Non-Equilibrium Materials (NEMat)
Research Experience for Undergraduates (REU)

Research Projects


Materials Far From Equilibrium

  1. High Capacity Nanocomposite Sulfide Glass Anodes for Lithium Batteries (Steve W. Martin) - Experimental
  2. Polymer Matrix Composites in Extreme Environments (Mike Kessler) - Computational
  3. Surfaces of Glassy metallic Alloys (Pat Thiel) - Experimental/Computational

Structures Far From Equilibrium

  1. Atomistic Simulation of Dislocations (Richard LeSar) - Computational
  2. Bio-Inspired Materials (Ashraf Bastawros) - Experimental
  3. Well Positioned, Highly Ordered DNA from a Capillary-Held Solution (Zhiqun Lin) - Experimental

Properties of Materials Far From Equilibrium

  1. Dielectric Properties of Perovskite Oxides with Non-Equilibrium Structures (Xaoli Tan) - Experimental
  2. Chemical synthesis of isotropic electromagnetically-active composites (Nicola Bowler) - Experimental
  3. Characterization of Metallic Glass Composites (Ersan Ustundag) - Computational

Materials Processing Far From Equilibrium

  1. Characterization of Rapidly Solidified Metallic Systems (L. Scott Chumbley) - Experimental
  2. Fundamental Properties of Crystal-Melt Interfaces (Ralph E. Napolitano) - Experimental
  3. Wear Resistant Boride Composites (Alan Russell) - Experimental

Materials Far-From Equilibrium

Figure 1. Comparison of the reversible capacities for various oxide anodes to that of our new SnS + GeS2 sulfide glass. The dramatic increase in the capacity compared to the oxide anodes by almost a factor of 3 in some cases is observed

1. High Capacity Nanocomposite Sulfide Glass Anodes for Lithium Batteries (Steve W. Martin) - Experimental

Rechargeable lithium batteries are comprised of three main components, an anode that stores the lithium in its charged state, a lithium ion conducting separator that serves to electrically insulate the anode from, yet provides a fast Li+ conduction pathway to, the cathode.

Reversible high capacity anodes have severe property requirements since they must be the storage system for the unreacted fuel that will power the battery. So far it has not been possible to use pure metallic Li anodes without many side reactions taking place, so one attempts to get closer to metallic Li to be able to safely store more energy in the battery. This is the reason why there is so far the unbroken paradigm of low energy density combined with good safety and good recyclability. The less energy there is stored, the less side reactions that take place and the more reversible is the battery.

In this research project, the hypothesis is being explored that the significantly higher conducting sulfide glasses (compared to oxide glasses) would be able to conduct significantly more Li+ into the glass structure and thereby significantly increase the energy density of the anode. SnS +GeS2 glass samples have been prepared where the concentration of the “active” SnS phase could be maximized in a sulfide glass that is very stable in air (most sulfide materials being chemically unstable compared to their oxide analogues). Figure (1) shows the capacity data of these glassy anodes.

These new sulfide glass anodes exhibit two important characteristics. First, the steady state capacity is nearly double that of the commonly used graphite carbon anodes, and second, compared to other inorganic anodes, the stability of the capacity with cycling (reversibility) is significantly higher. The cycling data are shown in Fig. (1) where the cyclic capacity of the SnS doped GeS2 glass anode is compared to the best commercial anodes known. Not only do the new GeS2 glassy anodes show the highest capacity, significantly higher that the highly commercialized and extremely well optimized carbon anodes, these materials also show the most reversible behavior of all anodes.

Example REU Experience:
The REU student will prepare, characterize, and then perform the electrochemical battery testing measurements on new chalcogenide glasses. The glasses will be prepared by weighing out, mixing by milling, melting, and casting, weeks 1-3. Basic glass characterization measurements, measuring the glass transition temperature, the density, and Infrared and Raman spectroscopy will be measured, weeks 4-6, to develop knowledge of the physical properties and chemical structure of the glass. Weeks 7-9 will be used to perform the electrochemical characterization measurements of the glasses.

