materials

Article Laser Treatment as Sintering Process for Dispenser Printed Based Paste

Moritz Greifzu 1,*, Roman Tkachov 1,2 , Lukas Stepien 1, Elena López 1, Frank Brückner 1,3 and Christoph Leyens 1,2

1 Additive Manufacturing and Printing, Fraunhofer-Institut für Werkstoff- und Strahltechnik, Dresden 01277, Germany; [email protected] (R.T.); [email protected] (L.S.); [email protected] (E.L.); [email protected] (F.B.); [email protected] (C.L.) 2 Institute of Materials Science, Technische Universität Dresden, Dresden 01062, Germany 3 Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå 97187, Sweden * Correspondence: [email protected]; Tel.: +49-351-83391-3606

 Received: 15 September 2019; Accepted: 17 October 2019; Published: 22 October 2019 

Abstract: Laser sintering as a thermal post treatment method for dispenser printed p- and n-type bismuth telluride based thermoelectric paste materials was investigated. A high-power fiber laser (600 W, 1064 nm) was used in combination with a scanning system to achieve high processing speed. A Design of Experiment (DoE) approach was used to identify the most relevant processing parameters. Printed layers were laser treated with different process parameters and the achieved sheet resistance, electrical conductivity, and Seebeck coefficient are compared to tube furnace processed reference specimen. For p-type material, electrical conductivity of 22 S/cm was achieved, compared to 15 S/cm in tube furnace process. For n-type material, conductivity achieved by laser process was much lower (7 S/cm) compared to 88 S/cm in furnace process. Also, Seebeck coefficient decreases during laser processing (40–70 µV/K and 110 µV/K) compared to the oven process (251 µV/K and 142 µV/K) for − − p- and n-type material. DoE did not yet deliver a set of optimum processing parameters, but supports doubts about the applicability of area specific laser energy density as a single parameter to optimize laser sintering process.

Keywords: laser sintering; thermoelectric; bismuth telluride; telluride; design of experiment; additive manufacturing

1. Introduction Thermoelectricity comprises several physical effects that occur in materials by means of voltage drop due to temperature gradient over the material (Seebeck effect) or development of temperature drops due to current flux (Peltier effect). These material properties are used in thermoelectric generators, e.g., as part of radioisotope generators to supply energy to deep space probes, when photovoltaic cells are no longer efficient. Probably the most familiar application for Peltier coolers is portable refrigerators. Currently, the most efficient materials for room temperature application are bismuth telluride and -based alloys. The standard route for producing generators/Peltier elements is compacting the material into disks, applying spark-plasma-sintering, cutting the disks into cubes, soldering them onto contacted Al2O3 substrates, lapping them, and soldering the top electrode. The approach is characterized by a considerable amount of manual work. An alternative approach to build up such devices is presented by additive manufacturing. Additive manufacturing describes a broad range of manufacturing technologies where material is added layer by layer to create a part. One of these processes is the SLM technique [1] (Selective Laser Melting), also called

Materials 2019, 12, 3453; doi:10.3390/ma12203453 www.mdpi.com/journal/materials Materials 2019, 12, 3453 2 of 15

LPBF (Laser Powder Bed Fusion). The powder is spread homogeneously by a blade or a roll on a bed and a laser, coupled to a scan optic introducing energy into that spread powder, usually melting and consolidating it. Through repeating this process of spreading and laser processing, a 3D part is generated. This technology is usually used to process structural materials as stainless steel [2], aluminum [3], or titanium [4]. However, they have also already been used to build up different such as half-Heusler [5], [6], and bismuth telluride [7]. The feasibility of laser processing of Bi2Te3 powder compacts was shown in recent work of El-Desouky et al. [8]. Formation of the correct phases for the used parameters was confirmed by XRD measurements. However, there it is stated that commercially available Bi2Te3 powders usually do not fulfil the required specifications for additive manufacturing, especially for LPBF, which is also mentioned in Reference [7]. Two papers were published by a research group around Mao [9] and Wu [10]. Mao has synthesized Bi2Te2.7Se0.3 by self-propagating high-temperature synthesis (SHS). A slurry was produced, mixing the ground material with alcohol, and then the slurry was spread on a substrate. For comparison, a sample of the same powder was produced by conventional SPS (spark plasma sintering). It was found that the chemical composition of the material was dependent on the laser energy density. Energies higher than 33 J/mm3, especially and , evaporated from the sample significantly. The strategy to compensate this effect was to enrich the starting composition of the compound, especially by using excess of tellurium. Good transport properties are measured, as shown in Table1, though the properties are measured after 36 h of thermal annealing at 400 ◦C. The figure of merit (ZT) value at room temperature was calculated to 0.65. In Wu’s paper, the slurry is modified for dispenser printing. Also, DSC (differential scanning calorimetry) analysis is performed for as-processed samples in comparison to those which received thermal annealing at 400 ◦C for 6 h. These measurements indicate material inhomogeneity after LPBF, which is compensated by annealing. A bulk sample was produced by stacking 75 layers. The sample was used to measure the transport properties given in Table1. Wu’s group has published on further experiments based on LPBF fabrication of thermoelectric materials like CoSb2.85Te0.15 [11], SnTe [6], or ZrNiSn [12].

Table 1. Overview over previous studies on laser treatment of bismuth telluride materials.

Seebeck Electrical Power Wavelength Ref. Material E (J/mm2) Coefficient Conductivity Factor (nm) sq. (µV/K) (S/cm) (W/mK2) Not Not Not [8] Bi Te (powder) 1070 0.6–1.4 2 3 measured measured measured [7] Bi Te (powder) 1070 1.3 166 600 1.65 10 3 2 3 − × − [9] Bi Te Se (dried slurry) 1064 0.3–11 1 145 700 1.47 10 3 2 2.7 0.3 − × − [10] Bi Te Se (printable paste) 1064 0.5–1.5 1 105.7 122 1.36 10 3 2 2.7 0.3 − × − 1 calculated from volumetric density that was given in the papers.

Problems pointed out by References [7,8] in processing bismuth telluride powder by LPBF occur when trying to homogenously spread the powder, due to its arbitrary shape and broad particle size distribution. In additive manufacturing, spherical powders and narrow diameter distribution are usually favorable. Using the approach of creating a paste circumvents these issues, since for dispensing processes these perquisites to the material are less severe. Precisely, the overall idea is to use a dispenser for printing electrode, and p-/n-type thermoelectric materials in order to build up the whole thermoelectric device. The dispensing process is suitable for such an approach since multi-color printing is possible. The problem is seen in a thermal process to consolidate the printed materials and to adjust the thermoelectric properties. In this work, laser processing is compared with more conventional furnace processing of bismuth telluride materials. The following Table2 summarizes the results of the work, where thermoelectric materials were prepared as printable pastes and then thermally post treated in furnaces. It can be clearly seen that the range of achievable thermoelectric properties is broad. At a first glance, comparing the two tables, laser processing seems to lead to much better properties, but it should be noted that Materials 2019, 12, 3453 3 of 15 after laser processing, the authors of References [9,10] annealed their specimen for several hours at 400 ◦C, which exceeds most temperatures in Table2. The achieved power factors shown in Table2 do not exceed a power factor of 7.28 10 4 W/(mK2). Achieving high electrical conductivity seems the × − most critical issue, since Seebeck coefficients seem quite reasonable.

