[NewCandle] New prospective matrix material for high temp LENR

[Horace Heffner]

On Dec 28, 2008, at 10:06 AM, Keith Nagel wrote:

> Indeed. I too have wondered whether cermet materials could
> be used for LENR type reactions, or just as novel electrodes
> for hydrogen generation.
>
> http://en.wikipedia.org/wiki/Cermet
>
> I am not too up on the vanguard of materials science to know where
> the thesis you linked fits in with the field; I gathered from a
> quick scan of the paper that the ductility of the alloy was the
> novelty but there could be more. That kind of dendritic growth
> is common in alloys in general; I've been reading much on  
> ferromagnetic
> alloys and SEM photos often show the familiar frond shaped growths.
> But perhaps a cermet with the metal component forming dendrites
> is new??? Does that account for the ductility?
>
> K.

The advancement is in composite metallic glasses, not cermets.

 From Wiki:

"A cermet is a composite material composed of ceramic (cer) and  
metallic (met) materials. A cermet is ideally designed to have the  
optimal properties of both a ceramic, such as high temperature  
resistance and hardness, and those of a metal, such as the ability to  
undergo plastic deformation. The metal is used as a binder for an  
oxide, boride, carbide, or alumina. Generally, the metallic elements  
used are nickel, molybdenum, and cobalt. Depending on the physical  
structure of the material, cermets can also be metal matrix  
composites, but cermets are usually less than 20% metal by volume."

My impression, from the various examples given, is that cermets have  
high resistance, and thus may not make good cathodes. "The metal is  
used as a binder for an oxide, boride, carbide, or alumina."  They  
are not metallic glasses and don't appear to be amenable to cold  
rolling and thermoplastic forming.  Their main application appears to  
be high temperature environments.

The glasses developed act more like metals than ceramics, and a low  
cost thermoformable glass replacement for metallic titanium is even  
available.  Starting on page 121:

"6.2 Background on Nanostructure-Dendrite Composites

Recently, titanium-based nanostructure-dendrite composites have been  
reported which are said to exhibit increased toughness over bulk  
metallic glasses (BMGs) and other nanostructured materials (see [1]).  
This work has spawned an
entirely new field of nanoeutectic-oriented metallurgy, with dozens  
of publications touting the benefits of such materials over BMGs  
(see, for instance, [2–14]). However, upon further investigation of  
this new field, we note that uniaxial compression tests are typically  
the only mechanical tests used to characterize the perceived  
toughness and enhanced plasticity of the nanostructured composites.  
The geometry of the compression test, where closing stresses and  
friction arrest slip, has resulted in the overstatement of toughness  
in these new materials. It appears that the nanostructure-dendrite  
composites are very similar to BMG composites, but with a eutectic  
matrix that failed to form a glass during rapid cooling. The  
crystallized matrix, or nanostructured eutectic as it is often  
called, is typically comprised of brittle [***** note the prior word!  
****] intermetallics, and fails with no apparent plasticity in  
bending or tension tests. When nanostructure-dendrite composites are  
loaded in an unconfined manner, as with the uniaxial tension test,  
cracks move unimpeded through the brittle matrix and global ductility  
is absent. In contrast, if the matrix is frozen as a glass, the  
length scale of the deformation is larger, allowing microstructural  
stabilization mechanisms to become possible. In the current work, we  
demonstrate several new titanium-based glass-dendrite composites, all  
of which exhibit at least 5 percent tensile ductility, low cost, and  
densities comparable to high-performance crystalline titanium alloys  
(4.97–5.15 g/cm3). We observe a remarkable similarity in mechanical  
properties (such as tensile ductility, yield strength, and fracture  
toughness) between the new glassy-composites and high-strength  
crystalline titanium alloys (such as Ti-6Al-4V) — all with a  
significantly lower Young’s modulus. The current work demonstrates  
that titanium-based BMG composites can be competitive with  
crystalline titanium alloys for structural applications where high  
strength and toughness are a necessity."

Page 123: "In the case where the glass matrix does not vitrify into a  
bulk glass, a brittle nanostructure-dendrite composite forms,  
accompanied by a large increase in G. Based on nearly two decades of  
research with BMGs, we note that a crystallized or partially  
crystallized metallic glass is always more brittle than a fully  
amorphous one."

"While the glass-dendrite composites have been shown to exhibit up to  
~ 13% tensile ductility [14], nanostructure-dendrite composites fail  
in an apparently brittle manner with no global ductility. Shear bands  
that form in the nanostructured eutectic matrix develop into  
catastrophic cracks at much shorter lengths than in BMGs, leading to  
failure on a continuous path through the matrix. The crystalline  
phases act as nucleation sites for shear bands, which typically form  
at lower stresses than in monolithic glasses. Despite claims of  
increased toughness and ductility in several publications on  
nanostructure-dendrite composites, tension tests are noticeably  
absent [1–14]. "

With this background, now to more specifically answer your questions.

The key to making high toughness composites with all the other  
required features is the creation of *soft* crystal dendrites, i.e.  
dendrites having a low shear modulus, lower than the glass in which  
they are imbedded.

P 34: "The advancement that allowed in-situ alloys to exhibit tensile  
ductility while ex-situ alloys do not is the understanding that the  
inclusions need to be softer than the glass matrix [23]. This  
fundamental concept was experimentally demonstrated in 2001, with the  
alloy Zr56.2Ti13.8Nb5Cu6.9Ni5.6Be12.5 (LM2), but the concept was  
largely overlooked."

Much of the rest of the work relates to how to form metallic glasses.  
The result is summed up on P. 14: "We have observed that the two most  
important criteria for bulk glass formation are deep eutectics and  
atomic size mismatch."

P. 133: "The low processing temperatures of the composites allow for  
rapid prototyping of parts and a cost savings over crystalline  
titanium alloys. "

Best regards,

Horace Heffner
http://www.mtaonline.net/~hheffner/