2. Polymer Matrix Composites in Extreme Environments (Mike Kessler) - Experimental

Fiber-reinforced polymer matrix composites (PMCs) are used in a wide variety of applications because of their high strength-to-weight ratios. PMCs play an important role in developing lighter, stronger, and more energy efficient systems .  Often these systems are subject to severe environments, where the inherently complex material response of PMCs over time may limit their effectiveness. The matrix materials used in thermosetting PMCs, which are amorphous polymer glasses, operate far from thermodynamic equilibrium.  Residual stresses developed during cure and physical aging affect various properties related to the density and strength in the material.  Understanding and controlling the complex structure-property-processing relationships for these non-equilibrium materials (especially under extreme conditions near the materials’ glass transition temperatures) is at the heart of the research efforts.  Recent research has focused on thermosets created through the cyclotrimerization of cyanate ester resins which can be designed to have a combination of high temperature stability and good processability.

Often, high-temperature polymer matrix composites are reinforced with continuous carbon-fibers.  The thermal expansion mismatch between the carbon-fiber reinforcement (αaxial ~ –0.5×10–6 K–1) and the polymer matrix (α ~ 20×10–6 to 100×10–6 K–1) is severe and causes significant thermal stresses in the matrix when temperatures fluctuate.  The thermal expansion mismatch also results in substantial residual stresses during cure which may lead to significant part warpage and matrix microcracking.  These residual thermal stresses, which contribute significantly to ultimate composite strength, exist both within isolated unidirectional laminae (intralaminar) as well as those that are caused by laminating together plies of different fiber orientations (interlaminar). 

A reduction in scale of the reinforcement from the micron to nanometer level serves to reduce the thermal stresses in the matrix by reducing the magnitude of the thermally induced displacements at the filler/matrix interface and distributing the thermal stresses more evenly through the matrix.  Since nano-reinforcements often exhibit better reinforcement than conventional fiber composites at the same volume fractions, the matrix can be efficiently modified without high filler content.     

Example REU Experience:
In this REU project, cyanate ester-based composites reinforced with two different kinds of nanosize particles: nano-particulate zirconium tungstate (ZrW2O8) and multiwalled carbon nanotubes (CNT) will be investigated.  Zirconium tungstate exhibits a negative thermal expansion (NTE) over a wide range of temperatures that is isotropic, reversible, and relatively large in magnitude (α = –9×10–6 K–1), while the CNTs display unprecedented strength and stiffness, high aspect ratios, and slight anisotropic NTE behavior.  It is expected that the addition of these nano-particles to cyanate esters will drastically reduce residual thermal stresses, increase dimensional stability, improve thermal properties (such as Tg), and improve mechanical properties. The REU student will conduct three primary tasks in her/his research: weeks 1-3, matrix selection and characterization; weeks 4-6, processing and characterization of cyanate ester nanocomposites; and weeks 7-9, design, processing, and testing of macro-nano composites.  Through the project, several fundamental scientific questions will be addressed relating to how the incorporation of nano-particles with NTE affects the processing properties of the BECY based resins and the resulting overall thermomechanical behavior of the composites.

3. Surfaces of Glassy metallic Alloys (Pat Thiel) - Experimental/Computational

Surfaces of metallic alloys are of tremendous technological importance since they determine and limit a material’s response to its environment in many situations. In recent years, special interest has developed in the surfaces of alloys that are structurally and chemically complex, because they have sometimes been reported to exhibit unusually good oxidation resistance, low adhesion, and/or low friction , . In this project, we propose to engage REU students in studies of surfaces of glass-forming metallic alloys. The research goal is to understand whether the disordered atomic arrangement affects the fundamental response of the clean surface to its environment.  The educational goal is to encourage students to understand materials at the nanoscale by using techniques and approaches that provide this exciting level of information, particularly scanning tunneling microscopy (STM).

