Frank Nabarro: The Architect of Crystal Defects
Frank Reginald Nunes Nabarro (1916–2006) was a titan of 20th-century condensed matter physics. A pioneer in the study of dislocations—the "mistakes" in the atomic lattice of solids—Nabarro provided the mathematical framework that explains why metals are malleable, how they fail under heat, and how they can be engineered for strength. His career spanned seven decades, during which he transformed the study of metallurgy from an empirical craft into a rigorous branch of theoretical physics.
1. Biography: From London to Johannesburg
Born on March 7, 1916, in London, Frank Nabarro was the son of a distinguished pathologist. He displayed early brilliance, attending Nottingham High School before winning a scholarship to Exeter College, Oxford. There, he achieved first-class honors in both Mathematics (1937) and Physics (1938).
His academic trajectory was shaped by his move to the University of Bristol to work under Sir Nevill Mott (who later won the Nobel Prize). It was here, in the late 1930s, that Nabarro began investigating the mechanical properties of crystals.
During World War II, Nabarro’s intellect was diverted to the war effort. He served in the Army Operational Research Group (AORG), where he applied mathematical modeling to ballistics and the effectiveness of anti-aircraft fire.
In 1953, Nabarro made a pivotal life decision: he moved to South Africa to become the Head of the Department of Physics at the University of the Witwatersrand (Wits) in Johannesburg. While many international scholars avoided South Africa during the apartheid era, Nabarro chose to stay, becoming a fierce advocate for academic freedom and transforming Wits into a global powerhouse for solid-state physics. He remained in South Africa until his death on July 20, 2006, at the age of 90.
2. Major Contributions
Nabarro’s work centered on the "imperfections" of crystals. While early 20th-century physics assumed crystals were perfect lattices, Nabarro proved that their real-world behavior is dictated by their defects.
The Peierls-Nabarro Stress
Building on the work of Rudolf Peierls, Nabarro developed a mathematical model to calculate the force required to move a dislocation through a crystal lattice. This "Peierls-Nabarro stress" explains the inherent friction an atom feels as it slides past another. This remains a fundamental concept in understanding the plasticity and hardness of materials.
Nabarro-Herring Creep
In 1948, Nabarro proposed a radical theory (later refined by Conyers Herring) regarding how materials deform at high temperatures under low stress. He suggested that atoms actually migrate (diffuse) through the bulk of the crystal grains. This process, known as Nabarro-Herring Creep, is essential for aerospace engineering, particularly in the design of jet engine turbine blades that must withstand extreme heat without stretching.
Precipitation Hardening
Nabarro provided the first theoretical treatment of how "misfit" between different types of crystals in an alloy (like copper in aluminum) creates internal strain that prevents dislocations from moving, thereby hardening the metal.
Mathematical Theory of Dislocations
He was among the first to apply rigorous topology and elasticity theory to defects, treating dislocations not just as physical gaps, but as mathematical singularities in a field.
3. Notable Publications
Nabarro was a prolific writer known for his clarity and encyclopedic knowledge.
- The Mathematical Theory of Stationary Dislocations (1947): A foundational paper that laid the groundwork for modern physical metallurgy.
- Deformation of Crystals by the Motion of Single Ions (1948): The seminal paper introducing what would become Nabarro-Herring Creep.
- Theory of Crystal Dislocations (1967): This 800-page monograph is widely considered the "Bible" of the field. It consolidated decades of research into a single, definitive volume.
- Dislocations in Solids (Series Editor, 1979–2006): Nabarro edited this multi-volume series (reaching 12 volumes by his death), which serves as the comprehensive record of all progress in dislocation research.
4. Awards and Recognition
Nabarro’s contributions were recognized by the world’s most prestigious scientific bodies:
- Fellow of the Royal Society (FRS): Elected in 1971.
- The Beilby Medal and Prize (1950): For his work on plasticity.
- The De Beers Gold Medal (1980): South Africa’s premier physics award.
- The Claude Harris Leon Foundation Award (1983).
- Order of Mapungubwe (Silver): Awarded posthumously by the South African government for his:
"excellent contributions to the field of physics and for his role in developing the sciences in South Africa."
- Honorary Doctorates: Received from the University of the Witwatersrand, University of Cape Town, and the University of Natal.
5. Impact and Legacy
Frank Nabarro is often called the "Father of Dislocation Theory." Before his work, the strength of materials was largely a matter of trial and error. Because of Nabarro, engineers can now predict how a new alloy will behave under stress, heat, or radiation.
Beyond his equations, his legacy lives on in the "South African School" of Physics. By choosing to build a world-class research center in Johannesburg, he ensured that South Africa remained connected to the global scientific community during its years of isolation. He mentored generations of physicists who went on to lead departments worldwide.
6. Collaborations
Nabarro was a highly collaborative researcher who bridged the gap between British and international science:
- Sir Nevill Mott: His mentor at Bristol, with whom he explored the fundamental limits of metallic strength.
- Conyers Herring: Their independent work on atomic diffusion merged to create the Nabarro-Herring Creep model.
- Alan Cottrell: A contemporary with whom he frequently corresponded to refine the theory of how dislocations interact with impurities (Cottrell atmospheres).
- The "Wits Group": At the University of the Witwatersrand, he collaborated with figures like J.H. van der Merwe, contributing to the theory of epitaxial growth (how thin films grow on surfaces).
7. Lesser-Known Facts
- The Musical Physicist: Nabarro was a gifted cellist. He often remarked that the discipline required for music was similar to that required for mathematical physics. He was a regular attendee and supporter of the chamber music scene in Johannesburg.
- Academic Activist: During the height of Apartheid, Nabarro used his position as a Fellow of the Royal Society and a senior university official to protest the "Extension of University Education Act," which sought to segregate South African universities. He fought to keep Wits open to students of all races.
- Working to the Last Breath: Nabarro never truly retired. He was known to spend eight hours a day in his office well into his late 80s. He was actively editing Volume 13 of Dislocations in Solids and responding to correspondence from young researchers just days before he passed away in 2006.
- The "Nabarro Stress" in Daily Life: He once joked that:
"while he spent his life studying stress in crystals, the most difficult stress to model was the administrative stress of running a university department."