Table 2. Overview of previous work on bismuth telluride materials that were thermally post treated in furnaces.

Seebeck Electrical Power Thermal Ref. Atmosphere Material Coefficient Conductivity Factor Processing (µV/K) (S/cm) (W/mK2) 4 12 h @ 350 ◦C n.a. Sb Te 152 315 7.28 10 [13] 2 3 × − 4 48 h @ 350 ◦C n.a. Bi Te 287 14 1.15 10 2 3 − × − 4 [14] 12 h @ 250 ◦C Ar Bi Te (+2% Se) 190 40 1.44 10 2 3 − × − 4 [15] 12 h @ 350 ◦C n.a. Bi Te (+1% Se) 170 100 2.89 10 2 3 − × − 4 [16] 12 h @ 250 ◦C n.a. Bi Sb Te (+8% Te) 280 23 1.80 10 0.5 1.5 3 × − 5 Bi0.5Sb1.5Te3 (+8% Te) 250 11 6.88 10− [17] 12 h @250 ◦C Ar × Bi 84 110 7.76 10 5 − × − 5 Sb2Te3 137 53 9.88 10− [18] 3 h @ 250 ◦C n.a. × Bi Te 132 4 6.97 10 6 1.8 3.2 − × − 4 Sb2Te3 103 200 2.12 10− [19] 3 h @ 250 ◦C n.a. × Bi Te 145 7 1.40 10 5 1.8 3.2 − × − 4 Sb2Te3 109 278 3.27 10− [20] 3 h @ 250 ◦C N2 × Bi Te 138 100 1.92 10 4 1.8 3.2 − × − 4 ZnSbO 225 30 1.52 10− [21] 10 min @ 500 ◦C Air × CoSb 49 211 5.05 10 5 3 − × −

Compared to furnace heating, which usually takes hours, laser scanning is instead a very fast process and surfaces of some square millimeters can be scanned within less than a second. Laser treatment would allow the exact control of energy to be introduced into the printed layers, which could not be done in furnaces, but would be important, since e.g. p- and n-type materials have different sintering/melting temperatures. Laser processing would thus allow parallel build-up of both materials. The thermoelectric community currently searches basically for new materials with high ZT values, since this increases the overall performance of Peltier or generator devices. Another focus is put on abundant or flexible materials [22]. From an engineering point of view, it is also very important to optimize and automatize the buildup of thermoelectric devices in order to decrease their cost, and thus for generators also the ratio of cost per generated power. Here, additive manufacturing presents a new and very promising alternative.

2. Materials and Methods A paste was developed for the experiments conducted in this investigation, made of p-/n-doped bismuth telluride, polyvinylpyrrolidone (binder), and terpineol (solvent). The raw material was acquired as particles (mesh size 800, hence particles smaller than 15 µm) from Leshan Kai Yada Photoelectric Technology Co. Ltd (Hangzhou, China). Purity is specified as 99.99%. A check of the chemical composition of the materials by EDX (Energy-dispersive X-ray spectroscopy) resulted in the distribution, shown in Table 6. Subsequently, the selenium doped bismuth telluride is referred to as n-type and the antimony bismuth telluride as p-type. For printing, a dispenser printer (Musashi Engineering Europe GmbH., Muenchen, Germany) by Musashi, the Shotmaster 500 was used. Dispensing is a printing method, where a paste material is applied through pressure onto a substrate from a cartridge via a needle with a specific diameter. Materials 2019, 12, 3453 4 of 15

The cartridge is moved by the printer into x-, y-, and z-direction over the substrate by the printer. Parameters like printing velocity, applied pressure, needle diameter, and gap between needle and substrate influence the result and are usually optimized through a parameter study for each printing paste. Al O substrates were used to print the layers on. The size of the printed layers was 7.5 7.5 mm 2 3 × or 10 10 mm. × A 600 W, 1064 nm, single mode, ytterbium fiber laser system with a beam parameter product (BPP) of 0.37 mm mrad from the company IPG Laser GmbH (Burbach, Germany) was used. It was coupled · to a 160 mm collimation (Coherent), a scanning system (IntelliScan20, Scanlab GmbH, Puchheim, Germany) and a 340 mm f–theta focusing lens (Sill Optics GmbH & Co. KG, Wendelstein, Germanz). All experiments were conducted out of the focal plane of the system, since working in the focus plane after first trials showed too-high energies which evaporated the printed material. The distance from focal plane was varied between 20 and 24 mm. A portable mini-glove box was used with an optical window on the top for the laser beam. This box is placed under the scanner and flooded with argon to achieve inert gas atmosphere. The residual oxygen content was measured with an Oxy HP 2.0 system from OWT GmbH (Hagen am Teutoburger Wald, Germany). Experiments in Ar-atmosphere were conducted with less than 20 ppm oxygen. Laser treatment was performed programming the laser-scanner system to emit a certain optical power P and moving the laser spot with the velocity v over the specimen’s surface. To treat a rectangular area, a hatch spacing sis defined. The laser then writes a line, switches off, is moved back to the starting point, shifted perpendicularly to the writing direction by the hatch spacing, switched on, and then writes a line again. This is repeated until the area is fully treated. The tube furnace, which was used to produce reference specimen to be compared with the laser treated specimen is a furnace from HTM Reetz GmbH (Berlin, Germany). It can be connected to dry air, nitrogen, argon, or forming gas atmosphere. The tube oven can operate at maximum heat rates of 10 K/min, maximum temperature of 1150 ◦C, and does not provide active cooling. The characterization of the printed surfaces after thermal treatment was done by measuring thermoelectric properties. Sheet resistance was measured a by Keithley 2001 multimeter (HTM Reetz GmbH, Ohio, USA) in a four-point measurement setup. The setup is equipped with spring-loaded pins having 1 mm spacing, positioned in a line, and centered on the specimen to be measured. Measurements were conducted in ambient conditions. The following equation was applied to calculate electrical conductivity σ from the measured sheet resistance Rsq.:

! ! 1 a d w − σ = Rsq. w C ; F (1) × × d s × s with w – sheet thickness, and C(a/d;d/s) and F(w/s) being geometrical correction factors. In this investigation, due to used geometries, the factors were C(a/d;d/s) = 4.0095 and F(w/s) = 1 [23]. To calculate conductivity, the layer thickness has to be measured. This has been done using a micrometer screw gauge. Pictures were taken with a Keyence VHX5000 optical microscope (Keyence Germany GmbH, Neu-Isenburg, Germanz). A commercial setup (SRX, Fraunhofer IPM, Freiburg, Germany) was used for the measurement of the Seebeck effect. All measurements were performed as stated in air or nitrogen. The applied current was kept below 10 mA in order to minimize joule heating effects. Seebeck coefficient measurements were carried out with temperature gradients of ca. 3 K. The sample was placed on two heaters. To set up a temperature gradient, one heater was activated while the other acted as the cold side. Temperature and voltage were measured by two thermocouples. Subsequently the triggering of the heaters was reversed so that the Seebeck coefficient was measured again. To ensure better electrical and thermal contact of the prepared thin films, carbon foil was applied. No further (sputtered) gold contacts were used whatsoever. The thermocouple distance for measuring the temperature and voltage was set to 3.0 mm. To ensure proper contact, U-I-curves were tracked and showed linear behavior. Materials 2019, 12, 3453 5 of 15