New studies of the surfaces of metallic glasses will be conducted that are motivated by a new technology developed to synthesize porous metallic glass monoliths with extremely high surface areas , . The synthesis process involves interaction between the glass and a so-called fugitive metallic phase, which creates the pores but is eventually removed. Clearly, both the synthesis process and the final properties are heavily dependent upon surface and interfacial phenomena.

Metallic glasses pose new challenges because they cannot be heated above the glass transition temperature (Tg) without causing irreversible changes in bulk and surface structure. Hence, the approach is to lightly clean the nascent glass, characterize its properties below Tg, then compare its properties after heating it above the crystallization temperature. This affords an excellent opportunity to compare surface properties of glasses and crystalline surfaces within a single sample. Studies of surfaces of metallic glasses are very rare to date, and so the topic seems to hold great opportunities for fundamental discoveries. Ultimately, the goal is to study a variety of transition metal films and determine their effect on the large-scale morphology of the surface upon devitrification, since this is key to the synthesis of the porous monoliths.

Example REU Experience:
In weeks 1-3, the students will have hands-on learning experiences with the ultrahigh vacuum equipment. They will work closely with a graduate student because of the technical complexity of the equipment. They will then have the opportunity to collect, weeks 4-6, and then analyze, weeks 7-9, STM data. It is anticipated that they will earn co-authorship on a research paper. This group is highly interdisciplinary within the University so the students will be exposed to an environment that is enriched by regular interactions with chemists, mathematicians, materials scientists, and physicists. Furthermore, they will be exposed to a scientific environment in which women typically play a major role, where for instance, 3 out of 5 of the group members are women (including the PI)

Structures Far-From-Equilibrium

1. Atomistic Simulation of Dislocations (Richard LeSar) - Computational

Dislocations are curvilinear non-equilibrium defects whose motion “carries” deformation through a lattice.  As deformation proceeds, the number (i.e., the total length) of dislocations increases and complex microstructures are formed.  Despite 70 years of studying dislocations, a detailed understanding of dislocation microstructure evolution and its relation to the mechanical properties of a material eludes us.  Recent advances in the experimental study of the 3-D distribution of dislocations as well as new capabilities for direct dislocation simulations  offer a new approach for both better understanding evolving dislocation microstructures as well as for designing and creating materials with a desired microstructure.

In this research, detailed 3-D simulations at the dislocation level will be used to map out the distribution of dislocations and the consequent materials response. Since the output of the simulations includes the position of all dislocation lines, that “data” will be used to develop key measures of the microstructure.  From the modeling results, features of the dislocation distributions that are key to the mechanical response will be identified and new theories and models of materials deformation developed. 

Example REU experience:
Atomistic simulations, such as molecular dynamics (MD) or Monte Carlo, are now commonplace – excellent computer codes are freely available and easy to use.  Past experience has shown that projects based on such simulations are well within the grasp of undergraduates and are an excellent learning tool for creating a deeper understanding or many issues in materials science.  A set of projects, based on MD simulations, will provide information that is essential for the projects described above.

The dislocation-level simulations require detailed atomistic-level models of certain key parameters.  For the purpose of this project, we will focus on calculating various material parameters (e.g., structure, elastic moduli) and properties of dislocations (e.g., mobility).  The overall goal of the project is for the student to use MD to examine the temperature and strain dependence of these properties in face-centered cubic metals. The student will be trained in a series of well-defined steps in the basic use of the methods, followed by a systematic determination of material properties. 

In weeks 1-2, the student will be introduced to computer simulations, computing in a cluster environment, and will be given computer codes with which to run trial calculations.  In weeks 3-4, the student will perform a series of atomistic simulations to calculate thermodynamic and structural properties of simple metals, using empirical potentials. In weeks 5-6, the student will use the atomistic code to model simple fracture at an atomic level (see, for example  [2]), monitoring dislocation development.  In the last two weeks of the study, the student will employ the parameters they determined in a large-scale dislocation dynamics simulation of plastic deformation.  All computer codes will be made available to the student.

