Substitution Of Ho Research Articles

Structural, microstructural, dielectric and ferroelectric properties of Ho3+ doped SrBi4Ti4O15 ceramics

The present work investigates microstructural, structural, ferroelectric and dielectric properties of Holmium, Ho3+, doped SrBi4Ti4O15 (SBT). Polycrystalline ceramics (SrBi4-xHoxTi4O15 SBHT, where x = 0, 0.02. and 0.03) were prepared by solid-state reaction method using high energy planetary ball mill. The surface morphology of all samples displayed dense microstructure and average grain size decreased with Holmium content increase. The XRD results of samples confirmed a single phase of SBT with orthorhombic crystal structure and a reduction in unit cell volume with the substitution of Holmium at B-site. The improvement in the structural and microstructural properties greatly enhanced the electric properties of SBHT. SBHT (x = 0.02) sample showed the improved ferroelectric properties (remnant polarization Pr = 1.63 μC/cm2 and Coercive field Ec = 0.51 kV/cm) making them suitable for ferroelectric applications. High Curie temperature (TC = 626 °C), elevated dielectric constant (εr = 236) and low loss (tanδ = 0.062) are obtained with the substitution of Ho (for x = 0.03) at B- site of SBT. These ceramics have high TC and improved thermal stability which makes these apt for the applications in high temperature sensors.

Dielectric, XPS, and ferromagnetic relaxation studies of Ho-substituted polycrystalline magnetic oxide materials

The impact of holmium (Ho) ions on lithium-rich spinel ferrites is studied through magnetic, morphological, and electrical transport properties. The X-ray photoelectron spectroscopy (XPS) analysis indicated the existence of all metal ions in the required valence state. Transmission electron microscopy (TEM) analysis exhibited nano-crystalline (∼50 nm) structures of these ferrites. The electrical resistivity measurements showed the higher resistivity values of the synthesized samples. The resistivity increased from 2.4 × 108 to 2.1 × 1010 Ω-cm with Ho inclusion, indicated an inverse relation with the size of grains. The Curie temperature (Tc) observed to decrease from 848 to 806 K with the incorporation of Ho3+ions. The activation energy (ΔEa) noted to vary between 0.21 and 0.23 eV. The drift mobility (μd) study revealed a decrease from 1.86x10−12to 6.14x10−14cm2V−1s−1by increasing Ho amount up to x = 0.06. A reduction in thermoelectric power (S) from -143 to -95 μ.V/K by increase in Ho concentration confirmed the Fe replacement by Ho on octahedral B-sites. The ferromagnetic resonance (FMR) investigations indicated a drop in FMR line-broadening (ΔH) from 2491 to 2000 Oe by the increase of Ho concentration, making it beneficial to reduce microwave losses. A non-linear variation in the resonance field (Hres) with the substitution of Ho was observed between 7359 to 7785 Oe.

Synthesis, characterization, dielectric and magnetic properties of substituted Y-type hexaferrites

Fabrication of a series of zinc–strontium hexaferrites, Sr2Zn2−xMnxFe12−yHoyO22 (x = 0.0–1.0, y = 0.0–0.1) was carries out by chemical co-precipitation route with the aim to enhance the saturation magnetization and coercivity and decrease the resistivity to make the materials applicable for recording media of high-density applications. The synthesized hexaferrites were interpreted by Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD). The results of XRD investigation certify the single magnetoplumbite phase formed in hexaferrites with regular crystallite particle size in the 31–38 nm range. Magnetic and dielectric properties of the hexaferrites were also observed. The dielectric parameters (dielectric loss also its tan loss and dielectric constant) were measured at ambient temperature and in 1.0 MHz to 3.0 GHz frequency range. These parameters of Mn–Ho-substituted Zn–Sr hexaferrites showed constant trend with frequency up to a certain value after that resonance sort of behavior was observed. In all the hexaferrites dielectric parameters initially decrease with frequency then become almost constant followed by resonance type behavior. The dielectric parameters of the synthesized samples decrease with dopant content initially but then increase up to maximum dopant level. The saturation magnetization (Ms) and remanence (Mr) increase as the dopant content increases due to magnetic substituents. The squareness ratios of all the nanomaterials synthesized in the present studies is less than 0.5. The coercivity (Hc) decreases by the substitution of Mn and Ho.

