What are the characteristics of copper telluride?

Green and renewable energy sources have attracted much attention because of concerns about pollution and limited oil sources.  Technology based on thermoelectric (TE) materials is appealing because it can realize direct conversion of waste heat to electric energy.  The dimensionless figure of merit (zT) is often used to evaluate the TE performance of a material;  a large zT is required to achieve a high energy conversion efficiency.  The zT is defined as S2σT/κ, where S is the Seebeck coefficient, σ is electrical conductivity, T is absolute temperature and κ is thermal conductivity (consisting of carrier thermal conductivity κc and lattice thermal conductivity κL). Application of TE technology is not widespread because of the limited zTs, which results in low conversion efficiency.  Improvement of materials’ performance is still the key to developing the TE technique.  Tuning the electron and phonon transports has greatly enhanced the zTs for many types of materials over the past decade.

Copper sulfide proved as excellent TE materials

Recently, copper selenide (Cu2-δSe) and copper sulfide (Cu2-δS) with liquid-like characteristics were shown to be excellent TE materials with exceptionally low thermal conductivity and high TE performance.  zTs as high as 1.5 in Cu2-δSe and 1.7 in Cu2-δS at 1000 K have been achieved, which are among the top values in bulk TE materials.  Cu2-δTe belongs to the same group of materials.  Because tellurium is heavier than sulfur and selenium, the thermal conductivity in telluride is usually expected to be lower than that in selenide or sulfide.  In addition, because tellurium is less electronegative, the chemical bonds for tellurides should be less ionic than those for sulfides and selenides, and the carrier mobility should be large in tellurides.  These two features make tellurides potentially important TE materials.  In fact, many of the state-of-the-art TE materials are tellurides, such as PbTe, Bi2Te3 and AgSbTe2. The reported high TE performance in Cu2-δSe and Cu2-δS indicates that a high zTs may also be achieved in Cu2-δTe.  However, recent studies showed that the zTs in Cu2Te is only approximately 0.3 at 900 K, which is much lower than those in Cu2Se and Cu2S.  Historically, the zTs in the tellurides have been reported to be higher than those in the selenides or the sulfides in classic TE materials, such as PbX (X=S, Se or Te)  and Bi2X3 (X=S, Se or Te).  The breakthrough of the zTs in Cu2-δX (X=S, Se or Te) is highly unusual.  By comparing the TE properties of Cu2-δX, we found that the abnormality is because Cu2Te has high electrical conductivity and low thermopower as compared with Cu2-δSe or Cu2-δS owing to its severe copper deficiency.  Although the stoichiometric chemical ratio of 2:1 for Cu and Te is used to increase copper levels as much as possible during the sample growth process, the Cu2Te bulk materials created using spark plasma sintering (SPS)—the same process used for Cu2-δSe or Cu2-δS—still have a marked copper deficiency as well as a low zTs.

TE technology is a fully solid-state technique, and its performance is determined primarily by materials’ density. A high density approaching a material’s theoretical density is typically required in bulk materials for high energy conversion efficiency to optimize the electrical transport properties. Partial covalent bonds and partial ionic bonds are the dominant chemical bonds in most TE materials. However, the atomic diffusion rates are usually low in these materials. SPS, or hot-pressing, is usually used to sinter powder materials to achieve high-density bulk samples; this technique has been used in nearly all bulk TE materials. These materials are different from metals and ceramics, which have high atomic diffusion rates that can be densified using direct annealing without extrinsic pressure (pressureless sintering). However, the extra SPS processes may slightly change the samples’ compositions as well as their physical properties. In Cu2-δX (X=S, Se, Te), even though S and Se elements have high vapor pressure, the chemical compositions of Cu2-δS and Cu2-δSe are easier to control than that of Cu2Te. This may be attributable to the chemical bonds. S/Se is smaller than Te, and the electronegativity difference between S/Se and Cu is larger than that between Te and Cu. Thus, the ionic bonding between Cu and S/Se is stronger than that between Cu and Te, and the self-compensation between the Cu vacancy and the anion vacancy is more efficient in Cu2-xS/Cu2-xSe than in Cu2-xTe. In addition, there are many stable and meta-stable phases in Cu2Te. Because these phases are very similar in structure and energy, a small change during the sample fabricating process, such as SPS sintering, can affect its structures and phases.