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Zeszyty czasopisma

Tom 120 (2023): Zeszyt 1 (January 2023)

Tom 119 (2022): Zeszyt 1 (January 2022)

Tom 118 (2021): Zeszyt 1 (January 2021)

Tom 117 (2020): Zeszyt 1 (January 2020)

Tom 116 (2019): Zeszyt 12 (December 2019)

Tom 116 (2019): Zeszyt 11 (November 2019)

Tom 116 (2019): Zeszyt 10 (October 2019)

Tom 116 (2019): Zeszyt 9 (September 2019)

Tom 116 (2019): Zeszyt 8 (August 2019)

Tom 116 (2019): Zeszyt 7 (July 2019)

Tom 116 (2019): Zeszyt 6 (June 2019)

Tom 116 (2019): Zeszyt 5 (May 2019)

Tom 116 (2019): Zeszyt 4 (April 2019)

Tom 116 (2019): Zeszyt 3 (March 2019)

Tom 116 (2019): Zeszyt 2 (February 2019)

Tom 116 (2019): Zeszyt 1 (January 2019)

Tom 115 (2018): Zeszyt 12 (December 2018)

Tom 115 (2018): Zeszyt 11 (November 2018)

Tom 115 (2018): Zeszyt 10 (October 2018)

Tom 115 (2018): Zeszyt 9 (September 2018)

Tom 115 (2018): Zeszyt 8 (August 2018)

Tom 115 (2018): Zeszyt 7 (July 2018)

Tom 115 (2018): Zeszyt 6 (June 2018)

Tom 115 (2018): Zeszyt 5 (May 2018)

Tom 115 (2018): Zeszyt 4 (April 2018)

Tom 115 (2018): Zeszyt 3 (March 2018)

Tom 115 (2018): Zeszyt 2 (February 2018)

Tom 115 (2018): Zeszyt 1 (January 2018)

Tom 114 (2017): Zeszyt 11 (November 2017)

Tom 114 (2017): Zeszyt 10 (October 2017)

Tom 114 (2017): Zeszyt 9 (September 2017)

Tom 114 (2017): Zeszyt 8 (August 2017)

Tom 114 (2017): Zeszyt 7 (July 2017)

Tom 114 (2017): Zeszyt 6 (June 2017)

Tom 114 (2017): Zeszyt 5 (May 2017)

Tom 114 (2017): Zeszyt 4 (April 2017)

Tom 114 (2017): Zeszyt 3 (March 2017)

Tom 114 (2017): Zeszyt 2 (February 2017)

Tom 114 (2017): Zeszyt 1 (January 2017)

Informacje o czasopiśmie
Format
Czasopismo
eISSN
2353-737X
Pierwsze wydanie
20 May 2020
Częstotliwość wydawania
1 raz w roku
Języki
Angielski

Wyszukiwanie

Tom 114 (2017): Zeszyt 1 (January 2017)

Informacje o czasopiśmie
Format
Czasopismo
eISSN
2353-737X
Pierwsze wydanie
20 May 2020
Częstotliwość wydawania
1 raz w roku
Języki
Angielski

Wyszukiwanie

15 Artykułów
Otwarty dostęp

Evolution of the polychromy in L’viv architecture in the period of the second half of the 17th century to the first half of the 19th century

Data publikacji: 23 May 2020
Zakres stron: 5 - 23

Abstrakt

Abstract

The article deals with the question of the evolution of polychromy in the L’viv architecture in the period of the second half of the 17th century to the first half of the 19th century. On the base of field and historical research, the author considers polychrome techniques, the colour palette, and the main principles and peculiarities of polychromy use on the façades and interiors of L’viv buildings of the Baroque and Classicism periods.