The program Minitab (v18, Minitab GmbH, Muenchen, Germany) was used to create and conduct the DoE study. In a factorial design DoE study, a number “n” of parameters is fed to the related software. The program then responds with a set of experiments to be conducted, which can be envisioned as a regular n-dimensional body with n = ”number of parameters”. The edges of this body are represented by specific combinations of the parameter values. Since for this form of DoE, linear relation between experimental parameters and response is a precondition, a central point was included into the study. The software then creates a linear model of influence parameters and response, assigning factors to the influence parameters which describe their impact on the response. This model is expressed by a linear equation containing all chosen factors and all combinations thereof, describing these factors influence on the response parameter. By stepwise backwards elimination of statistically non-relevant parameters, the model can be simplified and reduced to the relevant ones. The relevance of parameters can be seen in a Pareto Diagram. There, all bars crossing the red line are considered to have statistical significant influence on the response term. The red line is called the reference line and depends on the statistical significance level α. For DSC, equipment from Netzsch (Selb, Germany), namely the model DSC 404 F1 Pegasus was used. Experiments were conducted under argon atmosphere at heating and cooling rates of 10 K/min. Dry paste was put into Al2O3 pans, 21.41 mg for p-type and 37.66 mg for the n-type material. Imaging and EDX measurements were carried out with the scanning electron microscope JEOL 7800 (JEOL GmbH, Freising, Germany) with an Oxford Xmax80 EDX detector. An element mapping was performed on a polished specimen, which was sputtered with carbon.

3. Results In additive manufacturing, especially in the LPBF processes, the VED (volumetric energy density) is often used [9,24] as comprehensive parameter, even though it has been criticized as being too unspecific to describe the complex physical processes in the melt pool [24]. It is calculated from all relevant tunable parameters P E = (2) V v s h × × with P – laser power, v – scan velocity, s – hatch spacing, and h – thickness of layer. In contradiction to this, the area specific energy density Esq. was used in these experiments for comparing results of different laser process parameters with each other. It leaves out the layer thickness. The thickness of dispenser printed layers can actually be measured, but, anticipating results of this investigation, laser energy neither did propagate fully through the printed layers, nor was a clear border between treated and non-treated material visible. The area specific energy density Esq. is defined as

P E = (3) sq. v s × with P – laser power, v – scan velocity, and s – hatch spacing. This parameter has been used, e.g., also by El-Desouky et al. [8]. A first set of parameters for laser processing was defined and experiments were conducted to approve the processing window. Also, processing in ambient atmosphere and Ar atmosphere was done, to find out the effect of the presence of oxygen on the process. One batch of specimens was printed for these experiments. Layer thickness was not characterized at this stage, but only the sheet resistance of the differently processed layers was measured. In parallel, a DSC analysis was made to find out the melting point of p- and n-type material. The results of the initial laser experiment are plotted as graph of sheet resistance versus Esq. in Figure1. Materials 2019, 12, x FOR PEER REVIEW 6 of 16

energy neither did propagate fully through the printed layers, nor was a clear border between treated and non-treated material visible. The area specific energy density Esq. is defined as 𝑃 𝐸 = (3) . 𝑣×𝑠

with P – laser power, v – scan velocity, and s – hatch spacing. This parameter has been used, e.g., also by El-Desouky et al. [8]. A first set of parameters for laser processing was defined and experiments were conducted to approve the processing window. Also, processing in ambient atmosphere and Ar atmosphere was done, to find out the effect of the presence of oxygen on the process. One batch of specimens was printed for these experiments. Layer thickness was not characterized at this stage, but only the sheet resistance of the differently processed layers was measured. In parallel, a DSC analysis was made to find out the melting point of p- and n-type material. The results of the initial laser experiment are Materials 2019, 12, 3453 6 of 15 plotted as graph of sheet resistance versus Esq. in Figure 1.

10000

1000 /square)

Ω 100

10

1 sheet resistance ( sheet resistance 0.1 0.04 0.06 0.08 0.10 0.12 0.14 0.16 specific surface energy density (J/mm²) FigureFigure 1. Comparison1. Comparison of sheet of sheet resistance resistance of n-type of n-type bismuth bi telluridesmuth telluride when processed when processed in air (red in squares) air (red

andsquares) Ar (<20 and ppm Ar O(<202-content ppm O<220-content ppm; < blue 20 ppm; dots) atmosphere.blue dots) atmosphere. Power P = Power100 W, PScan = 100 velocity W, Scan v =velocity2.5–4 m v/s, = spot2.5–4 diameter m/s, spot d diameter= 562 µm, d hatch= 562 µm, spacing hatch h =spacing0.3–0.5 h mm. = 0.3–0.5 mm.

It isIt observedis observed that that a clean a clean Ar atmosphere Ar atmosphere leads to lead muchs to lower much sheet lower resistance sheet ofresistance around 1ofΩ /aroundsquare, 1 whileΩ/square, processing while in processing air leads to in about air leads 1000 toΩ /aboutsquare 1000 for low Ω/square and 100 forΩ low/square and for 100 high Ω/squareEsq.. Further, for high it appearsEsq.. Further, that it higher appears specific that higher energy specific density energy leads density to lower leads sheet to resistance, lower sheet especially resistance, in especially case of processingin case of in air.processing Due to thatin air. result, Due all to further that result, experiments all further were conductedexperiments in Arwere atmosphere. conducted in Ar atmosphere.Results of the differential scanning calorimetry of both materials are shown in Figures2 and3. In the rightResults part of of the the differential diagram for scanning n-type, a calorimetr clear meltingy of point both ismaterials visible at are almost shown 580 in◦C. Figure That peak 2 and wasFigure not seen 3. In in the Reference right part [13 ]of since the diagram the maximum for n-type, temperature a clear inmelting that investigation point is visible was at 450 almost◦C. For 580 the °C. p-typeThat materialpeak was in not the seen left in part Reference of the figure, [13] since the situationthe maximum is more temperature complex. in Two that peaks investigation are visible, was a first450 °C. one For at 410 the◦ p-typeC and amaterial second in at the 503 left◦C. part Madan of the et. figure, al. [13] the found situation a peak is for more their complex. p-type SbTwo2Te peaks3 at Materials425are◦C, visible, 2019 commenting, 12 , ax FORfirst PEER thatone REVIEW thisat 410 would °C and be the a second melting at temperature 503 °C. Madan of Te et. or Te-richal. [13] phases.found a The peak first 7for peakof their16 measuredp-type Sb in2Te this3 at investigation 425 °C, commenting probably correspondsthat this would to the be one the in melting Reference temperature [13]. of Te or Te-rich phases. The first peak measured in this investigation probably corresponds to the one in Reference [13]. 700 0.6 600 0.5

0.4 500

0.3 400

0.2 300

0.1 Temp. (°C) DSC (mW/mg) 200 0.0 100 -0.1 0 0 20406080100120 Time (min) FigureFigure 2. DifferentialDifferential scanning scanning calorimetry calorimetry (DSC) (DSC) results results for for dry dry p-type p-type paste paste (red (red dotted dotted line line shows shows temperaturetemperature curve). curve).