2. Bio-Inspired Materials (Ashraf Bastawros) - Experimental

Biological materials such as dentin, bone, and shells are essentially molecular composites comprised of soft phase (SP) proteins and ultra-high strength phase (UHSP)  bio-minerals. As organic-inorganic hybrid composites, natural materials show far superior mechanical properties such as stiffness and fracture toughness, as compared to their constituent materials. Previous studies have shown that these materials have many levels of hierarchy, which sometimes makes for a quite complex structure similar to staggered brick and mortar structure.  Nacre has polygonal platelets of aragonite interlock with their neighbors to form sheets which are stacked on top of each other in a staggered formation glued by protein. A more complex hierarchal structure is found in bone, wherein plate-like mineral crystals several nanometers in thickness and collagen molecules form mineralized collagen fibrils and fibers. The fibrils in bone further merge into various planar arrangements called lamellae 3-7 μm wide. In some cases, 3-8 lamellae come together in concentric layers around a central canal to form the microstructure known as an osteon or a Haversian system. Therefore, given the enormous variety and the complexity in the final structural form, it is remarkable to observe that the smallest building blocks in bio-composites are in the nanoscale. The thickness of mineral platelets in nacre is ~ 100 to 500 nm, ~ 15 – 20 nm in enamel, and only a few nanometers in bone and dentin.   

The bio-mineral used in these building blocks is as hard and brittle as classroom chalk, while the protein is as soft and malleable as the human skin. They have a stiffness ratio of about 1000. This, then, raises the question that despite such poor properties of its constituent materials, how does the final product i.e. the composite, end up as a hard and tough material? Recent studies have shown that the superior mechanical properties of these materials are largely an outcome of a highly optimized micro-architecture. For example, it has been hypothesized that the nanometer size scale of the mineral platelets is an outcome of fracture strength optimization while their aspect ratio is a result of optimizing the combined stiffness of the mineral-protein assembly. Moreover, beyond the linear behavior, the enhanced toughness of the mineralized tissues is plausibly an outcome of crack bridging mechanisms rather than diffused array of micro-cracking. From the above discussion, several common features have been identified in these natural layered structures: (i) The basic building block could be a simple level or hierarchy of multiple levels with scales ranges from sub-micron to several microns, (ii) huge differences in the UHSP/SP moduli with a modulus ratio of order 1000, and (iii) highly orthotropic properties.

The challenging quest is to find a synthetic pathway to artificial analogs of nacre and bones to produce superior engineering materials. To this end, this quest has been achieved in polymer-based system by sequential deposition of polyelectrolytes and clays. However, to the best of our knowledge it has not been achieved in metallic based systems.

Example REU Experience:
The goal of this research is introduce undergraduate students to mimicking natural layered structures to engineer metallic structures with high toughness and ultrahigh strength. The proposed research activities will focus on the design and testing of a bilayer structure, weeks 1-3, as an elementary structural level to understand the synergistic interaction between UHSP/SP. Elementary strength of materials principles will be employed to get tentative dimensions. The bilayer would be then produced by sputtering hyper-eutectic Zr-Pt amorphous alloy with various thicknesses on copper film (~ 10μm), weeks 4-6. The modulus ratio would be about 400, which is in the same order of magnitude of the natural layered structure. This particular system of amorphous Zr-Pt-Cu is chosen here because of its ease of synthesis and ability of patterning. Students would then proceed to examine the tensile behavior of such bilayer structure and the associated deformation mechanisms (as revealed optically or by SEM) and correlate their findings to the relative stiffness ratio of the bilayer, weeks 7-9.

