  We study the scaling and universal behavior of temperature-driven first-order
phase transitions in scalar models. These transitions are found to exhibit rich
phenomena, though they are controlled by a single complex-conjugate pair of the
imaginary fixed points of a $\phi^3$ theory. Scaling theories and
renormalization-group theories are developed to account for the phenomena.
Several universality classes with their own hysteresis exponents are found
including a field-like thermal class, a partly thermal class, and a purely
thermal class, designated respectively as Thermal Class I, II, and III. The
first two classes arise from the opposite limits of the scaling forms proposed
and may cross over to each other depending on the temperature sweep rate. They
are both described by a massless model and a purely massive model, both of
which are equivalent and are derived from the $\phi^3$ theory via symmetry.
Thermal Class III characterizes the cooling transitions in the absence of
applied external fields and is described by purely thermal models, which
includes cases in which the order parameters possess different symmetries and
thus exhibiting different universality classes. For the purely thermal models
whose free energies contain odd-symmetry terms, Thermal Class III emerges only
in mean-field level and is identical with Thermal Class II. Fluctuations change
the model into the other two models. Using the extant three- and two-loop
results for the static and dynamic exponents for the Yang-Lee edge singularity,
which falls into the same universality class to the $\phi^3$ theory, we
estimate the thermal hysteresis exponents of the various classes to the same
precisions. Comparisons with numerical results and experiments are briefly
discussed.
