**5.3 Stereology of composite grain size**

200 Corrosion Resistance

**50% 316L + 50% graphite 20% 316L + 80% graphite** 

**100% graphite** 

Table 4 contains the values of density for 316L steel sinter and graphite-steel composites. Addition of graphite to the material reduces density from the level of 7.16 g cm-3 for steel to the level of 2.35 g cm-3 for the sinter modified with 80% of graphite. Modification of steel sinter 316L with graphite allows for obtaining materials with reduced density. With respect to future applications of these materials for bipolar plates in fuel cells, the use of light materials will allow for achievement of one of the most essential goals of hydrogen technologies, i.e. reduction in generator weight. Addition of graphite to steel sinter impacts also on material hardness. Change in sinter hardness with concentration of graphite in the

Composites Densisty of sinter [g cm-3] Hardness 100% 316L 7.16 ± 0.38 79 ± 3.75 HRB 80% 316L + 20% grafit 6.93 ± 0.34 45 ± 4.15 HRB 50% 316L + 50% grafit 3.81 ± 0.19 35 ± 1.75 HRB 20% 316L + 80% grafit 2.35 ± 0.11 86 ± 4.30 HRF 100% grafit 1.97 ± 0.09 97 ± 4.85 HRF

Fig. 9. Microstructures of graphite-steel composites, magnitude 100 x.

Table 4. Density and hardness of graphite-steel composites.

composite is presented in Table 4.

Table 5 contains mean cross-sectional surface area, mean number of grains per mm2 of the surface and mean number of grains per mm3 of graphite-steel composites. Based on the data contained in the table, one should note that no effect of chemical composition of the sinter on mean grain size is observed. Mean grain diameter varies from 48 to 68 mm, whereas the greatest grain diameters are observed for the sinter with 50% proportion of graphite. The data are also presented in Fig. 10.


Table 5. Mean values of grain parameters in graphite-steel composites.

Fig. 10. Relationship between mean grain diameter in graphite-steel composite and proportion of graphite in the composite.

Properties of Graphite Sinters for Bipolar Plates in Fuel Cells 203

In consideration of the degree of wettability, the materials are typically divided into lyophilic materials, which have strong affinity for water (these materials attract water particles) (Fig. 12a) and the materials which repel water particles, termed lyophobic (Fig. 12b). Contact angle Θ provides a measure of wettability, which is an angle between the surface of a solid and tangent going through the point of contact of solid, liquid or gaseous phase determined for the liquid phase. It is conventionally adopted that solid bodies which are characterized by contact angles of Θ < 90° are wettable; these materials show high surface energy (if the liquid is water, these materials are termed hydrophilic). Materials which exhibit contact angle of Θ > 90° are regarded to be non-wettable (lyophobic or,

Rodzaj kompozytu Porosity [%]

 100% 316L 9.59 80% 316L + 20% grafit 14.43 50% 316L + 50% grafit 12.17 20% 316L + 80% grafit 11.09 100% grafit 10.73

Table 6. Comparison of porosity in graphite-steel composites.

**5.5 Investigations of sinter wettability** 

alternatively, hydrophobic = low surface energy).

Fig. 12. Diagram of wetting hydrophobic and hydrophilic materials.

In order to determine the effect of chemical composition of a composite on surface wettability, the analysis of wettability was carried out through evaluation of the value of Θ angle. The investigations concerned 316L steel sinter and sinters with addition of graphite. Fig. 13 presents contact angles evaluated for composite materials. A linear relationship between the proportion of graphite and surface wettability: contact angle increases with proportion of graphite in the composite. The highest contact angle was found for the sinter

#### **5.4 Composite porosity**

A variety of materials and methods of modification of the surface of materials used for bipolar plates points to rising interest in fuel cell technologies, with particular focus on the design of the cell. Main requirements concerning commercial use of materials for manufacturing fuel cells is the relationship between high corrosion resistance and low contact resistance, with low costs of manufacturing. Corrosion rate, contact resistance and wettability of material depend to some degree on material porosity. Therefore, the investigations of functional properties were started from the analysis of pore composition in the sinters included in the study.

Assessment of porosity concerned graphite-steel composites. 316L steel sinters with addition of graphite exhibit varied porosity depending on the proportion of graphite. Fig. 11a presents hysteresis for intrusion and extrusion pressure for mercury in 316L sinter. Narrow pressure hysteresis loop points to the presence of flat pores in the material. Similar profile of hysteresis for mercury intrusion and extrusion pressure was found for other composites included in the study. Fig. 11.b. presents distribution of pore diameters in the sinters included in the study. It should be emphasized that graphite-steel composites show pores with diameters which correspond to mesopores. Only in the sinter with 50% proportion of graphite no pores from the range of diameters corresponding to mesopores were found, whereas macropores with diameters over 0.08 μm were observed.

Fig. 11. Hysteresis of mercury intrusion/extrusion in graphite sinters and distribution of pores depending on the proportion of stainless steel in the sinter.

Table 6 presents the values of porosities evaluated based on microstructural examinations and tests using mercury porosimeter. The lowest porosity among the composites studied was found for 316L steel sinter (9.59%). Addition of graphite with the amount of 20% considerably enhances porosity of material compared to steel sinter. Other sinters, enriched with 50% and 80% of graphite, exhibit lower porosity compared to the sinter of 80% of 316L + 20% of graphite, but this is still the value higher than the value of porosity estimated for 316L sinter.

A variety of materials and methods of modification of the surface of materials used for bipolar plates points to rising interest in fuel cell technologies, with particular focus on the design of the cell. Main requirements concerning commercial use of materials for manufacturing fuel cells is the relationship between high corrosion resistance and low contact resistance, with low costs of manufacturing. Corrosion rate, contact resistance and wettability of material depend to some degree on material porosity. Therefore, the investigations of functional properties were started from the analysis of pore composition in

Assessment of porosity concerned graphite-steel composites. 316L steel sinters with addition of graphite exhibit varied porosity depending on the proportion of graphite. Fig. 11a presents hysteresis for intrusion and extrusion pressure for mercury in 316L sinter. Narrow pressure hysteresis loop points to the presence of flat pores in the material. Similar profile of hysteresis for mercury intrusion and extrusion pressure was found for other composites included in the study. Fig. 11.b. presents distribution of pore diameters in the sinters included in the study. It should be emphasized that graphite-steel composites show pores with diameters which correspond to mesopores. Only in the sinter with 50% proportion of graphite no pores from the range of diameters corresponding to mesopores were found, whereas macropores with diameters over 0.08 μm were

Fig. 11. Hysteresis of mercury intrusion/extrusion in graphite sinters and distribution of

Table 6 presents the values of porosities evaluated based on microstructural examinations and tests using mercury porosimeter. The lowest porosity among the composites studied was found for 316L steel sinter (9.59%). Addition of graphite with the amount of 20% considerably enhances porosity of material compared to steel sinter. Other sinters, enriched with 50% and 80% of graphite, exhibit lower porosity compared to the sinter of 80% of 316L + 20% of graphite, but this is still the value higher than the value of porosity estimated for

pores depending on the proportion of stainless steel in the sinter.

**5.4 Composite porosity** 

the sinters included in the study.

observed.

316L sinter.


Table 6. Comparison of porosity in graphite-steel composites.