3. Well Positioned, Highly Ordered DNA from a Capillary-Held Solution (Zhiqun Lin) - Experimental

The use of spontaneous self-assembly as a lithography and external fields-free means to construct well-ordered, often intriguing structures has received much attention due to the ease of producing complex, large-scale structures with small feature sizes. These self-organized structures promise new opportunities for developing miniaturized optical, electronic, optoelectronic, and magnetic devices. One extremely simple route to intriguing structures is drying mediated self-assembly of nonvolatile solutes (polymers, nanoparticles, and colloids) through irreversible solvent evaporation of a sessile droplet on a solid substrate. It is, in principle, a non-equilibrium process. However, the flow instabilities within the evaporating droplet often result in non-equilibrium and irregular dissipative structures, e.g., randomly organized convection patterns, stochastically distributed multi-rings, and so on. Therefore, fully utilizing evaporation as a simple tool for creating well-ordered structures that have numerous technological applications requires delicate control over the evaporative flux, the solution concentration, and the interfacial interaction between the solute and the substrate etc. 

Our objective is to develop a simple yet robust processing method for producing well-positioned nanostructured materials composed of DNA, possessing unprecedented regularity, in a precisely controllable manner, dispensing with the need for lithography techniques. Subsequent functionalization of highly ordered DNA can serve as multifunctional materials for a variety of potential applications in nanoelectronic devices. These ordered structures could be considered novel nanomaterials. Accordingly, they serve as ideal models for education in non-equilibrium nanomaterials science and engineering.

Example REU Experience:
In this project, the REU student will produce highly ordered, stretched DNA nanowires with unprecedented regularity in restricted geometries consisting of a spherical lens on a flat substrate - sphere-on-flat geometry. The central hypothesis of this research is that restricted geometries provide a unique environment for precisely controlling the flow within an evaporating droplet, which, in turn, regulates the structure formation. Restricted geometries of a sphere on a Si substrate will be used to produce a capillary-held droplet in weeks 1-3. The spatial-temporal evolution of formation of DNA nanowires during the solvent evaporation will be visualized by in-situ optical and/or fluorescence microscopy in weeks 4-6. The structures will then be characterized using optical microscope (OM), AFM, SEM, TEM in weeks 7-9.

Properties of Materials Far From Equilibrium

1. Dielectric Properties of Perovskite Oxides with Non-Equilibrium Structures (Xaoli Tan) - Experimental

Perovskites refer to oxide materials that are isostructural to the mineral CaTiO3, with a general chemical formula ABO3.  These ceramics are of great technological importance because of their wide applications in advanced engineering devices, such as capacitors, actuators, transducers, sensors, etc.  When two or more cation species share the B-site at a fixed ratio, a complex perovskite forms.  Examples include Pb(Mg1/3Nb2/3)O3, Pb(Mg1/2W1/2)O3, Pb(Fe2/3W1/3)O3, Ba(Zn1/3Ta2/3)O3, and Bi(Zn1/2Ti1/2)O3, among others.  If the two B-site cation species have a large difference in their sizes and/or charges, they tend to occupy the B-site lattice in an ordered fashion.  The degree of the B-site cation order, however, can only be maximized through extended high temperature annealing.  In other words, the nanostructure of these complex perovskites is a non-equilibrium one. 

For ceramics with nanostructures near equilibrium, long-range cation order is manifested as large (>100nm) chemical domains, as shown in Figure 2.  As the nanostructure gets farther away from equilibrium, these cation ordered domains get smaller. Eventually, the chemical domains may disappear in a completely disordered nanostructure. As a consequence of the changing nanostructure, the dielectric properties vary significantly. Figure 3 shows a diagram of the dielectric constant of a ceramic with different degree of cation order.  The peak dielectric constant increases dramatically as the long-range cation order is developed (the nanostructure approaches equilibrium).  