0.8 700

0.6 600

0.4 500

0.2 400

0.0 300 Temp. (°C)

DSC (mW/mg) -0.2 200

-0.4 100

-0.6 0 0 20406080100120

Time (min)

Figure 3. Differential scanning calorimetry (DSC) results for dry n-type paste (red dotted line shows temperature curve).

In a second step, using the results from the first laser experiments, the approach for optimizing the process was formalized by using design of experiment (DoE). Design of the study and its parameters are described in the subsequent paragraph. A new batch of specimens was produced by dispenser printing, including specimens for laser and tube furnace processing. At this step, sheet resistance and sheet thickness were measured in order to calculate electrical conductivity of the processed material. Also, the was measured for some specimens. Cross sections were examined by optical microscopy as well as the top view of the laser treated layers. The DoE study was performed for two different reasons. First, the previously done experiments raised doubts as to whether the integral parameter Esq. sufficiently describes the influence of laser radiation onto the sheet resistance of the treated layers. Secondly, an optimization of processing parameters is needed. The DoE study was designed in such a way that not only the influence of the Esq. on the sheet resistance, but also the influence of its singular factors of laser power, scan velocity, hatch spacing (see Equation (3)), and additionally, the distance from the focal plane could be analyzed. The parameter set for the laser for the DoE study is shown in the Table 3. The same parameters were used for experiments with n- and p-type material, besides the laser power, where the maximum applied power was increased up to 150 W. This was done due to the higher melting temperature observed in DSC. The sheet resistance of the laser treated surfaces again was chosen as response parameter.

Materials 2019, 12, x FOR PEER REVIEW 7 of 16

700 0.6 600 0.5

0.4 500

0.3 400

0.2 300

0.1 Temp. (°C) DSC (mW/mg) 200 0.0 100 -0.1 0 0 20406080100120 Time (min) Figure 2. Differential scanning calorimetry (DSC) results for dry p-type paste (red dotted line shows Materialstemperature2019, 12, 3453 curve). 7 of 15

0.8 700

0.6 600

0.4 500

0.2 400

0.0 300 Temp. (°C)

DSC (mW/mg) -0.2 200

-0.4 100

-0.6 0 0 20406080100120

Time (min) Figure 3. Differential scanning calorimetry (DSC) results for dry n-type paste (red dotted line shows Figure 3. Differential scanning calorimetry (DSC) results for dry n-type paste (red dotted line shows temperature curve). temperature curve). In a second step, using the results from the first laser experiments, the approach for optimizing the In a second step, using the results from the first laser experiments, the approach for optimizing process was formalized by using design of experiment (DoE). Design of the study and its parameters the process was formalized by using design of experiment (DoE). Design of the study and its are described in the subsequent paragraph. A new batch of specimens was produced by dispenser parameters are described in the subsequent paragraph. A new batch of specimens was produced by printing, including specimens for laser and tube furnace processing. At this step, sheet resistance and dispenser printing, including specimens for laser and tube furnace processing. At this step, sheet sheet thickness were measured in order to calculate electrical conductivity of the processed material. resistance and sheet thickness were measured in order to calculate electrical conductivity of the Also, the Seebeck coefficient was measured for some specimens. Cross sections were examined by processed material. Also, the Seebeck coefficient was measured for some specimens. Cross sections optical microscopy as well as the top view of the laser treated layers. were examined by optical microscopy as well as the top view of the laser treated layers. The DoE study was performed for two different reasons. First, the previously done experiments The DoE study was performed for two different reasons. First, the previously done experiments raised doubts as to whether the integral parameter Esq. sufficiently describes the influence of laser raised doubts as to whether the integral parameter Esq. sufficiently describes the influence of laser radiation onto the sheet resistance of the treated layers. Secondly, an optimization of processing radiation onto the sheet resistance of the treated layers. Secondly, an optimization of processing parameters is needed. parameters is needed. The DoE study was designed in such a way that not only the influence of the Esq. on the sheet The DoE study was designed in such a way that not only the influence of the Esq. on the sheet resistance, but also the influence of its singular factors of laser power, scan velocity, hatch spacing (see resistance, but also the influence of its singular factors of laser power, scan velocity, hatch spacing Equation (3)), and additionally, the distance from the focal plane could be analyzed. The parameter (see Equation (3)), and additionally, the distance from the focal plane could be analyzed. The set for the laser for the DoE study is shown in the Table3. The same parameters were used for parameter set for the laser for the DoE study is shown in the Table 3. The same parameters were experiments with n- and p-type material, besides the laser power, where the maximum applied power used for experiments with n- and p-type material, besides the laser power, where the maximum was increased up to 150 W. This was done due to the higher melting temperature observed in DSC. applied power was increased up to 150 W. This was done due to the higher melting temperature The sheet resistance of the laser treated surfaces again was chosen as response parameter. observed in DSC. The sheet resistance of the laser treated surfaces again was chosen as response parameter. Table 3. Parameter matrix for Design of Experiment (DoE) factorial design.

Material p-Type n-Type min max min max Parameter 1 +1 1 +1 − − Laser power (W) 80 120 80 150 Scan velocity (m/s) 2.5 5.0 2.5 5.0 Hatch spacing (mm) 0.3 0.5 0.3 0.5 Distance from focal plane (mm) 20 24 20 24

Alternatively, the electrical conductivity could also have been chosen as the response parameter, but it was known from the first set of experiments that the laser parameters would have impact on both sheet resistance and sheet thickness, from which the conductivity is calculated. In the next paragraph, the results of the second set of experiments, including the DoE study, are presented. From a Pareto plot, the importance of an influence factor on a result is shown. The longer a beam in the diagram, the higher the influence of the corresponding parameter. Materials 2019, 12, x FOR PEER REVIEW 8 of 16

Table 3. Parameter matrix for Design of Experiment (DoE) factorial design.

Material p-Type n-Type min max min max Parameter −1 +1 −1 +1 Laser power (W) 80 120 80 150 Scan velocity (m/s) 2.5 5.0 2.5 5.0 Hatch spacing (mm) 0.3 0.5 0.3 0.5 Distance from focal plane (mm) 20 24 20 24

Alternatively, the electrical conductivity could also have been chosen as the response parameter, but it was known from the first set of experiments that the laser parameters would have impact on both sheet resistance and sheet thickness, from which the conductivity is calculated. Materials 2019, 12, 3453 8 of 15 In the next paragraph, the results of the second set of experiments, including the DoE study, are presented. From a Pareto plot, the importance of an influence factor on a result is shown. The longer a beamFor thein the p-type diagram, material, the thehigher Pareto the plotinfluence in Figure of the4a showscorresponding that all terms parameter. of Equation (3) significantly influenceFor thethe sheet p-type resistance material, in the order:Pareto scanplot velocity,in Figure hatch 4a shows spacing, that laser all power, terms andof Equation combinations (3) thereof.significantly The distance influence from the the sheet focal resistance plane came in the out order: to have scan no velocity, significant hatch influence spacing, within laser the power, tested range.and combinations For the n-type thereof. material, The the distance Pareto diagramfrom the (Figurefocal plane4b) looks came more out complex,to have no indicating significant the hatchinfluence spacing within tobe the the tested most range. relevant For the parameter, n-type ma andterial, also the combinations Pareto diagram of hatch–spacing–distance (Figure 4b) looks more fromcomplex, focal plane,indicating distance the hatch from focalspacing plane–velocity, to be the mo andst relevant a triple parameter, influence consisting and also combinations of all parameters of excepthatch–spacing–distance distance from focal from plane. focal plane, distance from focal plane–velocity, and a triple influence consisting of all parameters except distance from focal plane.