Impact of holmium on structural, dielectric and magnetic properties of Cu–Zn spinel ferrites synthesized via sol–gel route

The mixed nano-ferrites materials Cu0.6Zn0.4HoxFe2−xO4 (0.00 ≤ x ≤ 0.12) were prepared via the sol–gel auto combustion technique. The TGA curve established the annealing temperature (500 °C) for phase formation. The single exothermic peak on the DSC plot occurred at 341 °C temperature. XRD patterns of these nano ferrites verified single phase formation of the FCC cubic structure. The lattice constant a was increased from 8.4244 to 8.4419 A and then its value decreased to 8.4319 A. Crystallite size was found in the range of 7 to 16 nm. The surface morphology of the samples was observed from the scanning electron microscope (SEM) images. The grain size was found within the range of 90 nm to 106 nm. Energy-dispersive X-ray (EDX) results showed the significant substitution of Ho in Cu–Zn spinel ferrites. Dielectric parameters such as dielectric constant, dielectric constant factor, and tan loss depicted a decreasing trend with Ho3+ ions concentration. The single-domain ferrimagnetism is confirmed by the squareness ratio, which is < 0.5. The Ms was found to be decreased while Hc was increased. The significant results of the dielectric as well as magnetic properties of these nano ferrites suggested their use in high-frequency devices applications.

Influence of Ho–Ni–Mn substitution on the structural and magnetic behavior of Ba–Sr Co2Z-type nanohexaferrites extension up to Mossbauer investigations

A series of Co2Z-type Ba–Sr nanohexaferrites Ba1.5Sr1.5Co2−zHozMnxNiyFe24−x−yO41 (z = 0.0, 0.05, 0.10, 0.15, 0.20, x = y = 0.0, 0.25, 0.5, 0.75, 1.00) have been synthesized using sol–gel auto-combustion synthesis route. The effect of Ho–Ni–Mn substitutions on crystallographic and magnetic properties of synthesized nanohexaferrites was investigated using XRD, VSM, and Mossbauer spectroscopy. Microstructural analysis showed single-phase crystal structures without any impurities and hexagonal with the space group P63/mmc. The average variation in crystallite size ranges from 43 to 60 nm with a slight increase in X-ray density and appreciable decrease in porosity was observed for different dopants. FE-SEM (Nova Nano SEM-450) substantiates the hexagonal structure and HR-TEM images assisted with SAED pattern confirm the crystalline quality and FWHM of the material, which significantly support the XRD results. FTIR spectra showed two characteristic metal stretching peaks in the range of 400–600 cm−1 due to the substitution of Ho–Ni–Mn. Magnetic measurements show maximum magnetic saturation (Ms) at 44.04 emu g−1 and elevated value of coercivity (Hc) 224Oe imparting typical characteristics of soft ferrite with high coercivity. Mossbauer analysis with least squares fit sextets of six distinguishable sites at room temperature for all samples substantially supports the results of VSM. The materials with large coercivity are useful in permanent magnet applications. The prepared composites could be useful for applications in microwave absorbing materials, magnetic storage, and the miniaturization of antennas for wireless communication devices.

Role of Ho Doping in Magnetization Mechanism of BiFeO3 Thin Films

BiFeO3 (BFO) as one of the considerable interesting multiferroic materials has been widely investigated for its fantastic science and potential application. Herein, effects of Ho doping on the structure, morphology, and magnetic properties of BFO thin films were systematically studied. X-ray diffraction (XRD) indicates a successful substitution of Ho at Bi site and a transition in crystal structure of Bi0.9Ho0.1FeO3 (BHFO) thin film. Raman scattering spectrum is further evidence of XRD analysis results. The Ho-doped BFO thin films possess uniform morphology. X-ray photoelectron spectroscopy (XPS) confirms that the substitution of Bi3+ ions with Ho3+ ions is beneficial. The remanent magnetization of BHFO thin films is about three times than that of the BFO thin films under a maximum magnetic field of 15,000 Oe. And we systematically discussed several possible reasons for the enhancement of magnetization.