Słowa kluczowe

  • evolution of polychromy in architecture
  • façades and interiors of the L’viv buildings of the Baroque and Classicism periods
  • polychromy techniques and the principles of their use
Otwarty dostęp

Anamorphic images on the historical background along with their classification and some selected examples

Data publikacji: 23 May 2020
Zakres stron: 25 - 40

Abstrakt

Abstract

Art based on optical illusions has accompanied the everyday life of a human being since ancient times, until today. Primarily, it played a specific role as an artistic game and manifested the artists’ own mastery, while often playing the serviceable role of an artistic advertisement. This study presents a detailed definition of anamorphic images together with their precise classification, and provides a description of the methods used for their construction. The problems discussed here have been presented on a historical background. The examples of particularly chosen anamorphic images have been presented together with their visualised images. The theoretical background, how one can create such anamorphic images, provides the basis for further design and development of anamorphic images to be created both in an urban space of a town, and in the interiors of public use..

Słowa kluczowe

  • transformation
  • anamorphic image
  • visualisation of anamorphic images
  • reflective surfaces
Otwarty dostęp

Active ingredients in paint strippers

Data publikacji: 23 May 2020
Zakres stron: 41 - 52

Abstrakt

Abstract

The papers presents the study on the application of acetals as active ingredients in paint strippers. The formulation was prepared and the ability to wrinkling of the shell was rated. The ability to wrinkling of the shell was rated by examining the force required to detachment the coating.

Słowa kluczowe

  • paint stripper
  • active ingredients
  • acetals
Otwarty dostęp

A comparative analysis of two methods for determining the influence of vibrations on people in buildings

Data publikacji: 23 May 2020
Zakres stron: 53 - 64

Abstrakt

Abstract

Different methodologies for the assessment of the influence of vibrations on people residing in buildings can be found in international standards. There are two dominant methods of assessment: the analysis of the RMS values in 1/3 octave bands and vibration dose VDV. In this article, these two methods of assessing the influence of vibration on humans are compared, not only on the basis of standard formulas but also on a real example. The influence of transport vibrations (busses, trams, trucks) on humans residing in traditional, masonry buildings was analysed. Conclusions resulting from a comparative analysis of the two methods of evaluation were formed.

Słowa kluczowe

  • passive perception of vibration
  • vibration dose
  • RMS analysis
  • ‘in-situ’ measurements
  • transport vibrations
Otwarty dostęp

Simple models for determination of the differences of ground and building foundation response spectra in LGC region

Data publikacji: 23 May 2020
Zakres stron: 65 - 77

Abstrakt

Abstract

The paper deals with the transmission of mine-induced vibrations in the LGC region from the ground to the foundations of typical buildings. The simple, easy-to-use models have been proposed for the determination of the differences (relations) between ground and foundation acceleration response spectra. Experimentally obtained acceleration response spectra from vibrations occurring simultaneously on the ground near the building and the building foundation were the basis for the verification of the accuracy of the proposed models.

Słowa kluczowe

  • mining tremors
  • transmission of ground vibrations to building foundation
  • response spectra transmission
Otwarty dostęp

The laser scanning in the digitization process of relictually preserved architectural details

Data publikacji: 23 May 2020
Zakres stron: 79 - 84

Abstrakt

Abstract

Protection of cultural heritage in recent times is of a very important significance. The priority task is to secure the existing facilities in such a way that they can now and in the future serve as a representations of the areas and be a treasure house of knowledge for both current and future generations. Through the development of new technologies and applying them by maintenance staff and those working to protect the national heritage, digitization has become widespread. Photogrammetry and laser scanning proved to be useful tools in a non-invasive process of heritage digitizing. In the article the author describes the process of creating a three-dimensional model of an architectural detail, the model of “acanthus leaf” which is the final product of the study. The methodology described by the author outlines the problems occurring during the measurement and subsequent data processing stages. The final product is an important database, useful when creating the reconstruction and renovation of similar facilities.

