List of reported
The thermoelectric material ZnSb has been studied intensively in recent years and has shown promising features. The other zinc‐antimonide compound, Zn4Sb3 has remarkable low thermal conductivity, but it is accompanied with phase transitions at moderate temperature and has inherent stability problems. Compared to that, ZnSb is relatively phase stable and has a relative high charge carrier mobility and Seebeck coefficient, thus yielding a decent power factor. Meanwhile, its thermal conductivity can be reduced by means of nanostructuring, thus giving a good figure of merit at moderate temperatures, 400–600 K. Many researchers have dedicated their efforts to study and improve ZnSb properties, and the figure of merit has been reported to be above one. Still, ZnSb as a thermoelectric material has features and behaviours that are not well‐understood. The behaviour and properties of its intrinsic defects are not understood, but have interested researchers in recent years. This chapter intends to offer a comprehensive review on ZnSb to the readers. By combining own experiences from research on thermoelectric materials, the authors address the prospect for improving the thermoelectric properties of ZnSb and the concerns of transferring lab results to manufacturing.
- zinc antimonide
- impurity band conduction
- intrinsic defects
The thermoelectric effect in ZnSb has been known for almost two centuries. The first documented encounter can be traced back to the original work of Seebeck on thermoelectric current generation on different materials and alloy pairs in 1819–1827 [1, 2]. Quantitative measurements of the Seebeck voltage of ZnSb have been carried out by Becquerel in 1866 , and thermoelectric generators using Zn‐Sb alloy were fabricated for practical purposes since 1870 . From the early twentieth century, many attempted to solve the crystal structure, but were barely successful [5–7], until Almin, finally, in 1948 determined the crystal structure of ZnSb together with CdSb . A large interest in ZnSb followed and was benefitted by the progress on semiconductors since the 1950s. Figure 1 shows a thermoelectric device made by ZnSb in the 1950s. It shows a solar thermoelectric generator prototype built by
Table 1 lists some reported figure of merit
|Report year||At temperature (K)||Sample||Reference|
|(2012)||0.9||659||Polycrystalline with Cu|||
|(2013)||1.15||670||Polycrystalline with Ag|||
|(2014)||0.9||635||Polycrystalline with Ag|||
|(2014)||0.8||600||Polycrystalline with Cu|||
|(2014)||1||630||Polycrystalline with Sn+Cd|||
|(2014)||1.5||673||Polycrystalline with Cu|||
|(2015)||0.8||700||Polycrystalline with Zn vacancies|||
2. Crystal structure of ZnSb
2.1. Crystallographic structure and covalent bonds
The crystallographic structure of ZnSb has been determined by Almin . According to this determination, ZnSb has orthorhombic crystal structure, oP16 and belongs to the space group Pbca no. 61. The structure of ZnSb has been studied and confirmed by different techniques, albeit gave slightly different interatomic distances [25–28]. Figure 2 shows the crystal structure of ZnSb that was generated by the structural data from Mozharivskyj .
The crystal structure can be viewed as a deformed zinc blende structure. The distorted edge‐sharing ZnSb4 tetrahedra generate a peculiar five‐fold coordination of each atom, as seen in Figure 2a: one of the same kind and four of the other kind. In the context of bonding and its relation to conduction, all atoms in the crystal structure are tied together in a network—a point, we will return to below.
Another way to systematize the structure is to group the atoms together in planar rhomboid rings of Zn2Sb2 that have short Zn‐Zn bonds connected to two different longer Zn‐Sb bonds, as seen in Figure 2a. These motifs are also tied together in a network that completes the crystal structure. The crystal structure can be recognized in atomic scale by scanning transmission electron microscopy (STEM), as shown in Figure 2b.