Example REU Experience:
In this project, the REU student will explore the structure-property relationships in perovskites other than Pb(Mg1/3Nb2/3)O3, such as Pb(Mg1/2W1/2)O3 and Pb(Fe2/3W1/3)O3. The powder processing (weeks 1-2), x-ray diffraction analysis (weeks 3-4), density measurements (week 5), grain size determination with SEM (week 6-7), as well as the dielectric characterization (weeks 7-9), are all suited for undergraduate research assistants. These tasks are relatively simple and do not require the operation of sophisticated instruments.  The preparation of the Pb(Mg1/2W1/2)O3 ceramic will involve the following steps. (1) Mix and mill MgO and WO3 powders, (2) calcine the mixture at 1000°C for 4 hours to obtain MgWO4, (3) mix and mill MgWO4 and PbO, (4) calcine the mixture at 900°C for 4 hours to obtain Pb(Mg1/2W1/2)O3 powder, (5) sinter the Pb(Mg1/2W1/2)O3 powder at 1200°C for 2 hours to obtain the Pb(Mg1/2W1/2)O3 ceramic. The degree of cation order can be controlled and manipulated at the sintering stage. Carrying out such laboratory practices, the undergraduate assistants will not only obtain hands-on experimental skills, but also be intrigued by the concrete images of those abstract concepts they learned in classroom.


2. Chemical synthesis of isotropic electromagnetically-active composites (Nicola Bowler) - Experimental

Chemically synthesized layered, electromagnetically-active, micro-particles dispersed in a supporting matrix of polymer can be used to create a material that can manipulate light. It has recently been demonstrated that a composite material formed by dispersing nickel-coated glass microbubbles (shown at left) in a wax matrix exhibits a simultaneous dielectric and magnetic resonance in the microwave frequency range (dielectric resonance is shown at right for two concentrations of coated microbubbles ).  Here, the dependence of the resonance on the non-equilibrium state of the metal layer in the particles will be investigated to determine how the resonance can be strengthened, in order to achieve simultaneous negative permittivity and permeability in the material over a certain frequency band.  A material with these properties has a negative index (NI) of refraction and is sometimes termed ‘left-handed’.  Presently it is possible to fabricate NI materials by creating a highly ordered array of elements that each resonate at a specified frequency .  Due to their ordered structure, these ‘meta-materials’ are inherently anisotropic, being able only to manipulate light incident from a small range of angles.

Example REU Project:
New multi-layered micro-particles will be synthesized chemically, weeks 1-3, and the crystal structure of the metal layer characterized, weeks 4-6.  The dielectric and magnetic parameters of composites formed from dispersions of these particles will be measured over a wide frequency range – weeks 7-9.  The proposed research will exploit techniques from multiple disciplines; physics, electrical engineering, chemical engineering and materials science.


3. Characterization of Metallic Glass Composites (Ersan Ustundag) - Computational

Bulk metallic glasses (BMGs) are attractive structural materials due to their unique mechanical properties such as high elastic strain (about 2%) and high strength (around 2 GPa). However, most BMGs experience sudden failure during unconstrained loading at room temperature, which weakens their potential for load bearing applications. This lack of ductility in BMGs has been addressed by the development of particulate, wire, and in-situ composites with considerable improvement of toughness. Among these, the most promising are the in-situ composites where a dendritic second phase, also called the β phase, develops via chemical portioning during the cooling process , , .  β phase BMG composites offer a unique opportunity to control the mechanical properties of BMG composites by manipulating their microstructure, namely the volume fraction, morphology and size of the precipitates. We have recently developed a pseudo-binary (quasi-equilibrium) phase diagram that can be used as a “processing map” for these composites . The present project will attempt to optimize the microstructure of these composites for further enhancement of their mechanical properties, especially under tension.

Example REU Experience:
The REU student will begin with training on BMG composites and software tools, weeks 1-3. He/she will then proceed with the analysis of diffraction patterns via Rietveld refinement to extract lattice strains in the β phase particles and employ a self-consistent model to predict the response of the composite to mechanical loading, weeks 4-6. Finally, he/she will use optimization algorithms to match model predictions with experimental data by modifying model input parameters, weeks 7-9. The result will be the in-situ constitutive behavior of both the β phase particles and the BMG matrix. By employing advanced computational tools in an interesting scientific problem, this project aims to train students in research methodology while exposing them to cyberinfrastructure tools that will be invaluable in their future professional endeavor.

