This new proposal outlines plans to exploit the hydrothermal method to explore new categories of materials with particular emphasis on extremely refractory oxides. The hydrothermal method employs water at extremely high temperatures and pressures and has proven to be an excellent technique for the preparation of a wide variety of inorganic materials as high quality single crystals. Technology was developed at Clemson to do reactions at substantially higher temperatures (750¢ªC) with more powerful and concentrated mineralizers (e.g.>25M hydroxide or halide) than any previously work in these labs. These conditions enable synthesis and good crystal growth on chemical systems heretofore too inert or refractory to be otherwise accessible. The specific categories of solids to be investigated are; 1) wide bandgap oxides, specifically the s block metal borates and beryllates; 2) ultrarefractory oxides like hafnates, thorates and yttrates; and 3) large single crystal ferromagnetic oxides for neutron diffraction study. The fundamental reaction chemistry will be systematically explored for each class of materials. Single crystals will be targeted in all cases. Our crystal growth can be divided into three broad regimes, namely crystals large enough for single crystal diffraction (0.3mm), crystals large enough for single crystal magnetic and piezoelectric characterization (1-2mm), and crystals large enough for optical device characterization (1cm). Each of these size regimes will employ an appropriate hydrothermal growth method. For example, for wide bandgap oxides to find use in deep UV optical applications, single crystals of 1 cm/edge will be required for characterization in prototype optical devices. In contrast, ferrimagnets specifically grown for single crystal magnetic structures on the nearby Oak Ridge Spallation Neutron Source will require single crystal 1-2 mm per edge.
NON-TECHNICAL SUMMARY: This work will employ a relatively obscure technology to grow single crystals of solids that are otherwise difficult to study. The solid crystals will have applications in many advanced technologies, especially lasers, advanced optics, magnetic materials and sensors. The field of crystal growth has all but disappeared from university research programs in the United States. This shortcoming has created a major gap in the skill set of onshore materials science capabilities. There is substantial evidence that this shortcoming is having a significant impact on national competitiveness in key materials. Furthermore this shortcoming has a significant negative impact on the students¡¯ research abilities. This is detrimental to scientific research and discouraging to students. A rational and systematic ability to grow crystals of sufficient size and quality for physical property evaluation leads to a dramatic improvement in the breadth of student development. Often patents result from this work and the resultant commercial application provides an important perspective to the students who are much thus better prepared to make better decisions about experimental design. Finally the technology is very ¡°green¡±, which raises the awareness of the students to societal impact of their science.
This project developed the hydrothermal method to grow interesting new single crystals for a number of applications. The hydrothermal method involves doing chemical reactions in water heated above its boiling point at high pressures. In this project we used water heated up to 650?C (1200?F) at pressures of 2 kbar (30,000psi). These conditions are experimentally demanding and it required considerable effort to develop the techniques through the course of this project. The technology is now in place and exciting new chemistry is unfolding. Intellectual Merits: Our focus is on inorganic single crystals especially oxides and fluorides. We explored new reaction chemistry and were able to isolate and characterize a series of new metal halides, metal oxides and metal borates. One interesting aspect of this research was the class of extremely refractory oxides, namely those that melt at exceptionally high temperatures. We were able to grow high quality single crystals of materials like thorium oxide (mp 3400?C), lutetium oxide (mp 2500?C) and strontium hafnate (mp 2750?C). These are some of the highest melting oxides known. They all have a variety of potential applications but many of their basic physical properties were not well known previously, due to the lack of pure high quality crystals. In several cases accurate structures were established for the first time and this work opened up considerable new chemistry of such exotic materials. A primary goal of this work was to investigate new crystals for optical applications particularly related to lasers. We were able to grow a series of new metal borates that enable the development of new solid-state lasers with deep UV (sub 266nm) wavelengths. Deep UV all solid lasers have a number of important applications including high-resolution photolithography, micromachining, laser surgery and chem/bio detection. The technical limitations are primarily materials related, and the development of the borate crystals in this grant represent a significant step forward. Several patents and licenses have emerged from this grant. One particularly exciting discovery from this grant is the development of multifunctional "smart" single crystals. We found that the hydrothermal method allows us to grow high quality epitaxial layers of doped materials, of any thickness, on single crystal seeds. Thus we can grow multiple millimeter layers of a doped material on an undoped seed. We can repeat this process as many times as we wish with different dopants in each layer. Since each dopant can perform a different optical function, we can grow a single crystal with multiple optical functionalities all in one small single crystal. This is a significant game changer for solid-state laser design. It can enable the design of new lasers with extremely high efficiency, power and beam quality. Normally a laser cavity requires several individual single crystals to perform the various functions. Each separately mounted piece adds size cost and inefficiency. This new technology enables custom designs of individual multifunctional single crystals that can replace the arrays of bulk crystals. Since our new technology enables laser engineers to custom design the single crystals with multiple specifications, they are "smart" single crystals. These discoveries opens the door for mass production of multifunctional laser crystals with exceptional capabilities for the first time. It is an optical analogy to the early days of microelectronics where several electronic functions were first imparted on a single crystal of silicon. Broader Impacts: At least six patent applications have emerged from this work, along with licenses to startups and SBIR proposals. We feel that this is an excellent example of fundamental inorganic reaction chemistry making a significant contribution to a field of advanced technology, in this case advanced laser design. A considerable number of graduate students (eight), several undergraduate chemistry majors and three postdoctoral fellows were partly supported over the course of this grant.