(a) (b)

FigureFigure 4. 4.Pareto Pareto plotplot forfor thethe reducedreduced statisticalstatistical models calculated by by Minitab, Minitab, (a (a) )p-type; p-type; (b (b) )n-type. n-type.

TheThe measured measured sheet sheet resistance resistanceRsq. Rwassq. was also also plotted plotted against againstEsq. in E Figuresq. in Figure5. For the5. For p-type the material,p-type thematerial, plot seems the toplot show seems some to potentialshow some decrease potential of resistancedecrease of for resistance growing laserfor growing energy, approachinglaser energy, a Materialsthresholdapproaching 2019, around12, x FORa threshold 1PEERΩ/square. REVIEW around For the1 Ω n-type/square. material, For the then-type relation material, appears the torelation be linear. appears In the9 toof same be16 diagram,linear. In the the calculated same diagram, electrical the calculated conductivity electrical is plotted, conductivity which will is plotted, be discussed whichlater. will be discussed later. 16 25 10 7

6 12

20 8 square) square) Ω/ Ω/ 5 15 6 4 8 10 4 3

conductivity (S/cm) 4 conductivity (S/cm) 5 2 2 sheet resistance ( sheet resistance sheet resistance ( 0 0 1 0.04 0.08 0.12 0.16 0.00 0.05 0.10 0.15 0.20 0.25 0.30 area energy density (J/mm²) area energy density (J/mm²) (a) (b)

FigureFigure 5. 5.SheetSheet resistance resistance and and conductivity conductivity vs. vs. EEsq.sq., ,((aa) )p-type; p-type; ( (bb)) n-type n-type (empty symbolssymbols correlatedcorrelated to toconductivity, conductivity, red red filled filled symbols symbols to to sheet sheet resistance). resistance).

ToTo create create benchmark resultsresults forfor furnace furnace treated treate specimens,d specimens, to beto comparedbe compared with with the laser the treatedlaser treatedones, threeones, three specimens specimens of each of each material material were were processed processed in a in tube a tube furnace furnace for for 30 30 min min at at 350 350◦ C°C in informing forming gas gas (4% (4% H 2H/96%2/96% N 2N).2). Thermoelectric Thermoelectric properties properties of of these these samples samples are are shown shown in Table in Table4. Here, 4. Here,the thermoelectric the thermoelectric properties properties are among are among the good the go onesod ones compared compared to results to results from from other other studies, studies, which whichare shown are shown in Table in2 Table. For p-type2. For p-type material, material, laser processing laser processing produced produced several samples several ofsamples sheet resistance of sheet resistanceequal or lowerequal thanor lower the furnace than processedthe furnace samples. processed For thesamples. n-type For material, the n-type this is material, absolutely this not theis absolutelycase, since not the the lowest case, sheetsince resistivitythe lowest is sheet measured resistivity as 2 Ωis/ measuredsquare at 0.2as J2/ mmΩ/square2. at 0.2 J/mm².

Table 4. Electrical characterization of furnace processed layers.

Sheet Seebeck Power Conductivity Thickness Material Resistance Coefficient Factor (S/cm) (mm) (Ω/square) (µV/K) (W/mK²) p-type 15.3 ± 1.5 2.4 ± 0.2 70.0 ± 3.0 251 9.64 ×·10−5 n-type 88.0 ± 3.7 0.3 ± 0.1 99.7 ± 1.3 −142 1.77 ×·10−4 The thickness of the layers was measured with a micrometer screw gauge. The graph in Figure 6 shows the normalized sheet thickness of the laser treated layers. The sheet thickness of the furnace specimens was used as a reference value. Using a normalized value in the graph was set, since all specimens were from the same batch, and thus contained the same amount of conductive material, and the pre-treatment thickness can be considered as equal. It can be seen that especially for high values of Esq., the thickness is growing up to almost the double of an oven-processed layer. According to Equation (1), increasing sheet thickness would decrease conductivity σ, which is obviously just true for bulk material.

Materials 2019, 12, 3453 9 of 15

Table 4. Electrical characterization of furnace processed layers.

Sheet Seebeck Conductivity Thickness Power Factor Material Resistance Coefficient (S/cm) (mm) (W/mK2) (Ω/square) (µV/K) p-type 15.3 1.5 2.4 0.2 70.0 3.0 251 9.64 10 5 ± ± ± ×· − n-type 88.0 3.7 0.3 0.1 99.7 1.3 142 1.77 10 4 ± ± ± − ×· −

The thickness of the layers was measured with a micrometer screw gauge. The graph in Figure6 shows the normalized sheet thickness of the laser treated layers. The sheet thickness of the furnace specimens was used as a reference value. Using a normalized value in the graph was set, since all specimens were from the same batch, and thus contained the same amount of conductive material, and the pre-treatment thickness can be considered as equal. It can be seen that especially for high values of Esq., the thickness is growing up to almost the double of an oven-processed layer. According to EquationMaterials 2019 (1),, 12 increasing, x FOR PEER sheetREVIEW thickness would decrease conductivity σ, which is obviously10 just of 16 true Materials 2019, 12, x FOR PEER REVIEW 10 of 16 for bulk material.

200 180 200 180 180 180 160 160 160 160 140 140 140 140 120 120 120 120 100 100 relative sheet thickness (%) sheet thickness relative

100 (%) relative sheet thickness 100 relative sheet thickness (%) sheet thickness relative 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.00 0.05 0.10 0.15 0.20 0.25 relative sheet thickness (%) relative sheet thickness 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.00 0.05 0.10 0.15 0.20 0.25 area energy density (J/mm²) area energy density (J/mm²) area energy(a densit) y (J/mm²) area energy(b) density (J/mm²) (a) (b) FigureFigure 6. 6.Sheet Sheet thickness thickness vs.vs. Esq. .((aa) )p-type; p-type; (b (b) n-type.) n-type. Figure 6. Sheet thickness vs. Esq. (a) p-type; (b) n-type. Also,Also, cross cross sections sections (Figure (Figure7) were 7) were prepared prepared from from p-type p-type laser treatedlaser treated samples samples for three for di threefferent Also, cross sections (Figure 7) were prepared from p-type laser treated samples for three Esq.different. There, E itsq. is. There, clearly it visibleis clearly that visible not thethat whole not the layer whole was layer molten, was molten, but only but a only thin a layer thin onlayer the on top. Also,differentthe notop. smooth EAlso,sq.. There, no surface smooth it isis clearly surface visible, visible is but visible, a that disturbed butnot athe disturbed one whole with layer one a high with was roughness. amolten, high roughness. but only a thin layer on the top. Also, no smooth surface is visible, but a disturbed one with a high roughness.