The interatomic distances for each bond are also annotated in Figure 2a. The bonds in ZnSb have been categorized in three groups, shown in Figure 2c and d: the bonds
Valence electrons in ZnSb have a certain distribution. Figure 3 shows the theoretical
The results show that a Zn atom transfers a small average fraction of 0.26 electrons to Sb along Zn‐Sb bond , which is expected due to the difference in Pauling electronegativity between Zn (1.65 eV) and Sb (1.96 eV). Essentially, the same transfer was found in the calculations of Benson
2.2. Electronic structure
2.2.1. Band calculations
The band structure of ZnSb has been calculated by
The zero energy position corresponds to the largest energy of filled states. Thus, the states below 0 correspond to the valence band, while those above correspond to the conduction band. The value of the band gap is severely underestimated by the computational approximations. By using more accurate methods but at the expense of increased computation time, such as HSE‐hybrid functional (Heyd‐Scuseria‐Ernzerhof), the band gap was calculated to be around 0.5 eV . This value is close to the experimental value for single crystal ZnSb. On the other hand, the shape of the bands may be less affected by computational approximations. The band diagram in Figure 4 shares some of the features found in most of the calculations listed. The band gap is indirect with the maximum of the valence band along the symmetry line Γ‐Χ and the minimum of the conduction band along Γ‐Z. One can see that the conduction band contains more satellites within 0.5 eV of the minimum than pockets of the maximum in the valence band. Thus, one can expect ZnSb would have acted better as an
It is worth mentioning the band diagram that was calculated by Yamada in 1978 by using pseudopotentials . The maximum of the valence band was found to be on the line from Γ‐Χ at
2.2.2. Experimental band gap
The band gap has been measured by different methods. Values of 0.5–0.53 eV for single crystal ZnSb, which were measured by optical absorption, have been reported [44, 45]. These values are essentially identical when considering the uncertainties, and 0.5 eV has been considered to be a reference value for single crystal ZnSb. Many research have also determined the thermodynamic band gap from the temperature dependence of the charge carrier concentration in the intrinsic regime. The charge carrier concentration is determined from the Hall coefficient, . A linear fit of vs. will yield the activation energy, which is half of the thermodynamic energy gap. (This assumes an effective density of states proportional to .). Values of 0.47–0.65 eV have been reported in the literature for single crystal ZnSb based upon Hall effect measurements [46, 47].
For polycrystalline ZnSb, the band gap is often reported to be around 0.31–0.35 eV [13, 48], which is smaller than that of single crystal material. It is justified to discuss whether the band gap really is different, or if it is due to that the idealizations of the measurement methods do not hold for polycrystalline material. One of the methods that has been used for estimating the band gap is to measure the temperature dependence of the Seebeck coefficient. By determining the temperature, , for the maximum of the absolute value of the Seebeck coefficient, , the band gap can be estimated from the Goldsmid formula, . This method works well for many semiconductors, for instance Zr‐NiSn half‐Heusler . However, there have been reports on some systems that the Goldsmid formula does not apply [51, 52]. ZnSb may be one of them. Guo and Luo have reported that the Goldsmid formula returned a value of = 0.3 eV for their polycrystalline ZnSb samples . However, Böttger
We should also keep in mind that heavily doping may influence the density of states near the band edges, forming tails that are extending into the band gap. The defect states in the band gap also influence the values. This is a well‐documented phenomenon (for Si) , even if the precision in a detailed quantitative understanding may be lacking. We have reported that the maximum of the Seebeck coefficient of ZnSb could be varied considerably by the presence of defect states in the band gap .
2.2.3. Effective mass and density of states
The idealized single parabolic band (SPB) model is convenient for analysing experimental results. The model has been successful for finding the optimum doping concentration of many thermoelectric materials [55–57]. When applied to Seebeck measurements on ZnSb with different doping concentrations, it has been observed that Pisarenko plot (the Seebeck coefficient vs. the charge carrier concentration) does not follow the SPB model with a single density of states effective mass , which was determined to be (0.42–0.49) × , where is free electron mass [15, 53], but rather a different mass fit for different ranges of doping concentrations. A previous study by Böttger
3. Electrical properties and doping effect
3.1. Electrical properties and scattering mechanisms
Table 2 summarizes the band gap, the charge carrier concentration and other electrical and thermal properties reported for ZnSb in the literature.