Słowa kluczowe

  • cloud points
  • digitizing
  • 3D model
  • relic
  • reconstruction
  • laser scanning
  • virtual cave
Otwarty dostęp

Vibrations of free-field and building caused by passages of the Pendolino train

Data publikacji: 23 May 2020
Zakres stron: 85 - 100

Abstrakt

Abstract

The objective of the paper was to conduct dynamic measurements of free-field vibrations in three profiles and establish decay relations of propagating vibrations. The sources of vibrations were passages of the Pendolino train with dedicated speeds from 40 to 293 km/h. In each profile, soil properties tests were carried out. Based on the data for a few selected speeds of the Pendolino train’s passages, the Vibration Reduction Coefficients (VRC) were determined at selected points. The paper is also focused on the analysis of vibrations of the selected one-storey masonry residential building located in the vicinity of one of the profiles and their evaluation of the harmfulness on the building structure using standard scales of dynamic influence.

Słowa kluczowe

  • free-field vibrations
  • dynamic investigation
  • speed of passage of a train
  • building vibrations
  • harmfulness of vibrations
Otwarty dostęp

Digital filters: hermitian, antihermitian, unitary and their applications

Data publikacji: 23 May 2020
Zakres stron: 101 - 112

Abstrakt

Abstract

The paper presents the relation between the three types of noncausal digital filters: unitary, Hermitian and antihermitian filters. A decomposition has been made of a causal filter into a Hermitian and unitary filter cascade, and the use of this decomposition to calculate the electric power quality of receivers has been highlighted. Reference has also been made to the analogy between the set of unitary filters and the unit circle in the Gaussian plane.

Słowa kluczowe

  • unitary and Hermitian digital filters
  • operators
  • convolution
Otwarty dostęp

Water sensitive design potentials in Paris, Berlin, and Budapest revisited

Data publikacji: 23 May 2020
Zakres stron: 113 - 123

Abstrakt

Abstract

Impacts of extreme weather events are increasing due to ongoing climate change. Floods in the urban areas not just damage properties but also threat human life. Coping with these challenges in metropolises of old Europe is difficult as the ex-istent building structures and combined rainwater and sewage systems originating from the 19th century. We compare three old European capital cities (Berlin, Budapest and Paris) regarding rainwater management concepts. We identify important spatial land use types of blue-green infrastructure and corresponding measures for the implementation towards a more water sensitive management by using the case study approach.

Słowa kluczowe

  • urban rainwater management
  • sustainable planning
  • european metropolises
Otwarty dostęp

Investigations into wastewater composition focusing on nonwoven wet wipes

Data publikacji: 23 May 2020
Zakres stron: 125 - 135

Abstrakt

Abstract

This paper summarizes the problems caused for wastewater systems by changing wastewater composition, especially the increasing use of nonwoven wet wipes and their disposal via the toilet. The nonwoven wet wipes do not disintegrate in the sewer system as paper-based toilet does, but accumulate to large agglomerations that can block the system and clog pumps. This not only reduces the operational safety and stability of the wastewater system, but also costs the operator a lot of money. The paper introduces an approach to estimate the composition of wastewater and presents first results for the city of Berlin.

Słowa kluczowe

  • wastewater composition
  • solids
  • wet wipes
  • clogging
Otwarty dostęp

Microplastics in urban water management

Data publikacji: 23 May 2020
Zakres stron: 137 - 146

Abstrakt

Abstract

The paper is discussing microplastics in the effluent of wastewater treatment plants and other discharges from urban areas. The aim of the discussion is to expose the challenges related to sampling and detecting plastic particles in treated wastewater. The different types of microplastics and a rough estimation of the amount of plastic which could end up in the aquatic environment is given. The work is based on a literature review of microplastic particles in treated wastewater and discusses their consequences on the aquatic ecosystem.

Słowa kluczowe

  • microplastic
  • particle
  • sample
  • analytic
Otwarty dostęp

An abstract nonlocal functional-differential second order evolution problem

Data publikacji: 23 May 2020
Zakres stron: 147 - 154

Abstrakt

Abstract

The aim of the paper is to prove two theorems on the existence and uniqueness of mild and classical solutions of a semilinear functional-differential evolution second order equation together with nonlocal conditions. The theory of strongly continuous cosine families of linear operators in a Banach space is applied. The paper is based on publications [1–9] and is a generalization of paper [6].

