With the film thickness increasing from 1,030 to 1,450 nm, the I c value increases more slowly. There is a little change with surface roughness for the two samples. However, much more a-axis grains appear in the 1,450-nm-thick film compared with the 1,030-nm-thick film. Apart from these, it is suggested that there is less oxygen content for the upper layers beyond 1,030 nm for samples F1450 and F2100. It is believed that the appearance of much more a-axis grains and the less oxygen content for the upper layers of thick films are the two main factors affecting the superconducting performances for samples F1450 and F2100. When the film thickness approaches to 2,100 nm, it is worth noting that there is nearly
no supercurrent increase with increasing film thickness compound screening assay from 1,450 to 2,100 nm. The phenomenon is first reported by Foltyn et al. [21]. They attributed it to a porous microstructure of the top layer. In our case, it is found that the gaps between a-axis grains will result in porosity inside the top layer. As a result,
the porosity inside the film and the gaps on the film surface will block the supercurrent for the 2,100-nm-thick film. Besides, the oxygen deficiency for the upper layer of the thicker film is another factor affecting the superconducting performances. For our GdBCO films, the superconducting performances are subject to three factors: a-axis grains, gaps between a-axis grains, and oxygen Trichostatin A price deficiency.
The stress and the roughness are not the main factors affecting the superconducting performances. Figure 8b shows the J c value of our studied films. It can be seen that the thinnest film, F200, exhibits the highest J c. The mechanism discussed above cannot explain why F200 has the 4��8C highest J c value. Van der Beek et al. [22] reported that a maximum in J c was obtained at a thickness between 100 and 200 nm. This result is similar to our studies. Foltyn et al. [8] attributed the very high J c for the thinnest YBCO films to the high density of misfit dislocations near the interface of the substrate and the above YBCO film. We believe that the high-level compressive stresses in F200 leads to the highest J c values. Figure 8 I c (a) and J c (b) measurements of GdBCO films with different thicknesses under optimized deposition conditions. Tao et al. [15] reported the J c of YBCO film to be 1.6 × 106 A/cm2 at 77 K and self-field with a thickness of 1.2 μm by sputtering method on buffered Ni-5 at.% W substrates. Tran et al. [23] found that the 0.2-μm-thick GdBCO film had the highest J c of 3.8 × 106 A/cm2 and the J c value decreased to 4.2 × 105 A/cm2 as the film thickness increased to 0.55 μm. From our results, the J c of the 1,450-nm-thick film can achieve as high as 2.0 × 106 A/cm2. At the same time, a nearly linear relationship between film thicknesses and I c has been found when the film thickness is below 1,030 nm.