|Reference||Report year||Structure||Dopant||σ (S)||ρ (Ω·cm)||µ (cm2/(V·s))||α (µV/K)||κ (W/(m·K))|
|||(1954)||–||2 at.% Sn + 0.1 at.% Ag||–||0.0019||–||–||210||–||–|
|||(1960)||Single crystal||–||2.8||3.57||350||5.1 × 1016||440||0.52–0.57||–|
|||(1961)||Single crystal||–||3||0.33||480||3 × 1016||550||0.49||–|
|||(1963)||Single crystal||–||1.6||0.63||248||4.1 × 1016||–||–||–|
|||(1963)||Single crystal||–||5.6||0.18||1150||3.1 × 1016||–||–||–|
|||(1964)||Single crystal||–||6.4||0.16||10||4 × 1018||110||0.53||1.1|
|||(1964)||Single crystal||–||19.7||0.05||384||3.2 × 1017||490||–||–|
|||(1964)||Polycrystalline||–||20||0.05||95.2||1.3 × 1018||375||–||1.3|
|||(1964)||Polycrystalline||0.1 at.% Ga||2.04||0.49||210||6 × 1016||675||–||–|
|||(1964)||Polycrystalline||0.1 at.% Cu||110||–||680||1.1 × 1018||–||–||–|
|||(1964)||Single crystal||–||–||–||3 × 1017||–||0.48||–|
|||(1966)||Single crystal||825||0.001||800||2 × 1016||182||–||3.7|
|||(1966)||Single crystal||Cu||42||0.02||660||4 × 1017||–||–||–|
|||(1966)||Single crystal||Cu||898||0.001||510||1 × 1019||–||–||–|
|||(1967)||Single crystal||Au||26.9||0.04||233||4.2 × 1017||404||0.47–0.65||–|
|||(1967)||Single crystal||In||37.5||0.03||326||7.7 × 1017||288||0.47–0.65||–|
|||(1967)||Single crystal||Te||51||0.02||379||8.3 × 1017||400||0.47–0.65||–|
|||(2010)||Polycrystalline||2 wt.% Ag||–||0.01||–||–||185||–||2.3|
|||(2013)||Polycrystalline||0.002 at.% Ag||–||0.0016||111||2.6 × 1019||181||–||1.5|
|||(2014)||Polycrystalline||Sn||180||–||–||4.9 × 1018||250||–||1.8|
|||(2014)||Polycrystalline||Cu||640||–||–||2.5 × 1019||134||–||–|
|||(2014)||Polycrystalline||0.15 at.% Cu||935||–||–||2.4 × 1019||142||–||–|
|||(2015)||Polycrystalline||3 at.% VZn||800||0.0012||66||7.9 × 1017||275||0.29||1.75|
Given the crystal structure, single crystal ZnSb exhibits anisotropic conduction, which is strongly dependent on the anisotropic effective mass . Böttger
There is much literature devoted to scattering mechanisms in semiconductors , while there are fewer reports dealing with that topic specifically for ZnSb. It is expected that ZnSb has similar behaviour to those semiconductors which have been much studied and follows similar trends. ZnSb appears to have favourably small polarity due to small electronegativity difference between Zn and Sb. This would in turn lead to a negligible polar optical phonon scattering compared to III–V and II–VI compounds where polar optical phonon scattering may be dominant. Roughly, transport in ZnSb is dominated by impurity scattering at low temperature, while at higher temperature, when lattice vibrations are stronger, longitudinal acoustic phonon scattering dominates (deformation potential scattering). The hole mobility varies with temperature as , thus a plot of vs. will give the scattering factor that implies the scattering mechanism. A value of -1 to -1.5 typically indicates that longitudinal acoustic phonon scattering dominates. In Figure 5a, the slopes of the Hall mobility approaches -1.5 as the doping concentration decreases. It indicates that acoustic phonon scattering dominates within the temperature range. The deviation from -1.5 for each individual curve is attributed to additional ionized impurity scattering . It is also seen that the higher hole concentration corresponds to an increase in ionized impurity scattering, which limits the hole mobility.
Not only the intentional dopants, but also defects, that are ionized or neutralized, screened or unscreened, contribute to scattering. Likely scattering centres are Zn vacancies, interstitials, internal strain, grain boundaries and dislocations. Figure 5b shows the resistivity for an unprocessed ZnSb ingot that was obtained directly from solidification and hot‐pressed ZnSb pellets with different dopant concentrations. The temperature coefficient, , for the unprocessed ingot is positive, which is commonly observed for metals and semiconductors in a certain temperature range where phonon scattering dominates. For the processed samples, either with or without Ag, the temperature coefficient is negative. The reason for the negative temperature coefficient could be the dominant Coulomb scattering due to charged defects or impurities.
