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DEVELOPMENT OF COMPOSITE MATERIALS FOR HIGH TEMPERATURE HEAT EXCHANGERS AND THERMIONIC GENERATORS

Objective


The project was aimed at the definition and the characterisation of advanced ceramic and metal ceramic materials suitable for structural and functional components of thermonic energy converters and heat exchangers. Coating techniques such as CVD and PS were developed to achieve multilayer structures exhibiting the required basic properties.

Methodologies suitable for the characterization of materials in high vacuum and at high temperature, and a qualified methodology for a special system such as the thermionic flame diode have been developed.

A device has been developed to measure emissivity, thermal conductivity and electrical resistivity. It can be used for the study and characterization of materials at high temperature and in vacuum.

Apparatus has been developed to measure the work function of materials used in thermionic conversion. It is efficient when used to examine samples which do not contain materials which react with caesium.

Leakage testing equipment for the study and assessment of thermionic diodes has been developed, which can be used to test systems in which sealing is required.
A testing station which simulates the real working conditions of the diode has been developed which is indispensable for the assessment and characterization of thermionic diodes.

A high temperature recuperator with nitride bonded silicon carbide tubes was built. Its thermal performance was assessed by means of experimental thermal cycles. The highest temperature of the hot fluid 1500 C. Such a ceramic heat exchanger can be used downstream in thermionic generators in a cogeneration system called total energy module for the generation of heat and electrical power.

Development work on thermionic convectors produced 2 metal-ceramic tri-layers. Chemical vapour deposition (CVD) and plasma spray (PS) were used for the coating of the substrate material. They were shown to be stable up to the required operating temperature. A series of tests showed that the assemblies were also able to meet the design specifications. Using these materials, thermionic diodes, corrosion test modules, and hot shells were all manufactured. A new type of caesium resistant metal-ceramic joint was created by metallizing alumina and vacuum brazing copper.

The basic properties required for the materials are ability to work at high temperatures (up to 1500 C); corrosion resistance in air and in the presence of hot combustion products; high thermal conductivity and good thermal fatigue. In addition, the material for the hot shell - or thermionic converter protecting structure must have electrical conductivity; low electronic surface barrier, high mechanical resistance against atmospheric pressure, and very high mean life.

None of the analysed commercial ceramics met the thermionic converter specifications, either in dimensional accuracy or allowable costs. CVD and PS technologies were studied for the application of coatings to substrate materials, or for free standing shapes.

The processes developed were CVD of tungsten, titanium nitride, silicon carbide, aluminium nitride and alumina; and PS of tungsten and molybdenum.

85 hot shells, 5 corrosion test modules and 4 water cooled thermionic diodes were manufactured. A nitride bonded silicon carbide was selected as a suitable material for the heat exchanger. It had a maximum operating temperature higher than the target value of 1350 C, a gas permeability low enough to be considered zero; thermal conductivity of 30 Wmk at 20 C; and a high thermal shock resistance (no evidence of cracking after quenching from 950 C to room temperature).

A 4-pass shell and tube cross counterflow ceramic heat exchanger was designed. A module composed of 8 tubes was built and subjected to thermal cycles to assess recuperator thermal performance with the result that air heating up to 1100 C exploiting flue gas heat at 1500 C was obtained.
THE GOAL OF THE PROJECT IS THE DEVELOPMENT OF MULTILAYER COMPOSITE MATERIALS IN ORDER TO MEET THE REQUIREMENTS OF BOTH HIGH TEMPERATURE HEAT EXCHANGERS AND TH ERMOIONIC GENERATORS.
SUCH MATERIALS HAVE TO PRESENT, AT OPERATING CONDITIONS, THE FOLLOWING CHARACTE RISTICS:
- HIGH MECHANICAL STRENGTH
- CORROSION RESISTANCE IN AIR AND IN COMBUSTION PRODUCT ENVIRONMENTS - THERMAL SHOCK AND THERMAL FATIGUE RESISTANCE
- HIGH THERMAL CONDUCTIVITY
- HIGH MEAN LIFE AND RELIABILITY

FOR THERMOIONIC APPLICATIONS THE DIODE MATERIALS MUST ALSO HAVE HIGH ELECTRICAL CONDUCTIVITY, COMPATIBILITY WITH THE INSIDE CESIUM ENVIRONMENT, GAS TIGHT AND SPECIAL ELECTRONIC SURFACE PROPERTIES.
THE SECOND PART OF THE ACTIVITY FORESEES THE REALIZATION, SETTING UP AND TESTIN G OF COMPONENT PROTOTYPES FOR BOTH THERMOIONIC GENERATORS AND HIGH TEMPERATURE HEAT EXCHANGERS.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

ANSALDO RICERCHE SPA*
Address
Corso Perrone 25
16152 Genova
Italy

Participants (3)

EINDHOVEN UNIVERSITY OF TECHNOLOGY
Netherlands
Address
Den Dolech 2
5600 MB Eindhoven
Ente per le Nuove Tecnologie l'Energia e l'Ambiente (ENEA)
Italy
Address
Via Anguillarese 301
00060 Santa Maria Di Galeria Roma
Xycarb BV
Netherlands
Address
4,Zuiddijk
5705 CS Helmond