Thompson Research Group |
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PROFESSOR THOMPSON’S RESEARCH PROGRAMS Tailoring Ceramic Microstructures The rock-salt structure TaC phase has a melting temperature near 4000 deg C and is one of the highest melting temperature materials known. This property makes it a candidate material for several ultra-high temperature structural applications. Near 2000 deg C, TaC exhibits a brittle-to-ductile transition and has been reported to exhibit ductility >30%; which is remarkable considering TaC is nominally considered a ceramic. By depleting the carbon content, the precipitation of a hexagonal Ta2C phase can occur. In a two-phase TaC+Ta2C region, a basket-weave microstructure can be processed offering several possibilities in tailoring the mechanical properties, such as fracture toughness, of this high temperature ceramic. To bring these materials into fruition, basic relationships between phase content, microstructure, constitutive behavior and macro-properties, such as elevated temperature strength and creep behavior, must be understood. To accomplish these goals, this program is providing atomic-scale to micron length-scale microscopy characterization of thermo-mechanical processed tantalum carbides. This work is supported by ARO- W911NF-08-1-0300.
Orientation Imaging Microscopy-based images showing the phase distribution of Ta2C and Ta4C3. I Atom probe map reconstruction showing an iso-concentration surface of Fe segregated to the grain boundary in a Ta(Hf)C specimen.
A 3D reconstruction a vacuum plasma sprayed tantalum carbide microstructure. The blue outline is a TaC grain and the grain laths are Ta2C precipitate phases. The multi-color, smaller spheres are pores. The 3D visualization allows us to understand how the precipitate phases form and evolve with processing within the TaC matrix. The reconstructed rendering was generated by serial section milling using a Focus Ion Beam (FIB). Magnetic-based Materials Research Professor Thompson is an active member of the multi-disciplinary Center for Materials for Information Technology (MINT, www.mint.ua.edu). His research group collaborates with several of the MINT faculty in magnetic-based research efforts. This offers ample cross-departmental interactions for Professor Thompson’s group working in multi-disciplinary team environments to solve next-generation magnetic-based material science and engineering challenges. The Materials Science of FePt: As a magnetic volume decreases in size, thermally induced fluctuations in the magnetization can occur; a phenomenon called superparamagnetism. For current magnetic materials, this ‘superparamagnetic barrier’ is rapidly being approached for any further reduction in the hard-disk drive bit size. To continue the bit size reduction, while maintaining stable magnetization for information storage, materials with high magnetocrystalline anisotropy is required. The L10 phase of FePt, shown below, is a candidate material for next generation magnetic storage because it’s high magnetocrystalline anisotropy. When FePt is processed, either as a thin film or self-assembled array of nanoparticles, the structure it nominally adopts a random solid solution face-centered-cubic (fcc) phase, A1, which is magnetically soft. A subsequent anneal at temperatures >500oC facilitates the A1 to L10 phase transformation. Unfortunately, annealing results in grain growth in thin films or sintering in nanoparticles destroying the narrow nanometer size distributions necessary for ultra-high areal storage density architectures.
The crystal structure phase transformation between A1 to L10 in FePt Currently, Professor Thompson’s FePt research addresses the following areas: 1) The influence of atomic segregation in the L10 ordering transformation and grain growth behavior (supported by NSF-CAREER-DMR-0547445) 2) Rapid thermal annealing to determine the time scales of ordering and grain growth (supported by MINT). 3) The origins of how compositional distributions develop during the synthesis of magnetic nanoparticles (FePt) and means to control the nucleation and growth sequence to minimize variations (supported by NSF-MRSEC-DMR-0213985). FePt-Research Highlights: (Please consult the ‘publication’ link for detailed, peer-reviewed results of these research efforts) Professor Thompson’s group has been able to provide experimental verification of Pt segregation to grain boundaries in as-deposited A1 FePt, shown below. The atom probe reconstruction has been verified by Fresnel contrast imaging in the TEM (images taken from Torres et al. MRS Proceedings 2008). I
A series of FePt nanoparticles have been extracted at different stages of synthesis and the size and composition from individual particles have been quantified by (S)TEM The results of this research has been able to determine that certain synthesis routes, with different elemental precursors, can yield very different compositional and size distributions from particle-to-particle. We have been able to show that the formation of a Pt-cluster seed with the subsequent addition of Fe in a two-step nucleation procedure produces a tighter distribution in composition and size
Exchanged Coupled Composites (gradient media films): One of the challenges of using highly magnetically anisotropic materials as small, thermally bits is that the required fields to ‘write’ (switch the magnetization) of the bit is much greater than fields available in conventional heads. This research thrust is to produce nanoscale granular features in which one end of the volume is magnetically soft (low magnetic anisotropy) and the other end magnetically hard (high magnetic anisotropy). By switching the soft end, it would be possible to nucleate and propagate a domain wall and switch the hard end. The concept was coined as an ‘exchanged coupled composite’ and originally involved making distinct multilayered film stacks of hard and soft materials. Our approach is to grade the composition.
Professor Thompson’s group works on the processing and fabrication of the thin film gradients by magnetron sputtering. These films are structurally characterized by transmission electron microscopy and atom probe tomography to quantify, at the atomic/nano-level, the quality and stability of the compositional gradient. In order to quantify the magnetic properties, the nanoscale anisotropic graded structure must be magnetically isolated from each other to avoid exchange coupling. This patterning of tens of nanometer size-scale features over large areas in regular arrays is a formable challenge. Professor Thompson’s group collaborates with several MINT faculty in their efforts to provide this level of nanoscale patterning of these films. This research has been supported by NSF-MRSEC-DMR-0213985. Ii Thin film Growth and Structure: As a thin film nucleates and grows on a substrate, the film will undergo a compressive-tensile-compressive stress state. This results from the initial embryonic formation on the substrate (compressive), migration of clusters to reduce surface energy (tensile) and then the adatom growth at the grain boundaries (compressive), as shown below.
