In MatSciEng: New Materials in Health and Safety

The MSDS diamond, including flammability, health, reactivity, and protective equipment information.

Despite best efforts and intentions, design and engineering for the lifetime of materials are sometimes incomplete.  In those cases, new materials can be introduced to refine and supplement the old materials or correct them entirely.  When health and safety are at risk, such new materials become the focus of international research and development effort.  In this post I've written about some new work and on-going investigation surrounding new materials in health and safety.

Nuclear Waste Storage in Clay 

A great deal of work is constantly on-going to study and monitor the safe storage of radioactive isotopes.  Care of ScienceDaily, Reich et al from the Gutenberg University Mainz have found that, in addition to other natural materials, natural clay materials can be used as storage for nuclear waste.  More specifically, it has shown that Opalinus Clay (a "Jurassic claystone" or weak mudstone) has the ability to sorb radioactive Plutonium (Pu) and Neptunium (Np) of select oxidation states from aqueous solutions.  For example, Neptunium, "...hardly diffuses through the clay, and even after a month is still almost where it started."  Longer investigations are always required when studying nuclear waste storage; for example, the radioactive half-life for Neptunium is 2.14 million years.

Replacement Coatings of Hexavalent Chromium (Cr)

Chromium metal exists most most commonly in the hexavalent oxidation state.  Hexavalent chromium is also a known human carcinogen via inhalation.  As a result, there is a great deal of motivation to find a replacement metal with similar properties, mechanical and otherwise.  However, the most common forms of hexavalent chrome are coatings - either in the electrolytic hard chrome (EHC) or decorative form - which possess a unique set of properties not easily replaced.  While some solutions do exist as partial replacements, a great deal of work is still on-going in aerospace, defense, and commercial markets to find a complete replacement for hexavalent chrome.   

Nanofiber Materials to Detect Chemical Hazards 

In a recent issue of the journal Advanced Materials [1] (also at ScienceDaily), Kelly et al have used porous Silicon (Si) filters as templates for the fabrication of carbon nanofibers.  These nanofibers have been demonstrated as sensors for hazardous chemical compounds.  More specifically, the nanofibers act as sensors for organic vapours which can cause neurological harm upon inhalation.  In application, such nanofibers can add supplementary information on whether activated charcoal air filters have expired or otherwise ceased to provide protection.

Composite Materials Extract Contaminants from Drinking Water 

Also over at ScienceDaily, a new composite 'multiphase' material has demonstrated the ability to extract radioactive and hazardous impurities - namely, radioactive iodide and arsenic - from drinking water.  Engineered by Drs. Pawlak and Venditti from NCSU, the material is made from natural and biodegradable material components hemicellulose and chitosan.  In addition, the internal structure of the 'multiphase' material is a foam, which possesses an increased surface area which likely helps increase the extraction efficiency.

Replacement of Cadmium (Cd) Metal Coatings

Similar to the work done to replace hexavalent Chromium, Cadmium (Cd) metal is also a target for replacement.  Cadmium is applied to a wide range of steel parts requiring corrosion protection, ranging from simple nuts and bolts to high-performance aircraft and defense components.  However, Cadmium itself is a toxic metal which can be released from the parts to the environment when in use.  Like Chromium, Cadmium has a unique set of material properties such as lubricity and contact resistance, as well as good corrosion resistance.  However, also like Chromium, complete Cadmium replacement still requires work as suitable material replacements have not been developed to adoptable levels.  Groups such as the Joint Cadmium Alternatives Team (JCAT) continue to drive study in this area.   

Nano-Materials can Detect and Neutralize Explosives 

Returning to ScienceDaily, work done by Prof. Apblett at the Oklahoma State University has focused on chemical-based sensing of explosives.  Apblett et al have sprayed thin coatings of catalytic Molybdenum (Mo) oxide nano-particles on various surfaces; in the presence of hydrogen peroxide-based explosives, the coatings undergo subsequent changes in colour and conductivity, both of which can be measured.  Adding additional spray, which reacts with the peroxide vapour, the coating can also disarm the explosive.  Additional studies performed by Apblett et al on Molybdenum oxides include sorbants for toxic metals, heavy metals, and radionuclides [2,3].

[1] T.L. Kelly et al. Carbon and carbon/silicon composites templated in rugate filters for the adsorption and detection of organic vapors.  Advanced Materials, Vol. 23 (2011), pp. 1776-1781.
[2] M. Chehbouni and A.W. Apblett. Molybdenum-oxide based sorbants for toxic metals.  Ceramic Transactions, Vol. 176 (2006), pp. 15-23.
[3] B.P. Kiran et al. Selective absorption of heavy metals and radionuclides from water in a direct-to-ceramic process.  Ceramic Transactions, Vol. 143 (2003), pp. 385-394.

Processing Limitations in Nano-Materials

Example Nanomaterials from Talapin et al at the University of Chicago (link).