Materials Processing Far From Equilibrium

1. Characterization of Rapidly Solidified Metallic Systems (L. Scott Chumbley) - Experimental

Metallic alloy systems subjected to rapid solidification processing (RSP) methods such as melt spinning often contain unique phases and structures due to the non-equilibrium conditions. In instances where the equilibrium phase is present, the scale of the resultant microstructure may be so reduced as to yield different, and often improved, mechanical properties as opposed to those seen when the same alloy is more conventionally processed. Often, thermodynamically stable phases predicted by the phase diagram are entirely absent, replaced instead with metastable phases whose formation is kinetically favored due to the imposed rapid cooling rate.  In the most extreme cases, the resultant structure may be entirely amorphous. This research project seeks to understand the fundamental science behind phase and structure development in as-solidified and heat-treated samples by studying two distinct alloy systems based on Aluminum, namely Aluminum-Silicon (Al-Si).

The Al-Si system is of industrial importance as it is the basis for lightweight, castable alloys suitable for applications in the automobile industry.  In this system alloys in the range 15-50 wt% have been rapidly solidified and examined, again using a combination of advanced characterization techniques.  Efforts in this area are aimed at developing a microstructural map of resultant phases and structural features which can be used to estimate the mechanical properties expected for any resultant microstructure.

Example REU Experience:  In this research project, the REU student will conduct their own independent experiments while at the same time being part of a larger research group preparing the amorphous alloys and characterizing them.  In the Al-Si study, the REU student will conduct experiments to determine the best procedure for producing large-scale solid parts of RSP material are needed that would involve independent research on different methods of consolidation and densification, weeks 1-3. This will then be followed by mechanical property measurements to determine whether the increases in mechanical properties seen due to RSP can be preserved when producing a bulk piece, weeks 4-6.  HRTEM studies of melt-spun Al-Si material are also critically needed, and while learning to operate an advanced TEM is probably beyond the scope of an undergraduate research project, assistance with sample preparation and computer analysis of the resultant images is easily within the expertise of a typical undergraduate, weeks 7-9.


2. Fundamental Properties of Crystal-Melt Interfaces (Ralph E. Napolitano) - Experimental

The phenomenon of morphological selection during the growth of a crystalline phase from its melt is one where the mechanisms of atomic attachment dictate the local migration of the solid-liquid interface and act in concert with the longer range processes of thermal and chemical diffusion in response to the forces which drive the overall freezing transformation. Complex patterns arise from the optimization of transport processes, where natural selection forces favor those morphologies which facilitate the most efficient redistribution of heat and solute. It is here that local thermochemical excesses arise from partitioning at the moving interface and where the dissipation of such imbalances must accompany further interface motion. Accordingly, much attention has been given to the underlying physics governing the collective optimization of these processes and to the resulting growth patterns particularly in highly driven systems where “far from equilibrium” conditions give rise to competition between metastable phases, non-equilibrium chemical compositions, and complex microstructures.

Evidence for the critical role of interfacial properties in the overall selection dynamics can be found in many examples of solid-liquid transformations, such as nucleation, eutectic solidification, and dendritic solidification. In each of these cases, some characteristic microstructural length scale is selected in accord with the partitioning of the driving forces required to overcome the intrinsic resistance to local curvature (ΔGr) and interface motion (ΔGk). Considering only this intrinsic response of the interface, the relevant components of the overall driving force can be written generally as  and  . Thus, we write the intrinsic component of interfacial undercooling, ΔTI, in terms of the two resistance parameters describing the capillary and kinetic contributions as
where   and   are known as the interfacial stiffness and interfacial mobility respectively, and both are generally anisotropic. While typically small in magnitude for nonfaceted metallic systems, the critical role of the energetic and kinetic anisotropy in the selection of dendritic morphology has been well established and related to a host of morphological dynamics that may result in steady, oscillating, or chaotic growth modes. Despite recent advances, what remains unclear is the physical origin of the anisotropic properties, their temperature and composition dependence, and the balance between energetic and kinetic factors. The object of this project is to experimentally measure these critical fundamental interfacial properties.