3.2. Doping effects
The most direct purpose of doping is to vary the charge carrier concentration. A broad range of dopant elements has been reported for ZnSb. The selection of dopant is often rationalized based on the same valence electron counting scheme that is applied to the elemental group IV, III–V or II–VI tetrahedrally bonded semiconductors. These considerations are applied for acceptors, while donors are challenging. One will have acceptors by replacing group I elements for Zn or group IV elements for Sb. It is expected that donors can be substitutes for group III elements on Zn sites and group VI elements on Sb sites. In all cases, there may be an issue with doping efficiency, i.e. not all the added dopant atoms will be electrically active. It is common and qualitatively well‐understood for other semiconductors that this inefficiency involves segregation (solid solubility limit), clustering of dopant atoms and/or agglomeration of complexes of dopant atoms and point defects. A theoretical calculation predicts that the optimum hole concentration for the thermoelectric efficiency of ZnSb is around 2 × 1019 cm-3 , which is achievable in
184.108.40.206. IZn—acceptors as elements of group I
CuZn, AuZn, AgZn all yield
220.127.116.11. IVSb—acceptors as the elements of group IV
Hole concentrations around (4–14) × 1018 cm-3 were obtained in materials with a content of (0.06–3) at.% Sn [53, 68]. The hole concentration variations with Sn doping concentrations where apparently opposite for these two studies. The highest hole concentration of 14 × 1018 cm-3 was obtained for content 0.1 at.% Sn  and yielded the highest mobility. It was suggested that two different doping mechanisms are effective in different temperature ranges involving different intrinsic defects, Sn on different lattice sites and their variation with temperature .
Hole concentrations of (2–2.5) × 1019 cm-3 have been reported by co‐doping of group I (0.15 at.% Cu or Ag)/IV (0.6 at.% Pb, Sn, or Ge)/Cd [20, 21]. The measured transport coefficients at different regions indicate two types of impurity acceptor: one embedded into Zn sites, and another into Sb sites. Here, Cd is not expected to act as an acceptor, but for increasing the phonon scattering and thereby reducing the thermal conductivity. A similar intended function has been applied by adding P to increase alloy phonon scattering in the Cu doped ZnSb [18, 69].
18.104.22.168. IIIZn—donors as elements of group III
Group III elements have been used as donors to yield
22.214.171.124. VISb—donors as elements of group VI
Some success with Te doping has been reported. Ueda
126.96.36.199. n‐Type to p‐type transition
4. Zn vacancies and intrinsic defects
The theoretical intrinsic charge carrier concentration of perfect ZnSb at room temperature is approximately 2 × 1014 cm-3 given by , where , and taking eV and × . However, the experimental measurements show that the charge carrier concentration of the best single crystals at room temperature is around (1–2) × 1016 cm-3 [15, 46, 61, 62] and up to ∼1018 cm-3 for polycrystalline samples without intended dopants [14, 45]. This deviation is considered due to the intrinsic defects, giving a net hole concentration. The most favoured intrinsic defects in ZnSb are Zn vacancies, which are believed to yield
The vacancy can in principle have any charge states, but only -2, -1 and 0 seem readily accessible by doping and temperature variation. The formation energy for VZn- in this configuration was calculated to be 0.32 eV. The net hole concentration in ZnSb without any doping was then calculated from the requirement of charge neutrality and assuming equilibrium number of vacancies in different charge states (-1 and -2 dominated). The net hole concentration in this configuration was calculated to be 8.8 × 1017 cm-3 at room temperature, which is in the range of the experimental Hall concentrations measured by Böttger
There have been many reports on changes in charge carrier concentration after heat treatments, both for single crystal and polycrystalline ZnSb. Many observed a slow recovery to the initial values of the charge carrier concentration [15, 20, 44, 45, 47, 61, 77]. Andronik
Recently, we have studied evaporation of Zn by thermogravimetry and Zn vacancy created during vaporization . The net hole concentration was measured to be about 6 × 1018 cm-3, corresponding to a vacancy concentration of about 0.03 at.%. Schematically, two processes in series were considered as:
Reaction (1) occurs when the VZn was created but within the dilute limit, while reaction (2) applies to a situation where the Zn vacancy concentration is beyond the solubility limit and ZnSb decomposes into Sb phase and Zn vapour.