Słowa kluczowe

  • nonlocal
  • second order
  • functional-differential
  • evolution problem
  • Banach space
Otwarty dostęp

On the uniqueness of solutions to parabolic semilinear problems under nonlocal conditions with integrals

Data publikacji: 23 May 2020
Zakres stron: 155 - 163

Abstrakt

Abstract

The uniqueness of classical solutions to parabolic semilinear problems together with nonlocal initial conditions with integrals, for the operator i,j=1nxi(aij(x,t)xj)+c(x,t)-t,\sum\limits_{i,j = 1}^n {{\partial \over {\partial {x_i}}}\left( {{a_{ij}}\left( {x,t} \right){\partial \over {\partial {x_j}}}} \right)} + c\left( {x,t} \right) - {\partial \over {\partial t}},, x=(x1,...,xn), in the cylindrical domain D:D0 ×(t0, t0+T) ⊂ ℜn+1, where t0 ∈ ℜ, 0 < T < ∞, are studied. The result requires that the nonlocal conditions with integrals be introduced.

Słowa kluczowe

  • parabolic problems
  • semilinear equation
  • nonlocal initial condition with integral
  • cylindrical domain
  • uniqueness of solutions
Otwarty dostęp

CuO/TiO2 and Cu2O/TiO2 nanoparticles influence on parameters of bulk heterojunction organic solar cells

Data publikacji: 23 May 2020
Zakres stron: 165 - 171

Abstrakt

Abstract

This publication presents a study conducted for related pyrazoloquinoline doped nanoparticles CuO/TiO2 and Cu2O/TiO2. The mixtures were used as the active material for the construction of photovoltaic cells.

Słowa kluczowe

  • photovoltaic cells
  • pyrazoloquinoline
  • efficiency
  • light absorbing materials
Otwarty dostęp

AC low frequency conductivity in PZT PFS ferroelectric ceramics

Data publikacji: 23 May 2020
Zakres stron: 173 - 185

Abstrakt

Abstract

Low frequency AC conductivity has been studied in ferroelectric ceramics PZT + PFS (Pb[(Fe1/3Sb2/3)xTiyZrz]O3 with x = 0.1 and y = 0.43, 0.44, 0.47) using Fourier transformation of charging and discharging currents. The results are interpreted in terms of fractal structure of the randomly generated clusters formed by sequentially correlated hopping paths of charge carriers.

Słowa kluczowe

  • PZT
  • AC conductivity
  • fractal structure conduction clusters
15 Artykułów
Otwarty dostęp

Evolution of the polychromy in L’viv architecture in the period of the second half of the 17th century to the first half of the 19th century

Data publikacji: 23 May 2020
Zakres stron: 5 - 23

Abstrakt

Abstract

The article deals with the question of the evolution of polychromy in the L’viv architecture in the period of the second half of the 17th century to the first half of the 19th century. On the base of field and historical research, the author considers polychrome techniques, the colour palette, and the main principles and peculiarities of polychromy use on the façades and interiors of L’viv buildings of the Baroque and Classicism periods.

Słowa kluczowe

  • evolution of polychromy in architecture
  • façades and interiors of the L’viv buildings of the Baroque and Classicism periods
  • polychromy techniques and the principles of their use
Otwarty dostęp

Anamorphic images on the historical background along with their classification and some selected examples

Data publikacji: 23 May 2020
Zakres stron: 25 - 40

Abstrakt

Abstract

Art based on optical illusions has accompanied the everyday life of a human being since ancient times, until today. Primarily, it played a specific role as an artistic game and manifested the artists’ own mastery, while often playing the serviceable role of an artistic advertisement. This study presents a detailed definition of anamorphic images together with their precise classification, and provides a description of the methods used for their construction. The problems discussed here have been presented on a historical background. The examples of particularly chosen anamorphic images have been presented together with their visualised images. The theoretical background, how one can create such anamorphic images, provides the basis for further design and development of anamorphic images to be created both in an urban space of a town, and in the interiors of public use..

