A Combined diffusion coefficient for gas mixture

[Snow_buggy]

TL;DR Summary

Is the diffusion direction from high concentration to low concentration and from high temperature to low temperature?

Hi. I am starting to do a gas mixture simulation. I learn the fundamentals from a paper. (doi: 10.1088/0022-3727/43/2/022001)
For a gas mixture (Fe vapor in Argon), the mole fraction of Fe vapor is calculated by,

and the diffusion flux JFe is given by,

x_ar is the mole fraction of Argon; T is the temperature; P is the pressure; E is the electric field.
It seems like Fe vapor diffuses from high concentration of Fe vapor to low concentration (grad x_Ar) and from high temperature to low temperature (- grad T).

If we calculates the mole fraction of Argon not Fe vapor, this paper (doi: 10.1007/BF01459700) says these coefficients follow this rule,

I don't understand why the temperature diffusion coefficient change the sign? Does it mean the argon diffuses from low temperature to high temperature?

 

[Chestermiller]

See Transport Phenomena by Bird, Stewart, and Lightfoot.