Magnetic phase transition and multiferroic phase separation in Ho1-xGdxMnO3

In the orthorhombic (o) RMnO3 compound, it is known that the magnetic state can change from the A-type antiferromagnetic (A-AFM) through a spiral spin order (SSO) into the E-AFM state with the decrease in the R ionic radius (rR). The magnetic phase transition from the E-AFM state to the A-AFM state is expected, but no experimental realization has been reported. In this study, this process in o-Ho1-xGdxMnO3 (x: 0–0.6) is reported. A substitution of Ho with Gd enables the transition of the multiferroic ground state from the E-AFM order (at x ≤ 0.1) to the SSO (at x > 0.4). Accompanied by the quenched disorder, a multiferroic phase separation state is demonstrated in the intermediate composition range of 0.1 < x < 0.4 near the phase boundary between the E-AFM and SSO. This phase-separated state is believed to enhance the ferroelectric polarization and magnetoelectric coupling. Our results demonstrate the dependences of the multiferroic properties of RMnO3 on rR and confirm that the multiferroic phase separation is an effective approach to enhance the multiferroic properties of RMnO3.

Phase transformation in crystal and magnetic structure and improved dielectric and magnetic properties of Ho substituted BiFeO3multiferroics

The changes in crystal and magnetic structure of BiFeO3 produced by partial substitution of Bi ions by Ho ions has been studied with powder X-ray diffraction, neutron powder diffraction, dielectric and magnetization techniques. The present study demonstrates that Bi1-xHoxFeO3(x = 0.05, 0.10, 0.15, &amp; 0.2) multiferroics shows change in crystal structure at x &amp;gt; 0.05. The sample with x = 0.05 exhibits rhombohedral structure (space group R3c), while the other three samples (x &amp;gt; 0.05) exhibit mixed phase with coexisting rhombohedral (R3c) and Orthorhombic (Pnma) structure. This change in the crystal structure is attributed to the distortion of FeO6 octahedra via substitution of Ho at phase boundaries. The magnetization studies indicate that doping of Ho in pristine BiFeO3 leads to enhancement in the ferromagnetic moment. We find that doping of Ho breaks the spin cycloid phase of BiFeO3 and creates a canted G-type antiferromagnetic structure in the hexagonal phase whereas the orthorhombic phase exhibits a collinear G-type AFM structure. The canting angle increases with increase in doping with Ho, leading to an enhancement in the ferromagnetic component in magnetization. Dielectric constant (ε′) and loss factor (tanδ) are measured in frequency range 1 kHz to 7 MHz and ε′ and tanδ show dispersion behaviour at low frequencies. The significant improvement in magnetization and dielectric properties is achieved by Ho substitution which in turn enhances the potential of BiFeO3 for multiferroics applications.

Large magnetic entropy change and magnetic properties in Gd60-xHoxFe20Al20 amorphous ribbons

The magnetic entropy change (ΔSM) and magnetic properties of amorphous ribbons Gd60-xHoxFe20Al20 (x = 10, 30 and 50) have been investigated. The partial substitution of Ho for Gd results in a monotonously decrease of phase transition temperature from 165 K to 65 K and an increase of maximal ΔSM from 5.44 J · Kg−1K−1 to 8.01 J · Kg−1K−1 under a field change of 0–5 T. Furthermore, a large value of relative cooling power (RCP) about 645 J.kg−1 is obtained in Gd10Ho50Fe20Al20 amorphous ribbons. This material could be a very good candidate for magnetic refrigerant around the liquid nitrogen temperature region. It is also found that the amorphous ribbons with the same chemical composition fabricated at different spinning rate show the similar temperature dependence of magnetization behavior, the maximal ΔSM as well as saturated magnetization, however, increases with the spinning rate. It could be attributed to the different rate of solidification of the alloy.

Vertical sections of Tb0.15Ho0.85Fey-Tb0.3Dy0.7Fey (y=1.85, 1.9, 2.0) in Tb-Dy-Ho-Fe system

