Demystifying metal whiskering

In 2005, a nuclear reactor at Millstone Power Station in the US was unexpectedly shut down. Technicians were stumped as to what the culprit might be, until they spotted under a microscope a thin metal filament spanning the surface of a computer card and bridging a conductive material. It was this filament that had shorted the circuit, triggering a false pressure signal that eventually shut down the reactor.

This was not the first time such filaments – nicknamed “whiskers” – had been observed. It was reported in1946 by H. L. Cobb that many electrical components fabricated during WWII failed due to whiskers protruding from the surface of electroplated cadmium. Nor was it the first incident whiskers had been held culpable for: they had crippled commercial satellites, military equipment, automobiles, medical equipment and computer servers and routers. Indeed, for their commonness and unpredictability, National Aeronautics and Space Administration of US has been tracking down metal whiskers for over seven decades. Many companies, e.g. the European Space Agency and the Compound Semiconductor Applications Catapult in the UK, have special teams devoted to mitigating the whisker plague. Notwithstanding, the physical mechanism for their formation and growth remains poorly understood. Wikipedia lists metal whiskering as a major unsolved problem in condensed matter physics, alongside High-T_c superconductors and amorphous solids.

Whiskers occur in many metals, including cadmium, tin, zinc, gold, silver, copper, aluminium, lead and indium as notable examples. They spontaneously sprout out after an apparently unpredictable incubation period, which could last from minutes to years even for the same metal. They grow on films as well as bulk metals. The growth is propped up by addition of atoms, which are transported across the sample and not just from nearby, to its root rather than to its top. In other words, a whisker emulates epitaxial nanowire growth, but with the growth surface inside the metal and pushing out of the host in the form of a single crystal.

Hitherto there is no quantitative understanding of metal whiskering, though a mixture of scenarios have been forwarded, mostly in the 1950s. Many of them, such as those based on crystal dislocations, have been firmly refuted by experiments. Currently, the most prevalent is the stress-relieving scenario, which hypothesizes that whiskers grow to relieve compressive stresses accumulated in a metal, in much the same way as a beam of water gushes out from a hole on a water bag being squashed. Though permeated, this mechanical scenario contradicts numerous empirical observations, such as the growth on stress-free bulk metals and even in samples under tensile stresses. In fact, stresses around a whisker root are random and tiny.

This studentship aims for a thorough quantitative understanding of the physical mechanism for the initiation and growth of metal whiskers, which will, in the ensuing years, provide a reliable physics-based whisker-elimination method. The research shall drastically diverge from the beaten track and take a hitherto untaken road ushered by recent work of the supervisor. It is strongly motivated by this fundamental question: why do whiskers grow on pure metals but not on pure non-metals? The hypothesis shall be investigated that spontaneous growth of whiskers originates from an electronic anomaly that ultimately fuels a crystallization process and launches out atoms to form whiskers. This novel electronic scenario, in contrast with any existing scenarios, distinguishes metals from non-metals and provides a potential unified explanation of empirical facts. Some preliminary studies have been carried out by the supervisor’s research group and they lend strong support to the hypothesis.

The studentship presents a unique opportunity for the candidate to pursue this ambitious project and nurture their research skills, broaden knowledge and further their career either in the academia or industry (auto, electronics, space, medical, etc.).

How to apply:

Applicants should apply to the Doctor of Philosophy in Physics and Astronomy with a start date of 1st January 2025. 

Applicants should submit an application for postgraduate study via the Cardiff University webpages (https://www.cardiff.ac.uk/study/postgraduate/research/programmes/programme/physics-and-astronomy) including: 

• your academic CV 

• Your degree certificates and transcripts to date including certified translations if these are not in English 

• two references, at least one of which should be academic. Your references can be emailed by the referee to physics-admissions@cardiff.ac.uk  

Please note: We are do not contact referees directly for references for each applicant

 • personal statement (as part of the university application form, or as a separate attachment, if you prefer. It has to provide a clear explanation of your research interest, preparation undertaken, and an understanding of the project. 

Your personal statement should be no more than 500 words, and address the following questions:

1. What are your scientific research interests and ambition? 

2. How has your academic and/or professional journey prepared you for PhD study? (for instance, give examples of work you particularly enjoyed, of challenges you overcame, of connecting with others about your work or ideas, of showing inventiveness, of developing new skills and knowledge)

3. Why do you think this project is important?”

The typical academic requirement is a minimum of a 2:1 physics and astronomy or a relevant discipline. 

Applicants whose first language is not English are normally expected to meet the minimum University requirements (e.g. IELTS 6.5 Overall with 5.5 minimum in sub-scores)

In the “Research Proposal” section of your application, please specify the project title and supervisors of this project.

In the funding section, please select that you will not be self-funding and write that the source of funding will be EPSRC. 

Once the deadline for applications has passed, we will review your application and advise you within a few weeks if you have been shortlisted for an interview. 

Eligibility :

EPSRC DTP studentships are available to home and international students. Up to 30% of our cohort can comprise international students, once the limit has been reached we are unable to make offers to international students. International students will not be charged the fee difference between the UK and international rate. Applicants should satisfy the UKRI eligibility requirements.