(a) (b) (c)

Figure 7. Cross(a) section of laser treated specimen:( b(a)) Esq. = 0.088 J/mm²; (b) Esq. = 0.154 J/mm²;(c) (c) Esq. = 0.206 J/mm². 2 2 FigureFigure 7. 7. CrossCross section of of laser laser treated treated specimen: specimen: (a) ( aE)sq.E =sq. 0.088= 0.088 J/mm²; J/mm (b) ;(Esq.b )= E0.154sq. = J/mm²;0.154J (/cmm) Esq.;( = c) 2 E0.206sq.In= Figure 0.206J/mm². J /8a–f,mm .top view images of the laser treated surfaces are seen, stating the applied energy density in the caption. Figure 8a–c shows the development for increasing Esq. for p-type material. HereInIn Figurealready Figure8 a–f,8a–f,at low top top temperatures, view view images images aof ofkind thethe of laserlaser clusteri treatedtreangted of surfaces material are can seen, be observed, stating stating the the with applied applied droplet-like energy energy densitydensityobjects in inappearing the the caption. caption. between FigureFigure 20–508 8a–ca–c µm. showsshows For thethe higher developmentdevelopment energies, the for increasingbuildup of EbridgesEsq.sq. forfor p-type between p-type material. material. these Here already at low temperatures, a kind of clustering of material can be observed, with droplet-like Heredroplets already can at be low seen. temperatures, At the highest a kindEsq., big of clusteringbubbles are of seen material with a can dimension be observed, of more with than droplet-like 200 µm, objectsobjectswith shiny appearing appearing metallic betweenbetween surfaces, 20–5020–50 while µµm. m.the Forrest higherof higher the surface energies, energies, appears the the buildup buildupdim with of ofnetwork bridges bridges likebetween between structures. these these dropletsFigure 8d–fcan be shows seen. theAt thedevelopment highest Esq .for, big the bubbles n-type are material. seen with Here, a di themension development of more thanseems 200 to µm, be withdifferent. shiny Figuremetallic 8d surfaces, looks like while an as-printed, the rest of driedthe surface surface, appears with some dim withvisible, network slightly like darker structures. lines. FigureFor higher 8d–f energies,shows the bubble development structures for appear, the n-type that grow material. larger Here,at higher the energy. development Bridges seems or network to be different.formation Figure is not 8dobserved. looks like an as-printed, dried surface, with some visible, slightly darker lines. For higher energies, bubble structures appear, that grow larger at higher energy. Bridges or network formation is not observed.

(a) (b) (c)

(a) (b) (c)

Materials 2019, 12, x FOR PEER REVIEW 10 of 16

200 180

180 160

160 140 140

120 120

100 100 relative sheet thickness (%) sheet thickness relative relative sheet thickness (%) relative sheet thickness 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.00 0.05 0.10 0.15 0.20 0.25 area energy density (J/mm²) area energy density (J/mm²) (a) (b)

Figure 6. Sheet thickness vs. Esq. (a) p-type; (b) n-type.

Also, cross sections (Figure 7) were prepared from p-type laser treated samples for three different Esq.. There, it is clearly visible that not the whole layer was molten, but only a thin layer on the top. Also, no smooth surface is visible, but a disturbed one with a high roughness.

(a) (b) (c)

Figure 7. Cross section of laser treated specimen: (a) Esq. = 0.088 J/mm²; (b) Esq. = 0.154 J/mm²; (c) Esq. = 0.206 J/mm².

In Figure 8a–f, top view images of the laser treated surfaces are seen, stating the applied energy Materials 2019, 12, 3453 10 of 15 density in the caption. Figure 8a–c shows the development for increasing Esq. for p-type material. Here already at low temperatures, a kind of clustering of material can be observed, with droplet-like dropletsobjects canappearing be seen. between At the highest20–50 µm.Esq. ,For big higher bubbles energies, are seen the with buildup a dimension of bridges of more between than 200theseµm, withdroplets shiny can metallic be seen. surfaces, At the highest while theEsq., rest big ofbubbles the surface are seen appears with a dimdimension with network of more likethan structures. 200 µm, Figurewith shiny8d–f showsmetallic the surfaces, development while the for rest the of n-type the surface material. appears Here, dim the with development network like seems structures. to be diFigurefferent. 8d–f Figure shows8d looksthe development like an as-printed, for the driedn-type surface, material. with Here, some the visible, development slightly seems darker to lines. be Fordifferent. higher Figure energies, 8d bubblelooks like structures an as-printed, appear, dried that growsurface, larger with at some higher visible, energy. slightly Bridges darker or network lines. formationFor higher is energies, not observed. bubble structures appear, that grow larger at higher energy. Bridges or network formation is not observed.

Materials 2019, 12, x FOR PEER REVIEW 11 of 16

Materials 2019, 12, (xa FOR) PEER REVIEW (b) (c) 11 of 16

(d) (e) (f) (d) (e) (f) Figure 8. Top view microscopy images of p-type material treated with (a) Esq. = 0.048 J/mm²; (b) Esq. =

0.080 J/mm²; (c) Esq. = 0.160 J/mm²; and n-type material treated with (d) Esq. = 0.053 J/mm²; (e) E2sq. = FigureFigure 8.8. TopTop view view microscopy microscopy images images of p-type of p-type material material treated treated with (a with) Esq. = (a 0.048) Esq. J/mm²;= 0.048 (b) JE/mmsq. = ; 0.107 J/mm²; (f) Esq.sq.2 = 0.200 J/mm². 2 sq. sq. 2 (b0.080) Esq. J/mm²;= 0.080 (c J)/ mmE =;( 0.160c) Esq. J/mm²;= 0.160 and J/mm n-type; and material n-type treated material with treated (d) E with = 0.053 (d) E sq.J/mm²;= 0.053 (e) JE/mm = ; 2 2 (e0.107) Esq. J/mm²;= 0.107 (f J)/ mmEsq. = ;(0.200f) Esq. J/mm².= 0.200 J/mm . Finally, the Seebeck coefficient was measured for three different Esq. for each material, 0.07, 0.10, andFinally,Finally, 0.13 J/mm² the the Seebeck Seebeck for p-type coe coefficientffi cientand 0.15, waswas measuredmeasured0.20, and for0.25 three J/mm² different diff erentfor n-type, EEsq.sq. forfor eachrespectively. each material, material, The0.07, 0.07, obtained0.10, 0.10, andandSeebeck 0.13 0.13 J/ mm J/mm²coefficients2 for for p-type p-type were and muchand 0.15, 0.15, lower 0.20, 0.20, and than and 0.25 for J0./ mmthe25 J/mm²furnace2 for n-type, for samples n-type, respectively. (shown respectively. in The Table obtained The 4). obtained Further, Seebeck for coeSeebeckn-typefficients coefficientsmaterial were much almost were lower nomuch deviation than lower for than theis seen furnacefor forthe differentfurnace samples samples E (shownsq.. For (shown inp-type, Table in 4better ).Table Further, Seebeck4). Further, for coefficient n-type for materialn-typeFigure material almost 9 is measured no almost deviation nofor deviation islower seen E for sq.is. di seenForfferent n-type,for Edifferentsq. .approximately For p-type,Esq.. For better p-type, 77% Seebeck ofbetter the coe SeebeckSeebeckfficient coefficientcoefficient Figure9 is of Figure 9 is measured for lower Esq.. For n-type, approximately 77% of the Seebeck coefficient of measuredfurnace fortreated lower samplesEsq.. For can n-type, be measured, approximately while for 77% p-type, of the the Seebeck Seebeck coe fficoefficientcient of furnace decreases treated to 16– samplesfurnace28%. cantreated be measured, samples can while be measured, for p-type, while the Seebeck for p-type, coeffi thecient Seebeck decreases coefficient to 16–28%. decreases to 16– 28%.

