Josef Remmel

Joining properties of a composite glass-ceramic sealant

When assembling planar solid oxide fuel cell (SOFC) stacks, an electrically insulating and gas-tight sealing material is required. Glass-ceramic sealants have been shown to be an appropriate material for this application in the past. In the present study, the investigations are focused on a composite material consisting of zirconia in a glass matrix based on the system of BaO-CaO-SiO 2 (BCS). The joining behavior with ferritic stainless steel is macroscopically observed by so-called ‘sandwiched’ samples made out of two steel plates (size 50 × 50 mm 2 ) with the glass sealant in-between. Dilatometric measurements are carried out, and the coefficient of thermal expansion is taken for varying amounts of zirconia in the composite material. The crystallization behavior of the sealant is investigated by differential thermal analysis. The microstructure of joined samples, submitted to different scenarios of thermal treatment, is characterized by optical and scanning electron microscopy. The joining properties strongly depend on the amount of filler material. Additions of 20 wt% zirconia in the glass matrix prove to be the optimal composition. The glass matrix tends to crystallize very slowly, giving the prospect of an elastic seal during the initial operation of a stack.

Brazing of metals to zirconia mechanically metallized with titanium

The mechanical metallization is a successful technique at laboratory scale and specially applied to oxide ceramics. Indirect brazing process of zirconia to metals is achieved using active-metal-free filler alloys on previously metallized ceramic. Stabilized zirconia ceramics were mechanically metallized with Ti and wetting conditions evaluated using commercial Ag-Cu and Au-Ni fillers with its respective thermal cycles. Better results were selected for brazing ceramic to metals in a high-vacuum furnace. Reliable vacuum tight ceramic/metal joints were obtained specially using the Ag-28Cu filler for results below 10-8 mbar.l.s-1; samples at the joint cross-section were examined by microstructural analysis techniques and energy dispersive X-ray analysis. Microhardness profiles were made across the joints interface where zirconia undergone a typical darkening effect during brazing. Microstructure at the braze region revealed a dark reaction layer and precipitation zone closely to metallized ceramic surface for zirconia/Ti-6Al4V joints due to chemical interactions between the individual components.

Joining of zirconia mechanically metallized with titanium

A brasagem direta e uma tecnica bem conhecida para uniao de cerâmicas entre si bem como aos metais, segundo a qual as ligas de adicao requeridas sao consideradas uma relativa desvantagem devido ao aumento nos custos do processo. Neste trabalho, a brasagem indireta da zirconia a zirconia mecanicamente metalizada com Ti foi alcancada com sucesso, usando-se ligas de adicao convencionais. A metalizacao mecânica especialmente aplicada aos oxidos cerâmicos e uma tecnica bem sucedida que foi patenteada pelo Forschungszentrum Juelich, Alemanha, e tem sido tema de pesquisas nos ultimos 10 anos. Esta metalizacao particular e feita a temperatura ambiente com baixo custo de producao, sendo ambientalmente segura. Zirconia parcialmente estabilizada com itria (Y-PSZ) foi mecanicamente metalizada com Ti e as condicoes de molhamento foram avaliadas usando as ligas de adicao Ag-28Cu e Au-18Ni. Estas ligas de adicao convencionais dissolvem parte do revestimento de titânio, tornando-se ligas ativas na temperatura de brasagem para reagir com o oxido de zirconio e acentuar o molhamento na superficie cerâmica. Os melhores resultados foram selecionados para os testes de brasagem em forno de alto-vacuo (< 3×10-5 mbar). A deteccao de vazamento de gas helio foi efetuada na interface cerâmica/cerâmica a temperatura ambiente; amostras de juntas estanques confiaveis foram examinadas por analise microestrutural e pela tecnica linescan atraves de analise por difracao de raios X na secao transversal das mesmas. Perfis de microdureza foram feitos atraves da interface da junta onde zirconia passou por efeito de escurecimento durante o processo de brasagem. Juntas brasadas de boa qualidade foram produzidas quando apenas uma das superficies cerâmicas em contato foi previamente metalizada, devido a difusao de titânio desta superficie para a outra face cerâmica nao-metalizada.

