Bob Mellen , inventor of the EDG Furnace
In 1964, after years of working as an aerospace and high-temperature engineer, Robert H. Mellen Sr. embarked on a journey to establish his own venture – the Mellen Company Inc. He foresaw the pivotal role that the crystal growth industry would play in the upcoming years and recognized the limitations imposed on crystal growers by existing furnace designs. Breaking away from the standard "one size fits all" approach, he aimed to craft a more efficient and controllable solution tailored to those engaged in cutting-edge research and technology development.
With the unwavering support of his wife, Ella, Bob Mellen laid the foundation of the Mellen Company Inc. He attentively listened to the requirements and applications of researchers, meticulously crafting furnaces by hand, and even assisting crystal growth researchers in devising their research processes. This intimate partnership and deep comprehension of ongoing crystal growth research enabled him to design systems that revolve around these unique processes, thereby eliminating a significant hurdle that researchers had been facing until then.
Empowering Crystal Growth with the Electrodynamic Gradient Furnace
Bob Mellen listened to the needs of researchers and tailored systems to fit their processes, a stark contrast to years of researchers adapting their processes to fit the systems. Bob fostered close collaborations with crystal growers. Through his own research, he identified the predominant challenge faced by these scientists: the difficulty in growing high-quality, pure crystals. Driven by his scientific fervor, Bob pioneered the creation of the first electrodynamic gradient (EDG) furnace to address this challenge.
The growth of single crystals, like gallium arsenide (GaAs) or cadmium zinc telluride (CZT), at high temperatures posed significant obstacles using conventional methods. Although crystals would form, they often ended up twinning or forming polycrystalline structures instead of achieving the desired single crystal purity. Bob collaborated closely with crystal growers and scientists to unravel the underlying causes of polycrystalline formation and other issues intrinsic to crystal growth, which had been grudgingly accepted by the industry up until this point.
During that era, most crystal growers used a Bridgman-style furnace for methods such as the Bridgman-Stockbarger or Czochralski. While these approaches suited certain crystals, sensitive ones like GaAs or CZT suffered from high failure rates in single crystal growth attempts. These techniques relied on seed crystals and translation motors to move the furnace or workcharge, initiating crystal formation. Bob determined that this movement of translation introduced minor vibrations, leading to polynucleation in the crystals. Any form of stress, thermal or physical, could trigger crystal formation and cause simultaneous spontaneous nucleation at multiple points. The conventional methods of the 1960s and earlier made it impossible to traverse the gradient without incurring vibrations, thus risking spontaneous crystal formation.
Having pinpointed a critical issue plaguing the crystal growth industry, Bob was determined to devise a solution. Drawing from his extensive research, he pondered over the fundamentals of crystal growth. Heat, unsurprisingly, stood as the cornerstone of crystal growth. At its core, heat transformed the solid material into a liquid state, while cooling facilitated crystallization. This concept was widely accepted, and Bob delved deeper into the mechanism of this heating process: heat flow.
Innovating Heat Flow for Flawless Crystal Growth: The EDG Furnace Breakthrough
Understanding a ‘center hot’ crystallization pattern is fundamental to the creation of an EDG furnace. To explain this concept, consider a hot beverage. When a hot beverage cools down, it first loses heat from its edges. Heat flows from the hotter liquid to the cooler surrounding air. If a hot drink is placed in a freezer, the liquid cools as a whole, however, upon reaching freezing point, areas with the least thermal mass encounter greater cooling (such as at the edges) and freeze first. Anyone who has removed and ice cube tray too quickly from a freezer has encountered this phenomenon: the outer shell of the ice cube solidifies, but the core remains liquid. Natural convective cooling follows a "center hot" crystallization pattern, whereby the center, with the highest thermal mass and lowest thermal losses, remains hot and molten while the edges crystallize. This can lead to polycrystalline structure formation, a flaw for crystal growth research. On the other hand, "center cold" crystallization is imperative for producing flawless single crystals. Center cold enables the crystal to form while the liquid follows the restructuring of the crystal, minimizing the risk of polynucleation.
One approach following the principles of center cold crystallization is the use of a seed crystal, as seen in the Czochralski method. After melting the material, a seed crystal is introduced to the liquid on a rod. The rod is then elevated and rotated to create a single crystal. Other methods involve melting the material and allowing it to cool to the freezing point before introducing the seed crystal, maintaining control over the growth process. Nevertheless, these methods have limitations, including limited control over all nucleation sites, a higher risk of impurities in the crystal, or the necessity for an immaculate crystal seed to initiate the process. Each method Bob investigated unveiled more insights into the interplay between heat flow and crystal growth.
Bridgman systems manage heat flow by using a static heat source (the furnace) and physically moving the work through the heat to achieve crystal melting and freezing. This approach yielded success for some crystals but encountered resistance from more delicate ones striving for single crystal formation. Recognizing the vibrational stresses inherent in the translation for the Bridgman system as the initial obstacle, Bob embarked on finding a method to enable dynamic heat flow across the work without necessitating physical motion.
Innovating Heat Flow for Flawless Crystal Growth: The EDG Furnace Breakthrough
His breakthrough solution involved shifting the temperature gradient rather than moving the work or the furnace. This innovation led to the creation of the first Electrodynamic Gradient furnace (EDG). The EDG system enables dynamic control of a precise temperature gradient within a furnace. Eliminating physical motion from the equation mitigates vibrations , resulting in the growth of pristine single crystals.
To accomplish this feat, Bob drew upon his years of expertise and research in thermodynamics. The inaugural EDG combined short heating zones with water cooling. These zones were subdivided into four quadrants, each individually regulated by dedicated thermocouples. This configuration enabled Bob and his team to not only measure and quantify temperatures but also comprehensively understand the heat flow throughout the entire system, encompassing the workpiece. The capacity to independently control each quadrant facilitated meticulous management of heating and cooling within the furnace.
Continuing Bob’s Legacy: Advancements in Thermal Flow and Furnace Design
In recognition of Bob's accomplishments, the Mellen Company Inc. received the IR100 award for the development of the EDG furnace in 1983. His efforts yielded three patents, signaling the dawn of further exploration in the realm of high-temperature research. The meticulous control and precise quantification of heat flow within the EDG furnace paved the path for Mellen Company researchers to delve deeper into crystal growth. Over time, Bob and his team published multiple papers, including "Anatomy of a Temperature Profile" and "Advances Toward Intelligent Processing of Electronic Materials," collaborating with and advising scientists across the industry.
Even in the present day, Mellen Company researchers and engineers draw from the insights into thermal flow garnered from the initial research to inform their furnace designs. This knowledge is seamlessly integrated into new furnace designs and accompanying accessories, such as Active Cooling Technology, Digital Shunting, and the Thermal Distortion Free Sightglass.
At the Mellen Company Inc., we remain dedicated to embodying Bob's commitment to partnership and success in every interaction with furnace operators. If you find yourself grappling with challenges in your high-temperature processes and require assistance, rest assured that we stand ready to provide support – reach out to us today.