Słowa kluczowe

  • transformation
  • anamorphic image
  • visualisation of anamorphic images
  • reflective surfaces
Otwarty dostęp

Active ingredients in paint strippers

Data publikacji: 23 May 2020
Zakres stron: 41 - 52

Abstrakt

Abstract

The papers presents the study on the application of acetals as active ingredients in paint strippers. The formulation was prepared and the ability to wrinkling of the shell was rated. The ability to wrinkling of the shell was rated by examining the force required to detachment the coating.

Słowa kluczowe

  • paint stripper
  • active ingredients
  • acetals
Otwarty dostęp

A comparative analysis of two methods for determining the influence of vibrations on people in buildings

Data publikacji: 23 May 2020
Zakres stron: 53 - 64

Abstrakt

Abstract

Different methodologies for the assessment of the influence of vibrations on people residing in buildings can be found in international standards. There are two dominant methods of assessment: the analysis of the RMS values in 1/3 octave bands and vibration dose VDV. In this article, these two methods of assessing the influence of vibration on humans are compared, not only on the basis of standard formulas but also on a real example. The influence of transport vibrations (busses, trams, trucks) on humans residing in traditional, masonry buildings was analysed. Conclusions resulting from a comparative analysis of the two methods of evaluation were formed.

Słowa kluczowe

  • passive perception of vibration
  • vibration dose
  • RMS analysis
  • ‘in-situ’ measurements
  • transport vibrations
Otwarty dostęp

Simple models for determination of the differences of ground and building foundation response spectra in LGC region

Data publikacji: 23 May 2020
Zakres stron: 65 - 77

Abstrakt

Abstract

The paper deals with the transmission of mine-induced vibrations in the LGC region from the ground to the foundations of typical buildings. The simple, easy-to-use models have been proposed for the determination of the differences (relations) between ground and foundation acceleration response spectra. Experimentally obtained acceleration response spectra from vibrations occurring simultaneously on the ground near the building and the building foundation were the basis for the verification of the accuracy of the proposed models.

Słowa kluczowe

  • mining tremors
  • transmission of ground vibrations to building foundation
  • response spectra transmission
Otwarty dostęp

The laser scanning in the digitization process of relictually preserved architectural details

Data publikacji: 23 May 2020
Zakres stron: 79 - 84

Abstrakt

Abstract

Protection of cultural heritage in recent times is of a very important significance. The priority task is to secure the existing facilities in such a way that they can now and in the future serve as a representations of the areas and be a treasure house of knowledge for both current and future generations. Through the development of new technologies and applying them by maintenance staff and those working to protect the national heritage, digitization has become widespread. Photogrammetry and laser scanning proved to be useful tools in a non-invasive process of heritage digitizing. In the article the author describes the process of creating a three-dimensional model of an architectural detail, the model of “acanthus leaf” which is the final product of the study. The methodology described by the author outlines the problems occurring during the measurement and subsequent data processing stages. The final product is an important database, useful when creating the reconstruction and renovation of similar facilities.

Słowa kluczowe

  • cloud points
  • digitizing
  • 3D model
  • relic
  • reconstruction
  • laser scanning
  • virtual cave
Otwarty dostęp

Vibrations of free-field and building caused by passages of the Pendolino train

Data publikacji: 23 May 2020
Zakres stron: 85 - 100

Abstrakt

Abstract

The objective of the paper was to conduct dynamic measurements of free-field vibrations in three profiles and establish decay relations of propagating vibrations. The sources of vibrations were passages of the Pendolino train with dedicated speeds from 40 to 293 km/h. In each profile, soil properties tests were carried out. Based on the data for a few selected speeds of the Pendolino train’s passages, the Vibration Reduction Coefficients (VRC) were determined at selected points. The paper is also focused on the analysis of vibrations of the selected one-storey masonry residential building located in the vicinity of one of the profiles and their evaluation of the harmfulness on the building structure using standard scales of dynamic influence.