80 80

60 60

40 40

-100 -100

Seebeck coefficient (µV/K) -120 Seebeck coefficient (µV/K) -120 0.08 0.12 0.16 0.20 0.24 0.08 0.12 0.16 0.20 0.24 specific energy density (J/mm²) specific energy density (J/mm²) Figure 9. Seebeck coefficient for laser treated bismuth telluride layers (red triangles p-type; black FigureFigure 9. 9. SeebeckSeebeck coecoefficientfficient forfor laser treated bismuth te telluridelluride layers layers (red (red triangles triangles p-type; p-type; black black squaressquaressquares n-type). n-type). n-type).

EDXEDX (Energy-dispersive (Energy-dispersive X-ray X-ray spectroscopy) spectroscopy) was was pe rformedperformed in order in order to check to check for deviationsfor deviations of of thethe elemental elemental composition. composition. Figure Figure 10 10 shows shows the the area area under under investigation. investigation. It is It a issection a section from from the the samesame cross cross cut cut as as shown shown in in Figure Figure 7b 7b treated treated with with Esq. E =sq. 0.154 = 0.154 J/mm². J/mm². Area Area 1 clearly 1 clearly lies liesin the in moltenthe molten section,section, while while Areas Areas 2 2and and 3 are3 are used used as asreferences references since since no nomolten molten particles particles can canbe identifiedbe identified in in them.them. Areas Areas 4 4and and 5 5were were slightly slightly highlighted highlighted du duringring EDX EDX analysis analysis for forhaving having higher higher tellurium tellurium contentcontent and and were were thus thus separately separately scanned. scanned.

Materials 2019, 12, 3453 11 of 15

EDX (Energy-dispersive X-ray spectroscopy) was performed in order to check for deviations of the elemental composition. Figure 10 shows the area under investigation. It is a section from the same 2 cross cut as shown in Figure7b treated with Esq. = 0.154 J/mm . Area 1 clearly lies in the molten section, while Areas 2 and 3 are used as references since no molten particles can be identified in them. Areas 4 and 5 were slightly highlighted during EDX analysis for having higher tellurium content and were thus separately scanned. Materials 2019, 12, x FOR PEER REVIEW 12 of 16

FigureFigure 10. 10. ScanningScanning electronelectron microscopy microscopy (SEM) (SEM) image im ofage area underof area investigation under investigation for energy-dispersive for energy-dispersiveX-ray spectroscopy X-ray (EDX) spectroscopy with 5 areas, (EDX) where wi elementalth 5 areas, composition where elemental was analyzed. composition was analyzed. The results of the elemental composition are shown in Table5 as a comparison of a furnace treated sampleThe results and the of dithefferent elemental areas ofcomposition the laser treated are shown sample. in Table Areas 5 2 as and a 3comparison are considered of a tofurnace be equal, treatedsince sample no deviation and the larger different than 0.1%areas is of seen. the laser Between treated the sample. oven sample Areas and 2 and Areas 3 are 2 and considered 3, there are to be slight equal,deviations since no not deviation exceeding larger 1.5%. than Two 0.1% percent is seen. less Between Sb is found the inoven the moltensample Areaand Areas 1, but 2 Bi and and 3, Te there also do are notslight deviate deviations more thannot exceeding 1%. Areas 1.5%. 4 and Two 5 deviate percent remarkably, less Sb is since found there in the is almost molten no Area Bi.The 1, but 4.7 Bi wt.% andmeasured Te also do in not Area deviate 4 are nearmore the than detection 1%. Areas limit. 4 and On the5 deviate other hand,remarkably, in these since areas, there the fractionis almost of no Sb is Bi. veryThe 4.7 high. wt.% Not measured seen in the in table Area are 4 are the near high the amounts detection of oxygen limit. On and the carbon, other also hand, found in these in Areas areas, 4 and the 5.fraction Figure of11 Sb shows is very the high. measured Not seen peaks in forthe Areatable 5,are where the high these amounts peaks are of clearlyoxygen visible. and carbon, The chemical also foundcomposition in Areas of4 and the purchased5. Figure 11 material shows is the shown measured in Table peaks6. for Area 5, where these peaks are clearly visible. The chemical composition of the purchased material is shown in Table 6.

Table 5. Mass fraction in % of the elements of p-type material after oven processing and in several

areas of a laser processed specimen (Esq. = 0.154 J/mm²).

Area 1 Area 4 Area 5 Oven Area 2 Area 3 Element (Near (Detail (Detail Processed (Bottom) (Bottom) Surface) Surface) Surface) Te 56.1% 58.8% 57.6% 57.7% 22.6% 7.3% Sb 27.8% 25.0% 27.2% 27.3% 72.7% 64.0% Bi 16.1% 16.1% 15.2% 15.1% 4.7% -

Materials 2019, 12, 3453 12 of 15

Table 5. Mass fraction in % of the elements of p-type material after oven processing and in several 2 areas of a laser processed specimen (Esq. = 0.154 J/mm ).

Oven Area 1 (Near Area 2 Area 3 Area 4 (Detail Area 5 (Detail Element Processed Surface) (Bottom) (Bottom) Surface) Surface) Te 56.1% 58.8% 57.6% 57.7% 22.6% 7.3% MaterialsSb 2019, 12, x 27.8% FOR PEER REVIEW 25.0% 27.2% 27.3% 72.7% 64.0% 13 of 16 Bi 16.1% 16.1% 15.2% 15.1% 4.7% - Table 6. Results of energy-dispersive X-ray spectroscopy (EDX) analysis, values in wt.%. Table 6. Results of energy-dispersive X-ray spectroscopy (EDX) analysis, values in wt.%. Element n-type p-type Element n-type p-type Bismuth 53.7 16.1 Bismuth 53.7 16.1 TelluriumTellurium 43.943.9 56.1 56.1 SeleniumSelenium 2.42.4 - - Antimony - 27.8 Antimony - 27.8

