This is the basic equation that des

This is the basic equation that describes the diffusion phenomenon. Fick's first law pointed out that in any concentration gradient driven diffusion system, the substance diffuses along the negative gradient direction determined from the concentration field, and the magnitude of the diffusion flow is proportional to the concentration gradient .. The diffusion equation is an image relational expression that describes the macroscopic diffusion phenomenon. It does not participate in the microscopic process of atomic motion inside the diffusion system, and the diffusion coefficient reflects the characteristics of the diffusion system. The concentration C in the diffusion equation is a function of position and time, and the diffusion coefficient D is a quadratic tensor theoretically containing 9 components, and is closely related to the structural symmetry of the diffusion system. The concentration distribution of diffusing species in a diffusing medium changes with time. Diffusion is always called unstable diffusion, and its diffusing flux changes with position and time. For the unstable diffusion, we can establish the second diffusion differential equation by starting from the equilibrium relationship of the materials.