The x(Tb0.15Ho0.85Fey)+(1–x)(Tb0.3Dy0.7Fey) (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9; y=1.85, 1.9, 2.0) samples were prepared by a vacuum arc furnace, and annealed at 1000 °C for 1 d and at 950 °C for a week. Three vertical sections of Tb0.15Ho0.85Fey-Tb0.3Dy0.7Fey (y=1.85, 1.9, 2.0) in the Tb-Dy-Ho-Fe system were determined using optical microscopy, scanning electron microscopy, energy dispersion X-ray spectroscopy, X-ray diffraction, and differential thermal analysis. These vertical sections consisted of two single-phase regions: L and (Tb,Dy,Ho)Fe2; four two-phase regions: L+(Tb,Dy,Ho)Fe3, L+(Tb,Dy,Ho)Fe2, (Tb,Dy,Ho)Fe2+(Tb,Dy,Ho)Fe3, and (Tb,Dy,Ho)Fe2+(Tb,Dy,Ho). The high Ho content of (Tb,Dy,Ho)Fey alloys led to the elevation of the peritectic temperature of L+(Tb,Dy,Ho)Fe3→(Tb,Dy,Ho)Fe2. The region of (Tb,Dy,Ho)Fe2 phase was shifted towards the side of rich-Ho with the Fe content increasing. It meant that the substitution of Ho for Dy or Tb had a marked effect on the solidification process of (Tb,Dy,Ho)Fe2 compounds.

Effect of Ho substitution on structure and magnetic property of BiFeO3 prepared by sol–gel method

We report the effect of the variation of concentration on the structure and magnetic property of the Ho doped BiFeO3 (Bi1−xHoxFeO3, x=0, 0.05, 0.1, 0.15 and 0.2) thin films. The result shows that all the peaks for BiFeO3, Bi0.95Ho0.05FeO3 and Bi0.9Ho0.1FeO3 thin films can be indexed according to the crystal structure of pure BiFeO3 by X-ray diffraction. A structural transition from the rhombohedral to orthorhombic occurred in the film with Bi0.85Ho 0.15FeO3 and Bi0.8Ho0.2FeO3. Raman spectroscopy indicates the substitution of Ho into the Bi site in BiFeO3 thin films. Scanning electron microscope and Transmission electron microscope are used to analyze the morphology and micro-structures of the thin films. The actual distribution can be easily observed in Energy-dispersive spectrometry elemental mappings. In comparison with the BiFeO3 thin film studies here, the Bi0.9H0.1FeO3 thin film shows the surprising remnant magnetization and saturation magnetization. The remnant magnetization of Bi0.9H0.1FeO3 thin film is nearly 8 times as large as that of BiFeO3 thin film. Moreover, the possible reasons for the enhancement of magnetization are discussed in detail.

Study of the electronic properties of Zn0.8–4xHoxOy (0.05 ≤ x ≤ 0.09) by X-ray absorption and photoemission spectroscopy

The electronic structure of Zn0.8–4xHoxOy (0.05≤x≤0.09) was investigated using X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopy (XPS). Samples were prepared by the solid state reaction method. Using X-ray absorption spectroscopy, the investigation of M4,5 absorption edge of Ho revealed that the electronic structure was directly influenced by the Ho concentration in the Zn0.8–4xHoxOy sample whereas the crystal structure properties showed little/no correlation to the substitution of Ho. The electronic structure differs substantially from those of the reference ZnO. The O K-edge spectra suggest that the combination of the Ho with ZnO enhances the effective charge of the O ions. A systematic study on the composition from lower to higher value of Ho dopant showed the blue shift in band gaps and is discussed in the view of the electronic structure of the Zn0.8–4xHoxOy samples. The inverse susceptibility (1/χ) against temperature curves is plotted to identify the magnetic contribution. Those curves indicate that the substitution of Ho into the ZnO compound causes a weaker antiferromagnetic (AFM) interaction.

Microstructural and Magnetic Properties of Holmium-substituted Yttrium Iron Garnets (Y3-xHoxFe5O12) Synthesized by Conventional Ceramic Method

Polycrystalline single-phase Holmium (Ho) substituted yttrium iron garnets (Y3-xHoxFe5O12), where (x = 0.1, 0.3, 0.6, 0.9, 1.5) were prepared by conventional ceramic technique. The crystalline structure and magnetic properties of the samples were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating samples magnetometer (VSM). XRD study showed well-defined peaks of Ho-YIG. X-ray diffraction data showed an increase of 66nm to 113nm in grain size with an increase in Ho composition from 0.1 to 1.5. The magnetic measurements showed that saturation magnetization (Ms) and retentivity increase with increasing Ho contents. There is a noticeable decrease in the coercivity with the increase of Ho composition. It is concluded that substitution of Ho makes the YIG better soft magnetic material which may be used in electronic devices for the microwave region as well as magnetic bubble domain type memories.