5. Impurity band conduction
In an idealized semiconductor, which is well‐approximated by pure Si crystals , the charge carrier concentration shows the so‐called freeze out at low (cryogenic) temperatures: the dopant atoms are not ionized and the charge carrier concentration goes towards zero, characterized by an infinite Hall coefficient in Hall measurements. However, the Hall coefficient in undoped ZnSb has shown a turning point at low temperatures, typically below 50 K, as seen in Figure 7, and then a decrease with further cooling, which is explained by impurity band conduction . Here, the term impurity band is most likely tied to defects, but observed phenomena are similar to what can be observed for high doping concentration in semiconductors. Impurity band conduction has been reported in many materials [80–82]. It was also observed for ZnSb by Justi
Let us here analyse sketchily the conditions for observing the characteristics shown in Figure 7. The specifics of this observation are given in . The changes in hole concentration as the sample was cooled is in principle similar to that of a textbook low doping concentration semiconductor, where the charge carriers are frozen out of the valence band. The valence band of the sample in Figure 7 will be nearly empty (for holes) at the lowest temperature. The holes are transferred to the acceptor‐based impurity band. However, the holes are mobile in the impurity band and contribute to conduction and Hall effect. Thus, the Hall coefficient decreases with cooling to the lowest temperatures. In order to get conduction in the impurity band, some donor compensating centres are needed. (Without donor level, the impurity band would be full of holes, i.e. empty for electrons and there is no conduction).
We turn to a situation where the doping concentration is much higher than that of the sample in Figure 7, and first consider the large difference in characteristic features of the change in the charge carrier concentration with temperature. Figure 8 compares the Hall concentration of the charge carrier and the Hall mobility (inset) at low temperature for high (0.3 at.% Cu) and lower concentration (no Cu), respectively (data in Refs. [52, 79]). One can see that the characteristic feature of impurity band conduction vanished in the highly doped sample at low temperature. The situation is qualitatively as follows: A highly doped sample is equivalent to a degenerate semiconductor, where the hole concentration is high and the Fermi level is located in the valence band. The native impurity band would be full of holes and so would the top of the valence band. At the lowest temperatures, the conduction will occur within the valence band. Therefore, one cannot have a similar change with temperature as in the case of undoped ZnSb where there was a vanishing conduction in the valence band at the lowest temperature.
5.1. The impurity band in ZnSb—its nature, origin and specific points of interest
The nature and theoretical treatment of general impurity band can be found in textbooks . The band states are considered to come from interactions, which set in for concentrations above a certain value of defect species, such as dopant atoms, impurities or intrinsic crystal defects.
The formation of impurity band is illustrated in Figure 9 for an
The mobility in the impurity band is typically small because the band is relatively narrow, and as a consequence, one would have a small dispersion curvature in and large effective mass. In a semiconductor, at low temperature, conduction can occur dominantly in the impurity band if there are some compensating levels, such that the impurity band is not fully occupied. At a particular higher concentration, there may be a smear of the impurity band and the conduction band, as illustrated in Figure 9c. Beyond this concentration, the Mott transition, which is a sharp transition from insulator to metal behaviour, can take place.
Usually, the Mott criterion for when the transition occurs is , where is the atomic concentration of impurities and is the Bohr radius of the impurity. There are several different treatments yielding essentially the same numbers . The most considered situations are those of high concentrations of shallow donors or acceptors. Here, the Mott criterion gives the insulator‐metal transition. However, one can create impurity bands with a certain energy anywhere in the band gap. Some of the concentrations for the Mott transition then approximately correspond to the concentration to have an impurity band. Rawat
With the doping concentrations for optimum thermoelectric performance that typically is ∼1019 cm-3, it is reasonable to expect that the formation of impurity bands is rather common in thermoelectric materials. Also, impurity band formation is just one of several high doping effects one should expect, such as the Mott transition, band tailing and band gap renormalization . Thus, one should discuss thermoelectric material in the framework of the theory related to heavily doped semiconductors.
