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low thermal conductivity of organic materials [21,30] .

Design of experiment for TE materials synthesis
During the optimization of synthesis of TE materials, experiments are performed to measure the effects of
experimental variables on responses. The optimization of preparation involves finding a combination of
variables that gives the best response. Thus, there are many types of experiments. DOE is a statistical way of
optimizing the response of experiments. An efficient DOE is a sequential approach wherein information
gained in the initial stage can be used to decide which factors may be kept constant or varied in later stages.
In this process, reducing the number of experimental variables to a more significant one is possible. Thus,
statistical DOE through fractional factorial design can provide an opportunity to minimize the number of
experiments by optimizing experimental conditions. The main advantages of DOE are efficiency, response
analysis, and simple interpretation of statistical outcomes.

The DOE has existed for quite some time, but its use is very new in synthesizing TE materials.
[31]
Selvaratnam et al. have reviewed the application of ML in general materials chemistry . Recent progress in
DOE and ML for predicting and controlling inorganic materials synthesis has been discussed by
[32]
Willamson et al. . Recent review articles on the use of DOE, ML, and AI for the synthesis of general solid-
state materials have been presented by Baum et al. .
[33]
The first example of the application of statistical DOE in TE materials synthesis is for the optimization of
the sintering parameters of K Bi Se S (for x = 0, 4, 6, and 8)-based TE materials by Kyratsi et al. .
[34]
8
13-x x
2
Sintering conditions must be optimized to obtain high-quality pellets of TE materials with the highest
density. In this work statistical DOE through fractional factorial experiment and Taguchi table has been
employed for identifying optimum parameters for sintering (duration, temperature, and pressure) for the
fabrication of high-quality pellets. Based on the ANOVA, it has been postulated that the density of the pellet
is more significantly affected by pressure than duration and temperature. The best condition for hot-press
sintering of these TE materials has been 80 MPa pressure, 530 °C temperature, and 90 min duration,
respectively, with a relative density of pellet approaching ~97%. These optimized sintering conditions have
been used for sintering the pellets of the entire K Bi Se S compounds Interestingly, the sample with x = 0
13-x x
.
8
2
exhibited a ZT value of 0.58 at 673 K.
Bi Te and its alloys, such as Bi Sb Te and Bi Te Se , have been used in TE technologies for decades.
3
3
x
2-x
2
x
3- x
2
However, Kanatzia et al. have employed factorial DOE via ANOVA for the ball-milling synthesis of Bi Te
2
3
[35]
for the first time . The analysis of parameters for ball milling, such as duration, rotation speed, and ball-to-
material ratio, respectively, suggests a strong influence of the duration and speed of ball milling on the TE
properties of nanocrystalline Bi Te . The ZT value of optimized ball-milled nanocrystalline Bi Te is 0.72 at
2
3
3
2
100 °C. It is expected that DOE can further improve the ZT value of Bi Te -based TE materials through
2
3
efficient optimization of ball-milling parameters.
Nuthongkum et al. applied RSM and statistical central composite design (CCD) for modeling and
optimizing thin film deposition parameters of thermoelectric Bi Te material using the radiofrequency (RF)
3
2
magnetron sputtering technique . This study has statistically analyzed the formation of Bi-Te thin films
[36]
considering several deposition factors, such as the flow rate of Ar gas and annealing temperature. The DOE
using RSM to find the optimal conditions for the targeted response utilizes mathematical and statistical
methods to develop models and evaluate factors. Thus, RSM plots suggest that the concentration of Te in
Bi-Te thin film would decrease if the annealing temperature is increased at a lower Ar flow rate. Using this
model, it has been determined that the optimized conditions for the deposition of good quality thin films