Example REU Experience:
In recent years, a method has been developed for reliable shape measurements and the first measurements of this anisotropy in metallic systems have been made. Accordingly, the REU student would work with a graduate student to further advance this method and to make additional measurements of this critical physical quantity under non-equilibrium conditions. Focusing on the composition and temperature dependence of the anisotropy, the student will use a state-of-the-art serial milling unit in conjunction with optical and scanning electron microscopy to generate three-dimensional images of selected particles within metallic microstructures. The student will analyze these images using digital imaging software and special codes written within the research group.

The REU experience described here will be hands on exposure to several experimental techniques, including metallography, optical microscopy, scanning electron microscopy, serial milling, temperature measurement, weeks 1-3, use of motion control hardware and software, image analysis, and 3D reconstruction, weeks 4-6. Finally, the student will be introduced to some theoretical aspects of microstructural selection in real materials and will gain an appreciation for the critical nature of fundamental interfacial properties, weeks 7-9.   


3. Wear Resistant Boride Composites (Alan Russell) - Experimental

Intermetallic composite materials comprised of AlMgB14 + TiB2, for example, being researched at Iowa State display high hardness (35-42 GPa) and extraordinarily high resistance to both sliding and erosive wear.  In many aggressive wear environments, these boride composites match or exceed the performance of much more costly super-hard materials (e.g., diamond or cubic BN), and they greatly out-perform the less costly cemented-tungsten carbide materials.  These composites have compositions at or near 55 vol. %TiB2 – 45 vol. % AlMgB14 fabricated by hot-pressing milled powders into two-phase specimens with sub-micron phase size.  
In many composite materials, the rule-of-mixtures (simple linear proportionality based on composition) predicts the best performance that can be expected for mechanical properties such as elastic modulus and ultimate strength.  These boride composites substantially exceed the rule-of-mixtures prediction for hardness which suggests that some microstructural or interfacial phenomenon is enhancing hardness.  The project is presently investigating whether these materials’ mechanical properties benefit from the so-called Veprek hardness enhancement ,  in which one phase has nano-scale dimensions and is surrounded by the second phase, thereby suppressing dislocation motion and serving as a barrier to long-range crack propagation.

Example REU Experience:
In addition to developing bulk materials comprised of AlMgB14 + TiB2, the borides can also be deposited as metastable, amorphous thin films onto substrates such as Si or steel. The REU student will be given thin-film specimens produced by other group members by sputtering onto Si and steel substrates.  The REU student’s task will be to characterize them by three methods. They will examine thin film roughness and coverage by scanning electron microscopy and profilometry.  This will require the student to be trained to use SE, BSE, and EDS capabilities of an SEM and to operate a profilometer.  The first specimens will be examined with one of the group’s graduate students, and when the REU student is ready to “solo” he/she can perform additional measurements alone, weeks 1-3. Next, they will measure the hardness of the deposited thin film by AFM nanoindentation.  As with the SEM training, the student will need to be coached on AFM operating procedures, perform the first measurements with one of the group’s graduate students or post-doctoral scientists, then work with the system on his/her own weeks 4-6. Finally, they will measure thin film adhesion with ultrasonic testing.  Intense ultrasonic impulses are brought to bear on the thin film/substrate specimen, and the amount of material removed by the impulses is measured by gravimetric analysis and in the SEM.  The ultrasonic testing will be performed with one of the staff scientists since it is probably unrealistic to expect an REU student to perform this test alone, weeks 7-9.