N15A-T010 TITLE: Powder Metallurgy for Advanced Thermionic Cathodes

TECHNOLOGY AREAS: Sensors, Electronics, Battlespace

ACQUISITION PROGRAM: PEO IWS 2, Above Water Sensors

OBJECTIVE: Develop processes to produce scandium doped tungsten powder for use in the production of advanced high current density thermionic cathodes.

DESCRIPTION: This effort seeks to identify, define (design), and demonstrate a process for the commercial scale production of scandium-doped tungsten powder consistent with the needs of the US domestic cathode industry in meeting Navy requirements. The process is required to provide metal powders with purity, particle size, uniformity and metallurgical content and structure suitable for cathode production and consistent with the latest advances in the field. Process controls that determine metal powder characteristics will be identified and designed into the process. Process repeatability (for consistent quality) will be a paramount consideration, followed in importance by cost and production capacity. The purpose of this effort is to prove a feasible process that will make advanced thermionic cathodes available to the microwave tube industry by providing the constituent metal powders, (not to produce finished cathodes).

Many existing Navy weapon systems rely on microwave vacuum electronics (microwave tubes) as the primary source of Radio Frequency (RF) power. This is especially true for many radar and electronic warfare (EW) systems. Future RF sensors, especially millimeter-wave EW systems, will require unprecedented performance in output power and bandwidth. The Navy has ongoing initiatives to develop capability in this frequency band. Proposed solutions based on vacuum electronics demand advanced, high current density cathodes (a collateral benefit is reduced life-cycle cost through improved reliability). Microwave tubes will exist in Navy systems for many decades to come due to two factors: 1) the sustainment of legacy systems, and 2) the deployment of future systems for which size, weight, and power-bandwidth make vacuum electronics the only viable option. The latter case includes future millimeter wave (MMW) sensors that will place extreme requirements on the MMW vacuum electron devices in the areas of electron beam current density, beam quality, and confinement (Ref. 1). In addition, legacy systems are under constant pressure to control life-cycle cost through increased reliability of expensive components. Often, this is the microwave tube.

Microwave tube performance and reliability depend fundamentally on the cathode. The vast majority are thermionic cathodes produced from the sintering of tungsten powder. Recent advances in cathode science have resulted in Scandate cathodes capable of providing extremely high current densities while promising long life due to operation at standard or reduced operating temperatures (Refs.2-4). These cathodes are produced from tungsten powder that has been chemically doped with small amounts of scandium � an element known to enhance thermionic emission. Once technically matured and commercially available, these cathodes will enable new microwave/MMW tube designs and make possible re-engineering of existing devices for enhanced life, leading to overall cost savings.

Research in the area of Scandate cathodes produced from scandium doped tungsten metal powder has been largely aimed at determining the optimum scandium content, metal powder size and sintering times to produce optimum cathode characteristics (mainly high current density). Consequently, metal powder constituents have been produced in extremely small batches (typically less than 10 grams), driven by research objectives. Scant attention has been given to the production of Scandate metal powders in amounts suitable for commercial cathode production where requirements of a few hundred (i.e. 300-500) kilograms of material per year are anticipated. Therefore, in order to make Scandate cathodes available in sufficient quantity, and at the required (and repeatable) quality, innovative methods for the production of scandium doped tungsten powder must be found. It should be noted that, as this is a revolutionary technology, reducing cost is an important, but secondary consideration. However, as a target, a material cost of no more than five times that of existing cathode-grade Tungsten powder is desired).

PHASE I: The company will define and develop a concept for the production of scandium-doped tungsten powder that meets the requirements as stated in the topic description. The company will demonstrate the feasibility of the concept in meeting Navy needs and will establish that the concept can be developed into a useful process for the Navy. Scaled process testing, analysis and modeling will establish feasibility.
The company will define and develop a concept for the production of scandium-doped tungsten powder that meets the requirements as stated in the topic description. The company will demonstrate the feasibility of the concept and will establish that the concept can be developed into a useful process for the Navy. Scaled process testing, analysis and modeling will establish feasibility.

PHASE II: Based on the results of Phase I and the Phase II contract statement of work, the company will develop a prototype process for the production of scandium-doped tungsten powder. The process will be evaluated to determine its capability in meeting Navy requirements for the production of scandium-doped tungsten powder. System performance will be demonstrated through prototype evaluation and modeling or analytical methods. Evaluation results will be used to refine the prototype into a design that meets Navy requirements. The company will prepare a Phase III development plan to transition the technology to commercial use to supply Navy needs.

PHASE III: The company will support the Navy in transitioning the technology for Navy use. The company will refine the production process for scandium-doped tungsten powder according to the Phase III development plan, for evaluation to determine its effectiveness in meeting Navy demand for microwave tubes. The company will support the Navy for test and validation to certify and qualify the process for Navy use and transition the production process for scandium-doped tungsten powder to a production facility for microwave tube cathodes.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The US domestic vacuum device industry supplies microwave tubes for a variety of scientific (fusion research), industrial (microwave heating) and communication (satellite uplink stations) applications. If successful, translation transition of technology developed under this effort to the industry is assured, as commercial and military microwave tubes are produced on the same production lines, using the same processes.

REFERENCES:
1. Li, Lili, et al. "Development of High-Current Sheet Beam Cathodes for Terahertz Sources", IEEE Trans. Electron Devices 56, May 2009: pp. 762-766.

2. Wang, Yiman, et al. "Development of High Current Current-Density Cathodes With Scandia-Doped Tungsten Powders", IEEE Trans. Electron Devices 54, May 2007: pp. 1061-1070.

3. Zhao, Jinfeng, et al. "High Current Density and Long-Life Nanocomposite Scandate Dispenser Cathode Fabrication Development of High Current Current-Density Cathodes with Scandia-Doped Tungsten Powders", IEEE Trans. Electron Devices 58, April 2011: pp. 1221-1228.

4. Zhao, Jinfeng, et al. "Scandate Dispenser Cathode Fabrication for a High-Aspect-Ratio High-Current-Density Sheet Beam Electron Gun", IEEE Trans. Electron Devices 59, June 2012: pp. 1792-1798.

KEYWORDS: Thermionic cathodes; scandate cathode; microwave tubes; vacuum electronics; powder metallurgy; tungsten metal powder

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