation and reduction of the PPy layer [97]. Figure 13.10 shows pictures of the clockwise movement of this artificial muscle due to PPy oxidation (a) and the counterclockwise movement due to reduction (c). 

Li metal originally suffers from the dendrite growth problem that causes shorter life and safety problem. This point is not discussed in this article as so many reports and reviews can be found elsewhere. Li electrode in Li-S battery has inherent problem. As stated in redox-shuttle mechanism of polysulfides in Fig. 1, reduction of polysulfides at the Li electrode surface is a problem. If polysulfides are deposited as Li2S, capacity is lost and Li electrode efficiency becomes worse. It is very important to prevent Li2S deposition on the Li electrode to maintain cycles. Solid electrolyte is a one solution however, brittle solid electrolyte caimot tolerate the volume change of Li electrode. Practical way to improve is thought to be a use of electrolyte additives. LiNOs is well known to depress the redox-shuttle phenomena [19]. The effect of this additive was concluded in the paper as a formation of protective layer on Li surface. LiNOs possibly oxidized the polysulfides and at the same time, formed the film by decomposition of itself. As a result, polysulfides were prevented from contacting with Li electrode. 

Fig. 18 The change in height of a 16.8 [im PPy(DBS) film over time. The volume in the oxidized state under electrochemical cycling did not return to the same volume before cycling there was an irreversible increase in height. This corresponds to an irreversible volume change in a direction out of the plane of the film (z-direction) (Reprinted (adapted) with permission from Smela and Gadegaard (2001). Copyright (2001) American Chemical Society)...

A serious complication of using profilometry to measure volume change becomes apparent when attempting to make measurements on thin films (<10 pm). Here the applied force from the stylus can lead to anomalous results such as a smaller PPy (DBS) film thickness in the expanded (reduced) state than in the contracted (oxidized) state. This is due to the lower stiffness (Young s modulus of elasticity) in the reduced state compared to the oxidized state resulting in the stylus sinking into the softer polymer. This is not a problem for relatively thick films provided absolute values are not required profilometry can serve as a useful tool to observe the actuation of thick conducting polymer films. 

This technique allows the real-time volume change to be mapped in situ in the out-of-plane direction. This has shown that the PPy(DBS) film fliickness increased by over 35 % in the reduced state compared to the oxidized state (Fig. 24). This compares with an in-plane strain of 2 % found in prior studies using bilayers, showing that the volume change of PPy(DBS) is anisotropic. The anisotropic strain of PPy(DBS) supports the view that it has a lamella structure with planes orientated parallel to the substrate (Wemet et al. 1985). 

---