Figure 11. Peaks of the energy-dispersive x-ray spectroscopy (EDX) analysis of Area 5. Figure 11. Peaks of the energy-dispersive x-ray spectroscopy (EDX) analysis of Area 5. 4. Discussion 4. Discussion According to the results shown in Figure1, laser processing of bismuth telluride-based materials shouldAccording be better doneto the in inertresults atmosphere. shown in ThisFigure is actually 1, laser done processing by the researchers of bismuth of telluride-based all publications foundmaterials on laser should processing be better of done such kindin inert of materialsatmosphere. [9,10 This]. Nonetheless, is actually done it is notable by the thatresearchers processing of all in ambientpublications air results found in on conductive laser processing layers of at such all, sincekind of furnace materials processing [9,10]. Nonetheless, in ambient airit is leads notable to fullythat processing in ambient air results in conductive layers at all, since furnace processing in ambient air oxidized layers. That a resistance is measurable at all is suspected to be possible due to the much leads to fully oxidized layers. That a resistance is measurable at all is suspected to be possible due to shorter processing times and thus much more limitation time for oxidation processes. the much shorter processing times and thus much more limitation time for oxidation processes. Higher laser energies (80–120 W), even if a defocused laser is used, do not lead to the desired faster Higher laser energies (80–120 W), even if a defocused laser is used, do not lead to the desired process in comparison to the work in References [9,10] where 3–10 W equipment is used. The intensity faster process in comparison to the work in References [9,10] where 3–10 W equipment is used. The and Esq. of laser radiation and processing parameters is comparable, but still, the layers fabricated here intensity and Esq. of laser radiation and processing parameters is comparable, but still, the layers were not homogeneously molten. This is probably due to the much faster scan speeds and thus due to fabricated here were not homogeneously molten. This is probably due to the much faster scan less time in which each segment of a layer is irradiated, so that energy cannot propagate into the depth speeds and thus due to less time in which each segment of a layer is irradiated, so that energy cannot of the printed layers. propagate into the depth of the printed layers. On the other hand, in the two quoted papers, after laser processing, 6 h or even 24 h of annealing On the other hand, in the two quoted papers, after laser processing, 6 h or even 24 h of at 400 C was performed. That temperature already presents 70% of the endothermic peak found in annealing◦ at 400 °C was performed. That temperature already presents 70% of the endothermic peak Reference [13] and in this work, and there might be already sintering activities. So, nothing was known found in Reference [13] and in this work, and there might be already sintering activities. So, nothing was known about the thermoelectric properties of as laser treated layers, yet. The finding that VED is not a sufficient parameter to describe results of laser sintering or melting processes for additive manufacturing was published in Reference [24]. The results of the here presented investigation would support that assumption, extending it to area specific density and thermoelectric parameters as properties to be achieved. In Figure 5, Figure 6, and Figure 9, where sheet resistance, sheet thickness and Seebeck coefficient are shown, it is visible, that for same value of Esq. different of the just named properties are measured.

Materials 2019, 12, 3453 13 of 15 about the thermoelectric properties of as laser treated layers, yet. The finding that VED is not a sufficient parameter to describe results of laser sintering or melting processes for additive manufacturing was published in Reference [24]. The results of the here presented investigation would support that assumption, extending it to area specific density and thermoelectric parameters as properties to be achieved. In Figure5, Figure6, and Figure9, where sheet resistance, sheet thickness and Seebeck coefficient are shown, it is visible, that for same value of Esq. different of the just named properties are measured. N- and p-type materials reacted quite differently to the laser treatment and also statistical analysis via DoE resulted in very different influence parameters. For p-type, the statistical model seems to be comparatively clear and includes all parameters, which are also part of Equation (3) to calculated Esq.. Nonetheless, it is shown that scan velocity has a higher influence on the sheet resistance than power or hatch spacing. Unfortunately, the Pareto diagram for the n-type material looks much less clear for the tested parameters. The structures in Figure8d–f, where top views of the n-type laser treated specimen are shown, remain on balling effects that are encountered for too high scan speeds in LPBF. If the findings in Reference [24] for LPBF are applicable to the process and materials presented here, slower scan speed at lower laser power at the same Esq. might lead to better results. However, the findings in Reference [24] might not be directly applicable. In LPBF, metals are irradiated, while in the work presented here, they are composites of metals and organic compounds. Another reason for the archived results could also be that either energy propagation in composites is poor due to few contacts between metal particles, or also due to evaporating organics. Inhomogeneity and increased sheet thickness of the laser processed layers presents a major problem when characterizing electrical conductivity. That parameter does not depend on geometry and thus would have allowed better comparison to results, achieved by other groups. Since for the furnace and laser experiments, the same batch of printed specimens was used, at least for the work presented here the results of the sheet resistance are comparable. Decrease of Seebeck effect in the shown samples is matter of discussion. The depression of Seebeck due to better conductivity is unlikely, since especially for n-type, the conductivity is worse for laser process compared to furnace treatment and for p-type, the conductivity is still in the same range. Also, for p-type, the samples that were treated at modest Esq. show higher Seebeck coefficient. EDX analysis was performed to check if the chemical composition was changed during laser processing, e.g., due to evaporation of elements as indicated in Reference [10]. This was not precisely found in the analysis as the chemical composition was in principle similar to furnace-treated samples. Only in some tiny droplets on the surface of the scanned specimen was there a completely different composition, where almost no Bi is found, but the mass fraction of Sb is much higher. The high oxygen and carbon contents on the surface are not quantifiable by this method. The peaks are too high to be attributed to cross section preparation or EDX measurement itself, but on the other hand, it is assumed that here condensed organic material that was evaporated during laser process is seen. As for the Seebeck effect, a change of charge carrier density is suspected to happen due to segregation of elements, which would not be detected by EDX.

5. Conclusions Laser processing as thermal treatment was performed on n- and p-type bismuth telluride-based materials. It was shown that such treatment should be performed under inert atmosphere. Optimum processing parameters were not found for both materials, but it was shown, even though the materials can be considered similar, that they still need different treatment parameters. Material is molten on the surface of the printed layers, while in lower regions the particles appear as printed. DoE can be an approach to optimize these parameters, since the area specific laser energy density Esq. does not sufficiently describe the process. Higher laser power does not necessarily allow faster processing speed. DoE should be applied to processing windows with lower power and scan speeds, where in previous studies good results were found. Electrical conductivity was found to be slightly better (p-type) or Materials 2019, 12, 3453 14 of 15 worse (n-type) for laser treatment compared to furnace processing. The depression of Seebeck effect in as-laser treated specimen remains subject of investigation. The material properties are thus in a similar range for laser and furnace processing. Nonetheless, laser treatment is considered to be a promising approach due to its high processing speeds in comparison to furnace processing. Also, as shown, melting of the used material is induced by the laser which is not the case for the furnace process, where even sintering has not been found. Melting or sintering should actually enhance the electrical conductivity. Also, using a laser for processing the materials on temperature sensitive substrates is more likely than using a furnace.

Author Contributions: The work was distributed among the authors as follows: conceptualization: M.G. and L.S.; investigation: M.G. and R.T.; writing—original draft preparation: M.G.; writing—review and editing: R.T., L.S., E.L., F.B., and C.L.; supervision: C.L.; project administration: M.G.; funding acquisition: E.L., C.L. Funding: This research was funded by BUNDESMINISTERIUM FÜR BILDUNG UND FORSCHUNG (Federal Ministry of Education and Research of Germany), grant number 03ZZ0207B. Acknowledgments: The authors would like to thank the following contributors to this work: Fabian Lange for Seebeck measurements, Anja Mittag and Andrea Ostwaldt for preparation of cross sections specimen, Jörg Bretschneider for EDX analysis and discussion of its results, Robert Baumann for assistance with laser equipment. For assistance in planning DoE study and technical support with Minitab software the authors thank Maria Barbosa. For preparation and execution of DSC analysis, the contribution of Axel Marquardt from Technische Universtät Dresden, Chair of Material Engineering is acknowledged. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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