Minima in the strength of MnZn ferrites
- PDF / 312,135 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 57 Downloads / 206 Views
L.J.M.G. Dortmans TNO Institute of Applied Physics, POB 595, 5600 AN, Eindhoven, The Netherlands
G. de Withb) Laboratory for Solid State and Materials Chemistry, Eindhoven University of Technology, POB 513, 5600 MB Eindhoven, The Netherlands
M.J.M. de Graaf Ferroxcube R&D Center Eindhoven, POB 80058, 500 KA, Eindhoven, The Netherlands (Received 2 April 2001; accepted 29 September 2001)
Like many other oxides, MnZn ferrite shows a considerably decreased strength in a humid environment. Conventionally one explains this behavior by subcritical crack growth, i.e., a kinetic effect affecting the oxygen–metal bond breaking rate at the crack tip via a reaction with water. The dissociation of water in a proton and hydroxyl ion is considered as crucial. However, it appears that also other gases have a detrimental effect on the strength. In this paper the effect of H2O, H2S, NH3, NO, and CO on the strength of MnZn ferrite have been studied experimentally. For water a clear minimum in strength occurs at relative low partial pressure. For the other gases the data also indicate a minimum. An adsorption model taking into account the presence of competitive adsorption mechanisms, involving dissociative and nondissociative adsorption of the adsorbate and the (non)dissociative adsorption of the ever present N2, is presented. This model can explain the experimentally observed features well. Although kinetic effects induce subcritical crack growth in MnZn ferrites, it is concluded that adsorption is an important strength lowering effect for MnZn ferrite when exposed to active gases.
I. INTRODUCTION
An important aspect of the strength and fracture toughness of MnZn ferrites is its sensitivity to humidity. This has been explained by the effect known as subcritical crack growth (SCG).1 This implies that, at stress intensities lower than the critical value (the fracture toughness), a crack is extended by bond breaking reactions with a reactive species, in most cases H2O. For silica glass, showing the same susceptibility, it has been shown that an agent must have two structural characteristics to promote SCG by a bond-breaking reaction.2,3 Like H2O, it must be able to donate a proton, or another small positively charged fragment, like a methyl radical in the case
a)
Present address: Philips Centre for Manufacturing Technologies, POB 218, 5600 MD, Eindhoven, The Netherlands. b) Address all correspondence to this author. J. Mater. Res., Vol. 16, No. 12, Dec 2001
http://journals.cambridge.org
Downloaded: 15 Mar 2015
of methanol. Besides this, it must have a lone electron pair, enabling it to perform a nucleophilic attack on the cations in the material. The second explanation for this effect is a decrease in surface energy caused by adsorption on the “fresh” fracture surface.4,5 This will lower the fracture toughness and strength. This mechanism has in the past been proposed for the fracture of hexaferrites.6 The only requirement for activity in this mechanism is a good adsorption on the fracture surface. The relevance of these two
Data Loading...
