1. Schematic diagram of the cutting positions for the structure and performance characterization of samples

2. Schematic diagram of the electrical performance test of Gd/BST gradient composite

3. XRD patterns of Gd/BST gradient composites with different aging time

4. (a) Secondary electron image (SEI), (b, c) backscattered electron images (BEI) and (d) EPMA elemental mapping images of the samples after aging for 12 d

5. Change of thickness of the diffusion layer of Gd/BST heterojunction interface with aging time

6. EPMA element line analysis from Gd ball to BST matrix for (a, b) as-prepared sample and (c, d) sample after aging for 12 d

7. Temperature dependence of (a) electrical conductivity and (b) Seebeck coefficient for forward (solid icon) and reverse (hollow icon) testing of Gd/BST gradient composites for different aging time with insets showing change of electrical conductivity and Seebeck coefficient with aging time at 300, 350 and 500 K

8. Temperature dependence of (a) thermal conductivity, (b) carrier thermal conductivity, (c) lattice thermal conductivity, and (d) ZT for forward (solid icon) and reverse (hollow icon) testing of Gd/BST gradient composites with different aging time with insets showing changes of thermal conductivity, carrier thermal conductivity, lattice thermal conductivity, and ZT with aging time at 300, 350 and 500 K

9. Cooling performance of the Gd/BST gradient composite device with different aging time under different forward currents

Table 1. Cooling performance of Gd/BST gradient composite device with different ageing time/K