## 3 Results and Discussion
### Fabrication of Long Tubular Products  
The estimation of the bonding performances of tubes formed by diffusion bonding processes, was made in long tubular parts by joining three-unit tubes. Diffusion bonding is subject to a time-temperature relationship. In this study, the diffusion bonding was performed under the condition of 5 h in holding time. For a given holding time, the optimal temperature to achieve the desired bonding quality was determined through the results obtained at the five different temperatures, 1427 \({}^{\circ}\)C, 1438 \({}^{\circ}\)C, 1441 \({}^{\circ}\)C, 1443 \({}^{\circ}\)C, and 1454 \({}^{\circ}\)C.  
Two of the most important factors are the uniformity of temperature and the gas atmosphere in the heating zone. With too rapid heating to the designated temperature for diffusion bonding, the refractory brick and heating coil could be damaged. Figure 11 shows the controlled heating schedule proposed for the diffusion bonding of stacked unit tubes. If the nitrogen atmosphere is maintained at temperatures above 760 \({}^{\circ}\)C, the only heating elements in the heating system become nitrided, which shortens the heating element life span. At temperatures above 760 \({}^{\circ}\)C, the circulating hydrogen gas in the heating zone does not deteriorate the heating system and is safely emitted and burned at the outlet. Therefore, the constant heating rate in the heating zone is maintained until 760 \({}^{\circ}\)C, under a nitrogen atmosphere. Above 760 \({}^{\circ}\)C, the nitrogen atmosphere is substituted with a hydrogen atmosphere to activate the heating zone. During atmospheric change, the temperature of the heating zone is reduced about 30 \({}^{\circ}\)C, but the temperature can be stabilized with an increasing content of hydrogen gas. When the hydrogen content exceeds 99%, the temperature increases linearly to the designated temperature for diffusion bonding. Figure 12 shows the microstructural evolution with diffusion bonding temperatures. From the microstructures shown in Figure 10, the temperature for the diffusion bonding has been determined. Figure 10: Supporting device to maintain straightness of stacked tubes: (**a**) conceptual view; (**b**) three round molybdenum bars are located inside of the stacked tubes; (**c**) set-up view. Figure 11: Processing cycle for diffusion bonding to make long tubular parts. Figure 12: Microstructural evolution with diffusion bonding temperatures: (**a**) joining of binder metals starts; (**b**) width of binder layer decreases due to self-weight; (**c**) further decrease in the width of binder layer; (**d**) creation of a bonding line; (**e**) diffusion bonding of tungsten powder along with the bonding line; (**f**) bonding line disappears; (**g**) growth of tungsten powder (optimal bonding); (**h**) over-growth of tungsten; (**i**) pore generation with binder disappearance. Table 1 shows the test result of diffusion bonding experiments obtained at various temperatures. At temperatures below the designated temperature for diffusion bonding, joining between tubes is insufficient and tubes can easily separate due to insufficient joining. At the designated temperature for diffusion bonding, joining of binder metals starts and the joining line between binders starts to decrease due to the self-weight of the tubes. Then, the movement of tungsten powder becomes active and joining lines disappear. Perfect joining can be accomplished at this stage. At temperatures above the designated temperature for diffusion bonding, tungsten powders coalesce, and irregular tungsten powders also increase. Furthermore, the binder starts to disappear and pores are generated. The microstructures obtained from the diffusion bonding test clearly show the evolution of the microstructure. If the designated temperature is well-maintained during diffusion bonding, the surfaces pressurized by self-weights start to join by using applied thermal energy. A sound product was obtained at 1443 \({}^{\circ}\)C (Case D), as shown in Figure 13a.