In the recent issue of the journal Nanotechnology (also covered at ScienceDaily), Kelly [1] has written a very interesting article concerning processing limitations of some small-scale engineering designs and constructions.  From ScienceDaily:

The overall goal when entering nanotechnologies into the market is low-cost, high-volume manufacturability, but at the same time, the materials' properties must be highly reproducible within a pre-specified limit, which Kelly states cannot happen below the 3 nm limit when trying to make arrays. 

The top-down approach to manufacturing, which Kelly states is limited, uses external tools to cut and shape large materials to contain many smaller features.  Its alternative, the bottom-up approach, involves piecing together small units, usually molecules, to construct whole materials -- much like a jigsaw puzzle -- however this process is too unpredictable for defect-free mass production of arrays.

As outlined by Kelly, standard manufacture operates under what is called 6-sigma yield - which essentially means there is a manufacturing defect rate of only 0.00034%. Those extremely tight tolerances are simply not met by the respective manufacturing processes of most nanotechnologies.  However, there can be two ways to (re)consider these limitations.

The first is that - given the need for the technology - the scales of production will follow.  Moore's Law is a well-known example.  In short, due to continuous improvement in fabrication equipment and manufacturing technology - much of which was not around when Moore's "Law" was first proposed - the number of transistors possible on a single chip has doubled every two years from the 1970's to the present day.  Based on this, it is reasonable to propose that additional improvements in manufacturing technology will help raise future nanotechnologies out of the "unmanufacturable" category.     

The second way to consider these limitations is with reference to another complex system - the human body - which also possesses a great deal of complex functionality and manufacturing.  For example, our own DNA replication and copying processes would not meet 6-sigma yield if not for clever DNA proof-reading and correcting mechanisms.  In addition, some degree of DNA error is manageable because redundancy exists.  Redundancy might not sound like an ideal situation for large-scale fabrication, but it is a useful tool to help maintain proper system functionality even if a small percentage of components are defective.  And, thermodynamically speaking, if there will be an unavoidable amount of error, then redundancy may be the path of least resistance.  Furthermore, if you accept defects will exist - if not from manufacturing, then due to in-situ operation - the design process changes from a defect-free nanotechnology to one which may perhaps even benefit from a certain concentration of them - much like the way doped semiconductors operate and interact.  But of course, this may not be possible for some nano-materials and their applications, e.g. quantum dots requiring precise confinement energies.

Ultimately, as Kelly writes, some nanotechnologies are unfortunately bound to remain on a lab bench:

Many results in nanoscience can be shown to be intrinsically unmanufacturable in terms of ideas for applications in electronic or optoelectronic components, and so will remain as scientific curiosities.

Time will tell which are the most manufacturable and which are so in a sustainable manner.  

[1] MJ Kelly. Intrinsic top-down manufacturability. Nanotechnology, Vol. 22 (2011), 245303.

Nanomaterials in Construction

My previous post painted a general overview of the potential of materials science and engineering, such as in building science and sustainability.  In this post, I'll be presenting a few examples.

Via ScienceDaily: Researchers from Rice University and UCLA have recently reviewed examples combining nanomaterials and construction materials, or Manufactured NanoMaterials (MNMs).  A few of the many examples provided in their 2010 paper include (see [1], below): Carbon nanotubes in concrete for mechanical durability and crack prevention; Titanium dioxide nanoparticles in cement for self-cleaning properties, and in glass for solar energy-collecting properties; Iron oxide nanoparticles in concrete for abrasion resistance; Copper nanoparticles in steels for corrosion resistance; and silver nanoparticles in paintings for antibacterial properties.

Before full-scale application can be implemented, a number of issues must first be addressed.  In [1], for example, the authors outline "12 Principles of Ecologically-Responsible Construction Nanotechnology", which aim to address valid concerns such as MNM toxicity, carcinogenicity, contamination, disposal, and waste.  Also important to consider is the cost of these materials.  It has been suggested (see [1]) that that cost of MNMs will decrease due to 1) small additive ratios of the nanomaterial components, 2) further general development of nanomaterials, and 3) increase in production quantities.

In short, at the research level there is a great deal of nanomaterials science and engineering for construction, and there is a long list of potential as well as demonstrated advantages offered by MNMs.  Reviews on the subject include:

[1] J. Lee, S. Mahendra, and P.J.J. Alvarez.  Nanomaterials in the Construction Industry: A Review of Their Applications and Environmental Health and Safety Considerations.  ACS Nano, Vol. 4 (2010) pp. 3580-3590.
[2] Z. Ge and Z. Gao.  Applications of Nanotechnology and Nanomaterials in Construction.  First International Conference on Construction in Developing Countries (ICCIDC-I) (2008), pp. 235-240.  (Available On-Line.)
[3] W. Zhu, P.J.M. Bartos, and A. Porro.  Application of Nanotechnology in Construction: Summary of a State-Of-The-Art Report.  Materials and Structures, Vol. 37 (2004) pp. 649-658.