"The hydraulic resistivity is 18.2m Ω. Cm", what does m Ω. Cm mean? What does cm mean?

"The hydraulic resistivity is 18.2m Ω. Cm", what does m Ω. Cm mean? What does cm mean?

Not m Ω. Cm, but m Ω * cm
Cm is cm, of course
The resistance is inversely proportional to the cross-sectional area of the conductor and directly proportional to the length. Therefore, the unit of resistivity should be resistance unit × area unit / length unit = resistance unit × length unit
The resistivity of water and electricity is 18.2m Ω * cm, which means that the resistivity of water is 18.2m Ω. That is to say, the resistance of water with cross-sectional area of one square centimeter and length of one centimeter is 18.2m Ω

What is the guiding significance of resistivity index for the use of materials,

Resistivity index is one of the basic characteristics of materials, which quantitatively reflects the conductivity of materials. Under the corresponding conditions, it can also reflect the doping level of materials, the mobility of carriers, the uniformity of materials, single crystal or polycrystalline, etc
(for reference)

Effect of temperature on resistivity of impurity semiconductor
The description of the effect of temperature on the resistivity of n-type semiconductors in the book is roughly divided into the first decrease, then increase and then decrease. It is divided into the low temperature region, the saturation region and the intrinsic region. I don't know how to explain this trend, and what's the matter with the three regions

It should be like this:
For ab segment, the temperature is lower, and the influence of temperature on the impurity is greater than that on the semiconductor itself. At this time, the carriers in the semiconductor are mainly ionized by the impurity. When the temperature increases, the number of carriers increases and the resistivity decreases
For the BC segment, the ionization of impurity is completed, that is, it is saturated. However, the intrinsic excitation is not significant, the effect of carrier decreases, the effect of lattice vibration increases, and the resistivity increases with temperature
After the C point, the intrinsic excitation becomes the main factor, so the resistivity decreases with the increase of temperature

The resistivity of semiconductor materials is greatly affected by external conditions (such as temperature, light, etc.)

External conditions, such as light and temperature, will affect the material. Different materials have different conductive mechanisms, which have an impact on the resistivity. Generally speaking, it is the impact on carrier mobility and concentration