12. $k$ is a real number, let $f(x)=\frac{x^{4}+k x^{2}+1}{x^{4}+x^{2}+1}$. If for any three real numbers $a, b, c$ (which can be the same), there exists a triangle with side lengths $f(a), f(b), f(c)$, then the range of $k$ is $\qquad$.