When it comes to ZnSb specifically, it has been estimated that the impurity band may exist for the impurity concentrations that are well within the observed doping concentrations. We have determined the critical impurity concentration in ZnSb to be about 6 × 1017 cm-3 in previous study, which is around the acceptor concentrations used in the model to fit the low temperature measurements . For the doping concentration of 1018–1019 cm-3 in ZnSb, one can expect impurity bands to form. For undoped or very lightly doped material, we suggested that point defects, especially Zn vacancies can be expected to be involved in impurity bands. From the discussion of vacancy formation in Section 4, it appears likely that a high concentration of vacancies can be created by annealing, even to some extent they could combine with any other possible impurities or defects in impurity band. The impurities will likely be dependent on the specific dopants and the preparation technique. For example, oxygen is expected to be an impurity in ball‐milled material and the amount introduced will depend upon the atmosphere during processing. Presently, one cannot make a conclusion about the importance of oxygen in this context and the solid solubility of oxygen is unknown. Fedorov
5.2. Impact of impurity band on thermoelectric properties
An impurity band will have an effect on the transport properties and the thermoelectric device properties. It may not be immediately transparent how. The conduction in the impurity band is perhaps a minor effect in this context. The most important effect may be on the Seebeck coefficient.
5.2.1. Effect on conduction
The effect of impurity bands on the electrical conductivity is expected to be largest when holes in the valence band (for
5.2.2. Effect upon Seebeck—effective density of states mass
The density of states effective mass may be affected by an impurity band. The density of states may be changed by several high doping effects including the formation of impurity bands. Qualitatively a smear of the impurity band and valence band is expected. Thus, even though the conduction of impurity band is often only observable at low temperature, the Seebeck coefficient can be affected above room temperature. The details to calculate the transport coefficients can be found in Ref. . The Seebeck coefficient is sensitive to the position of the Fermi level and how the density of states varies with energy. Both these factors can be affected by an impurity band. There have been reports on change of density of states effective mass with varying doping concentration . From a Pisarenko plot, one can find the density of states effective mass by fitting measured data. In Ref. , we obtained the best fitting by assuming an impurity band. Further, it was shown that camel‐shaped curves of Seebeck coefficient with temperature, which is unusual in ZnSb, could be modelled by a temperature‐dependent impurity band. It has been suggested that if one could engineer the energy structure of impurity band, then one could have a tool to enhance the thermoelectric performance .
Another impact of the impurity band may be on the
6. ZnSb synthesis techniques
Common synthesis methods for most of bulk thermoelectric materials can be categorized into three groups according to different processing steps, namely stoichiometric melts (SM), powder metallurgic method (PM), pseudo‐pulverized and intermixed elements sintering method (Pseudo‐PIES), as shown in Figure 10.
Polycrystalline ZnSb ingots can be synthesized by the so‐called SM methods, i.e. melting of the elemental zinc and antimony followed by solidification in air or quenching in cold water. The purity of the starting elemental zinc and antimony materials has a significant impact on the resulting electrical properties. For example, starting materials with purities of 99.99% and 99.9999% allows to obtain the charge carrier concentration of ∼1019 and ∼1016 cm-3, respectively, on undoped polycrystalline ZnSb . Since ZnSb does not melt congruently, solidification will result in a mix of the phases, Zn4Sb3, ZnSb and Sb. This mix can be homogenized by sufficient heat treatments to reach the thermodynamic equilibrium state with a single uniform ZnSb phase [14, 15]. Another problem with the solidification is that the sample contains cracks that significantly influence on the electron transport .
The solidified ingots are often milled into fine powder and pressed to pellets, which is a procedure that includes the basic ingredients of standard powder metallurgy (PM). Milling offers access to nanosized grains, thus providing possibility to enhance the thermoelectric properties. Earlier studies show that different milling techniques led to a trend of grain size as 80.0, 44.6 and 32.4 nm for manually grinding, dry‐milling and wet‐milling , as well as 10 nm for cryo‐milling , respectively.
The PIES method (pulverized and intermixed elements sintering method) has been introduced into preparation of thermoelectric (Bi/Sb)2(Te/Se)3 materials, where all the elements are initially mixed and milled to fine powder before hot‐pressing (no melting process) . The electrical conductivity of the sample that was synthesized by this method has been reported about 5 times higher than that for the SM sample . Distinguished from a typical PIES method, we often used pseudo‐PIES for ZnSb, which partially mixed the dopant element with SM ingot in ball‐milling, and then processed hot‐pressing.