Sulfur‐replaced Fenton systems: can sulfate radical substitute hydroxyl radical for advanced oxidation technologies?

AbstractSulfate radicals (SO4•−) and hydroxy radicals (HO•) are the major radicals used in advanced oxidation technologies (AOTs) for the removal of contaminants. Although SO4•− reacts with organic or inorganic compounds with rate constants comparatively lower than that of HO•, AOTs based on SO4•− (abbreviated as SR‐AOTs) have gained lots of attention due to the selective oxidation and non‐pH‐dependence. A series of systems using persulfate (PS) or peroxymonosulfate (PMS) instead of H2O2 is designated as a sulfate radical‐Fenton system or sulfur‐replaced Fenton system (SR‐Fenton). Comparisons and analogies between Fenton (Fenton‐like) systems and SR‐Fenton systems are made and some new SR‐AOTs systems without PS or PMS are introduced. The possibility for the substitution of HO• by SO4•− for AOTs is discussed. Most likely in the future, efforts will be concentrated on product‐oriented AOTs with the purpose of recovery of chemical products rather than mineralization of organic contaminants, producing greenhouse gas CO2. Moreover, such SR‐Fenton system may be more atomically economical. © 2014 Society of Chemical Industry

Enhanced ferromagnetic properties in Ho and Ni co-doped BiFeO3 ceramics

The magnetic properties of polycrystalline Bi1-xHoxFe1-yNiyO3 (x = 0, 0.1; y = 0, 0.03), which were prepared by the solid-state method, have been investigated. The powder X-ray diffraction reveals that all the samples are polycrystalline and show rhombohedral perovskite structure. The micro-Raman scattering studies confirm that Bi0.9Ho0.1Fe0.97Ni0.03O3 has a compressive lattice distortion induced by the simultaneous substitution of Ho and Ni ions at A and B-sites, respectively. From the magnetization dependences at room temperature, Bi0.9Ho0.1Fe0.97Ni0.03O3 has enhanced magnetization (0.2280 emu/g) and low coercive field (280 Oe). It was revealed that the Ni dopant plays an important role for the improved ferromagnetic properties and the Ho dopant favors the magnetic exchange interactions in the co-doped ceramic.

Self-ignited high temperature synthesis and enhanced super-exchange interactions of Ho3+–Mn2+–Fe3+–O2−ferromagnetic nanoparticles

The present work is focused on the effect of Fe(3+) replacement by rare earth-Ho(3+) ions and their influence on the properties of MnFe2O4 ferrite. The Ho(3+) substituted MnFe2O4 ferrite samples with chemical formula MnHoxFe2-xO4 were synthesized where substitution concentration of Ho(3+) was 0.0, 0.05, 0.1 and 0.15. The samples were synthesized by the self-ignited sol-gel method using the nitrates of the respective elements. Powder X-ray diffraction, transmission electron microscopy, infrared spectroscopy, vibrating sample magnetometer (VSM) and electrical measurements were employed to characterize the structural, magnetic and electrical properties of these ferrite nanoparticles. The cations distribution between the tetrahedral (A-site) and octahedral sites (B-site) has been estimated by XRD analysis. It is found that substitution of Ho(3+) ions favorably influenced the magnetic and electrical properties. Magnetic measurements were carried out at 77 and 300 K. Saturation magnetization and coercivity increased from 54.57 to 71.6 emu g(-1) and 172 to 766 Oe, respectively, with increasing the Ho(3+) substitution. The change in magnetic properties may be explained with the increase of A-O-B (FeA(3+)-O(2-)-HoB(3+)) super exchange interactions and the anisotropy constant. The electrical properties show that the pure sample has lower resistivity with respect to any Ho(3+) doped one. The conduction mechanism is used to interpret electrical measurements. Results of the presently investigated samples with enhanced saturation magnetization, coercivity and remanence ratio indicate that the Ho(3+) doped MnFe2O4 nanoparticles can be a useful candidate for the application in high density recording media.