By measuring the local curvature of the wafer during the growth, the real-time, in-situ stress state can be measured. This measurement is done by measuring the shift in spatial position of a laser reflected off the growth surface and relating this shift to the stress state by Stoney’s equation.
Professor Thompson’s research group is investigating the influence of multi-component compositions on the stress state of thin films during growth and how segregation at grain boundaries control the stress state. Our recent results have shown that, as a function of composition, the film stress can be tuned to be either tensile, compressive or ‘zero-stress’ state. This research is supported by NSF-CAREER-DMR-0547445.
Heterostructures: Heterostructures are materials which comprise alternating layers of two or more different materials. At the interface of these composite structures, a large compositional gradient over just a few atomic layers can exist which provides for several unique driving forces for interfacial stability. With the ever growing impact of nanomaterials in devices, the role of interfacial stability between two materials has become increasingly important. Professor Thompson’s research provides fundamental materials characterization of the interfacial structure which is used to understand phase stability. By doing so, the research provides basic structural knowledge on how interfaces can be tailored for improved applicability. Heterostructured Magnetic Thin Films: Giant Magneto-Resistance (GMR) and Tunneling Magneto-Resistance (TMR) devices, commonly referred to as spin values, are magnetic sensors used in ‘reading’ bits in magnetic media hard-drives. The basic principle of operation is measurement of the electronic resistance change between two magnetic layers separated by either a nonmagnetic metallic spacer layer for GMR or an insulator oxide layer for TMR. The passage of the electrons between the two magnetic layers is by normal conduction for GMR and quantum mechanically tunneling for TMR. In each of these devices, the chemical abruptness of the interfaces in the thin film stacks is critical. Professor Thompson’s group works in collaboration with MINT to provide atomic scale analysis of these devices. The results of which are directly correlated to modeling predictions, property performance and the prediction of new materials or processes that can optimize these devices. Below is an example of a GMR stack. This research was supported by NSF-ECS-0529369 and MINT.
Phase Stability in Multilayer Thin Films: Several technically important devices, such as transistors, wear resistance coatings, magnetic sensors and photovoltaic devices, are based on the stacking of different thin film materials on top of each other. When the thicknesses are reduced, unique size effects can result. In some cases, these changes can be contributed to crystallographic transformations in the layers. These transformations can be rationalized and predicted using basic thermodynamic arguments between volumetric and interfacial free energy changes. As the thickness is reduced, the volume is reduced and consequently the contribution of the interfacial energy can be comparable, and even dominate, volumetric free energy changes. Using these thermodynamic arguments, predictive phase diagrams can be constructed that provide ‘phase regions’ as a function of the layer spacing, lambda, and volume fraction. Currently, this work has been developed for metallic multilayers.
High Temperature Shape Memory Alloys Shape memory alloys (SMA) are a unique class of materials which can
recover deformation induced at some lower temperature by heating through
a given transformation temperature. Consequently, this deformation recovery
operation can act as a source of work by having the material recover
against an applied load making them candidates for compact, low profile,
solid-state actuators and smart material applications. To date, the
exploitation of SMA is limited because the recovery phenomenon occurs
at low temperatures (<100oC). By increasing the transformation temperature,
SMA would have immediate usage in several higher temperature service
aerospace applications of interest to NASA. Previous research reports
have shown that macro-alloying NiTi with Pt, Pd, Hf, Zr and Au, can
increase the transformation temperature between 600oC – 1000oC
for particular elements and amounts. This alloying results in the precipitation
of several metastable nanometer-sized secondary phases. The phase and
microstructural evolution of these precipitates and their influence
on the shape memory effect are not fully understood or characterized.
To fully optimize the processing-properties-microstructure relationships
of high temperature SMA, we are conducting a detailed microscopy-based
study. The results of which will allow the materials to fully exploit
their properties. This program is providing unprecedented 3D atomic
level imaging of these materials using a recently acquired Local Electrode
Atom Probe at the University of Alabama (UA). The UA program (Professors
Thompson and Weaver) are collaborating with Dr. Ronald Noebe at the
NASA Glenn Research Center (GRC).
PROFESSOR THOMPSON’S AND GENERAL USER-BASED RESEARCH LABORATORIES Thin Film Processing and Annealing Laboratory: • AJA International ATC-1500 Sputtering chamber • Laboratory furnace (Tmax 1000 deg C) with encapsulation for inert/vacuum annealing
Specimen Preparatory Laboratory: • TEM preparation equipment:
Professor Thompson’s research group members are active users of UA’s Central Analytical Facility (CAF, www.caf.ua.edu). Professor Thompson was the Principle Investigator for a team of faculty members for the FEI Tecnai F20 Supertwin TEM (NSF-DMR-0421376) and laser attachment to the Imago Scientific Instruments Local Electrode Atom Probe, LEAP (NSF-DMR-0722631) housed in the CAF.
Professor Thompson is a member of the CAF external faculty board and has been part of faulty teams leading institutional infrastructure investments, such as the base-LEAP instrument and the FEI Quanta 3D dual electron and focus ion beam (FIB) instrument, at UA. Below are some of the tools available for use. Below is the suite of analytical microscopy equipment available in general shared user facilities at the University of Alabama. Professor Thompson’s group heavily uses several of these instruments.
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