There are different kinds of compaction techniques that follows powder metallurgy and have been used for fabrication of ZnSb samples. The most common ones are cold‐pressing (at room temperature with ultra‐pressure 2–10 GPa [83, 91]), hot‐pressing (>450°C with pressure of 20–300 MPa [16, 19, 20, 35]), and spark plasma sintering (SPS) (the electrical current is passed through the sample with 5 min reaction time at 350–450°C [83, 92]). One important difference among hot‐presses is the manipulation of secondary phases; both removal and proportioning are possible, and obviously depends upon the temperature and duration, but also on details of the instrument design and the environment of the ZnSb powder. Xiong
Another important consideration of the synthesis technique is the ability to produce large amounts of thermoelectric materials in a cost‐effective way. Considering that one of the favourable aspects of ZnSb from a commercial point of view is the low materials cost, there have been several efforts where the cost efficiency of the synthesis technique is important [92–94].
7. From laboratory to fabrication
ZnSb practical devices have been produced [9, 95], and there has been a promising achievement on thermoelectric performance of ZnSb in the laboratory. However, it is still challenging to transfer the achievement from laboratory to modern manufacturing. Progress in synthesis from different points of view have also to go through many tests regarding machinability, mechanical stability, thermal stability, thermal cycling and long‐term stability, as well as compatibility with targeted fabrication techniques. Several of these issues are expected to contribute to—as well as benefit from—a further fundamental understanding of ZnSb, when practical solutions on short and long timescales are targeted. On a short to medium timescale, ZnSb can take advantage of new fabrication technologies that has been developed, but using traditional approaches for the device functionality. On a longer perspective, ZnSb may also be brought further into the explorations of new nanotechnology approaches to improve the performance of possible future generations of thermoelectrics.
One hindrance towards an ideal thermoelectric module made entirely of ZnSb is the inability to synthesis of stable
The thermal stability of the synthesized ZnSb needs to be tested and probably be improved. This is one area where both fundamental studies and practical solutions may enter. ZnSb samples are subjected to Zn evaporation at high temperatures. The evaporation depends naturally very much on the ambient and the surface conditions. It is possible that protective layers can be applied to minimize evaporation. The situation has some similarities to that of several binary electronic materials, such as III–V materials, where one of the elements have a much higher vapour pressure, than what can be tolerated at the desired processing temperature. For GaAs, dielectric films SiO2 and Si3N4 have been used for the purpose of preventing arsenic evaporation. A similar approach with a conformal deposition of a protective dielectric layer may be advantageous for ZnSb. Thermal stability also has to do with the thermal generation of point defects and their diffusion at elevated temperatures. The understanding of the phenomena is unsatisfactory from an academic point of view, in particular on the level of defect chemistry and electronic structure, but there are many experimental observations of the simple electrical parameters. Several authors have reported that after a heat treatment of ZnSb, the electrical conductivity and the charge carrier concentration increased, while the Seebeck coefficient decreased. The change was followed by a slow recovery towards the initial values at room temperature [15, 20, 44, 45, 47, 61, 77]. The characteristics can be related it to the VZn‐ZnI Frenkel pair formation at elevated temperatures, and the recovery caused by their slower recombination at lower temperature. The vacancy concentration was linked to hole concentration. It was rationalized that these hysteresis effects would not be significant at high doping concentrations . The doping effect on the vacancy concentration was then not considered. A detailed understanding of the vacancies, interstitials and their energy levels, ionization and formation energies is needed to understand the influence for higher doping concentration. The influence of more complicated defects can also be a large challenge. There are also reports on various temperature‐cycling phenomena [52, 96, 97], involving the doping atoms and energy levels of these. Some of these effects may differ for different synthesis details.
The thermal stability is referring to all properties of the material, including the thermal conductivity. We have reported grain growth induced by heating, particularly in nanostructured bulk samples . The grain growth will naturally induce a change of thermal conductivity due to the dependence on phonon scattering. To which extent, it constitutes a practical problem depends upon the targeted operation temperature. Approaches to minimize grain growth usually consist of adding atoms that segregate in grain boundaries, thereby preventing grain growth. This is an area that needs further study.
There are various routes that can make ZnSb a part of long‐term exploration to improve thermoelectrics by nanostructures (not just nanograins). Some of them may use ways of depositing films of ZnSb, such as by MOCVD, sputtering [98, 99] and electroplating [11, 100],
The authors acknowledge support by the Norwegian Research Council under contract NFR11‐40‐6321 (NanoThermo) and the University of Oslo. The authors also thank to Dr. Patricia Almeida Carvalho for her contribution to transmission electron microscopic imaging.
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