Structural and magnetic properties of the Zn0.8−4xHoxOy (0.05 ≤ x ≤ 0.10) compounds prepared by solid state reactions

Zn0.8−4xHoxOy (0.05 ≤ x ≤ 0.10) diluted magnetic semiconductors were prepared by the solid state reaction method. We have studied the structural properties of the samples by using the XRD, SEM, and EDX techniques. The SEM results clearly demonstrate that Ho3+ ions are quite well substituted for Zn2+ in the ZnO lattice, and the grains of the samples are very well connected to each other and tightly packed. From the XRD and EDX spectra of the samples, it has been concluded that the substitution of Ho causes no change in the hexagonal wurtzite structure of ZnO. According to our M–H and M–T measurements paramagnetism has been observed for all the samples from our attainable lowest temperature of 10 K to 300 K. Furthermore, the trend of the AC-susceptibility (χ) versus temperature curves, measured under an AC-magnetic field of 10 Oe, also support our conclusion about the paramagnetic contribution in the Zn0.8−4xHoxOy compounds explored in this study. In order to clearly see the paramagnetic contribution, and whether there is also a ferromagnetic or antiferromagnetic contribution or not the inverse susceptibility (1/χ) against temperature curves are also plotted. Those curves indicate that, the substitution of Ho into the ZnO compound causes, in addition to the paramagnetism, a weaker antiferromagnetic (AFM) interaction.

Effect of Ho substitution on the ionic conductivity of La2Mo1.7W0.3O9 oxygen ion conductor

Partial substitutions of Ho at the La-site of La2Mo1.7W0.3O9−δ were carried out. Compound La2−xHoxMo1.7W0.3O9−δ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.5) has been synthesized by solid state reaction technique. The specimens were characterized by XRD, SEM, DSC for crystal structure, surface morphology, phase transition and ac impedance spectroscopy for conductivity and other electrical parameter determination. Partial substitution of Ho at the La-site of La2Mo1.7W0.3O9−δ, increases the conductivity within the substitutional range, x ≤ 0.2. The phase transition of La2Mo2O9 is suppressed in doped compound and a transition from Arrhenius to VTF behavior of temperature dependence of conductivity is observed around 500 °C. The conductivity is found to be high in the intermediate temperature region and at high temperature the conductivity of La2−xHoxMo1.7W0.3O9−δ (0.05 ≤ x ≤ 0.2) is almost similar with that of La2Mo2O9. The decrease in energy barrier enhances the thermally assisted process to start at lower temperature.

Enhanced magnetoelectric properties in Bi 0.95Ho 0.05FeO 3 polycrystalline ceramics

Polycrystalline samples of Ho doped BiFeO 3 were prepared by solid state reaction method and effect of partial substitution of Ho on dielectric, magnetic and ferroelectric properties was studied. High temperature dielectric results show two dielectric anomalies both in ɛ and tan δ, out of which, anomaly at higher temperature (∼400 °C) could be ascribed to antiferromagnetic Néel temperature which, is a signature of magnetoelectric coupling. The magnetic moment is greatly improved and the maximum magnetization was found to be 0.736 emu/g. Saturated ferroelectric hysteresis loops were observed for Bi 0.95Ho 0.05FeO 3 with remnant polarization ( P r ) = 1.59 μC/cm 2, maximum polarization ( P max ) = 2.56 μC/cm 2 and coercivity ( E c ) = 5.45 kV/cm. We have conducted comprehensive magnetoelectric and magnetodielectric properties at room temperature. Magnetic field induced ferroelectric hysteresis loop observed in Bi 0.95Ho 0.05FeO 3 is of prime importance.

Effect of Ho Doping on the High-Temperature Thermoelectric Properties of Ca&lt;sub&gt;3&lt;/sub&gt;Co&lt;sub&gt;4&lt;/sub&gt;O&lt;sub&gt;9&lt;/sub&gt;-Based Oxides

Ca3-xHoxCo4O9 (x=0.0, 0.15, 0.3, 0.45) samples were prepared using solid reaction and the effect of Ho doping on their high thermoelectric properties were investigated. The substitution of Ho for Ca resulted in an increase of both thermopower and electrical resistivity which could be attributed to the decrease of hole concentrations. The Ho-doped samples had lower thermal conductivity than Ca3Co4O9 due to their lower electronic and lattice thermal conductivity. The largest ZT values were attained in Ca2.7Ho0.3Co4O9 sample.