Recent Results of Stack Development at Forschungszentrum Jülich

Since the mid-nineties several generations of SOFC stacks have been designed and tested incorporating the anode substrate-type cells developed in Julich. The 6th generation, the so-called F-design stacks, with metallic interconnect has been the ‘work horse’ used for testing materials, cells and manufacturing processes in cell and stack development since its introduction in the year 2001. Stacks with up to 60 layers have been operated in recent years, delivering up to 13 kW of electric power. The ferritic parts are made of the commercial steel type CroFer22APU.

Demonstration of a 4-Cells SOFC Stack Under Different Experimental Conditions

Forschungszentrum Juelich (FZJ) has developed over the past years a well-known SOFC stack technology for a planar cell configuration. 1.5 mm thick anode supported largearea cells based on a NiO/YSZ (anode) and a YSZ (electrolyte) system are integrated in a stack using bipolar plates made of a special ferritic stainless steel acting as current collector and gas distributor Materials, processes, and surface treatments have been selected and optimized to reduce mismatch between different components and to achieve high power densities. The recent SOFC stacks of F-design, fuelled with humidified hydrogen and air has demonstrated a power density as high as 0.6 W/cm 2 at 800°C and a stable behavior up to 8000 h of operation. These tests are usually performed in standard conditions, using a low value of fuel utilization and a precise control of gas composition, fuel, and airflow rates and temperature distribution. It seems quite difficult to maintain the same strict control of these parameters when the stack operates in an actual device. Therefore, it is of interest to investigate the effect of different experimental conditions on the electrochemical performance that can simulate stack behavior as integrated in a real system. For example, an actual fuel cell system is designed to operate with fuel utilization in the range between 70% and 85% due to efficiency considerations. In addition, variations of the electric load and of the corresponding flow rates must be expected that give rise to temperature changes and gradients within the stack. Part of these effects might be detrimental for the stack service life, and can be limited by a well-designed balance of plant of the system. In this contribution, results of a series of tests on a 4-cells SOFC stack of F design, manufactured by FZJ and conducted in a new SOFC test bench by Eurocoating, are presented. This facility allows the investigation of the performance of stacks up to 1 kW as a function of several experimental parameters, including the amplitude of compressive loading, inlet gas temperature and pressure, flow rate, and fuel utilization.

Soldagem por Difusão de Aços Inoxidáveis para Fabricação de Trocadores de Calor Compactos

A uniao de materiais diferentes ou componentes com geometrias complexas muitas vezes requer processos de unioes especiais, como por exemplo, uma fonte de energia de alta densidade como laser ou feixe de eletrons para soldagem por fusao ou mesmo um processo de soldagem no estado solido. Em particular, este trabalho descreve os experimentos recentes da aplicacao da soldagem por difusao no estado solido (SDES) para diferentes tipos de acos inoxidaveis: austenitico AISI 316L, duplex UNS 31803 e superduplex UNS 327250. A soldagem por difusao foi realizada sob alto vacuo a 1050 °C, durante 60 minutos, empregando uma pressao uniaxial de 18-35 MPa. A qualidade da uniao foi avaliada por microscopia optica e testes mecânicos. Os resultados preliminares indicam a necessidade de otimizacao dos parâmetros de processo para os acos duplex e superduplex. Os resultados positivos alcancados no presente trabalho permitiram a fabricacao de prototipos de trocadores de calor compacto em aco inoxidavel, utilizando o novo metodo de fabricacao, desenvolvido por LABTUCAL / UFSC, atraves do qual o processo de corte a jato de agua e aplicado para confeccionar com precisao os canais do nucleo do permutador de calor.