Słowa kluczowe

  • free-field vibrations
  • dynamic investigation
  • speed of passage of a train
  • building vibrations
  • harmfulness of vibrations
Otwarty dostęp

Digital filters: hermitian, antihermitian, unitary and their applications

Data publikacji: 23 May 2020
Zakres stron: 101 - 112

Abstrakt

Abstract

The paper presents the relation between the three types of noncausal digital filters: unitary, Hermitian and antihermitian filters. A decomposition has been made of a causal filter into a Hermitian and unitary filter cascade, and the use of this decomposition to calculate the electric power quality of receivers has been highlighted. Reference has also been made to the analogy between the set of unitary filters and the unit circle in the Gaussian plane.

Słowa kluczowe

  • unitary and Hermitian digital filters
  • operators
  • convolution
Otwarty dostęp

Water sensitive design potentials in Paris, Berlin, and Budapest revisited

Data publikacji: 23 May 2020
Zakres stron: 113 - 123

Abstrakt

Abstract

Impacts of extreme weather events are increasing due to ongoing climate change. Floods in the urban areas not just damage properties but also threat human life. Coping with these challenges in metropolises of old Europe is difficult as the ex-istent building structures and combined rainwater and sewage systems originating from the 19th century. We compare three old European capital cities (Berlin, Budapest and Paris) regarding rainwater management concepts. We identify important spatial land use types of blue-green infrastructure and corresponding measures for the implementation towards a more water sensitive management by using the case study approach.

Słowa kluczowe

  • urban rainwater management
  • sustainable planning
  • european metropolises
Otwarty dostęp

Investigations into wastewater composition focusing on nonwoven wet wipes

Data publikacji: 23 May 2020
Zakres stron: 125 - 135

Abstrakt

Abstract

This paper summarizes the problems caused for wastewater systems by changing wastewater composition, especially the increasing use of nonwoven wet wipes and their disposal via the toilet. The nonwoven wet wipes do not disintegrate in the sewer system as paper-based toilet does, but accumulate to large agglomerations that can block the system and clog pumps. This not only reduces the operational safety and stability of the wastewater system, but also costs the operator a lot of money. The paper introduces an approach to estimate the composition of wastewater and presents first results for the city of Berlin.

Słowa kluczowe

  • wastewater composition
  • solids
  • wet wipes
  • clogging
Otwarty dostęp

Microplastics in urban water management

Data publikacji: 23 May 2020
Zakres stron: 137 - 146

Abstrakt

Abstract

The paper is discussing microplastics in the effluent of wastewater treatment plants and other discharges from urban areas. The aim of the discussion is to expose the challenges related to sampling and detecting plastic particles in treated wastewater. The different types of microplastics and a rough estimation of the amount of plastic which could end up in the aquatic environment is given. The work is based on a literature review of microplastic particles in treated wastewater and discusses their consequences on the aquatic ecosystem.

Słowa kluczowe

  • microplastic
  • particle
  • sample
  • analytic
Otwarty dostęp

An abstract nonlocal functional-differential second order evolution problem

Data publikacji: 23 May 2020
Zakres stron: 147 - 154

Abstrakt

Abstract

The aim of the paper is to prove two theorems on the existence and uniqueness of mild and classical solutions of a semilinear functional-differential evolution second order equation together with nonlocal conditions. The theory of strongly continuous cosine families of linear operators in a Banach space is applied. The paper is based on publications [1–9] and is a generalization of paper [6].

Słowa kluczowe

  • nonlocal
  • second order
  • functional-differential
  • evolution problem
  • Banach space
Otwarty dostęp

On the uniqueness of solutions to parabolic semilinear problems under nonlocal conditions with integrals

Data publikacji: 23 May 2020
Zakres stron: 155 - 163

Abstrakt

Abstract

The uniqueness of classical solutions to parabolic semilinear problems together with nonlocal initial conditions with integrals, for the operator i,j=1nxi(aij(x,t)xj)+c(x,t)-t,\sum\limits_{i,j = 1}^n {{\partial \over {\partial {x_i}}}\left( {{a_{ij}}\left( {x,t} \right){\partial \over {\partial {x_j}}}} \right)} + c\left( {x,t} \right) - {\partial \over {\partial t}},, x=(x1,...,xn), in the cylindrical domain D:D0 ×(t0, t0+T) ⊂ ℜn+1, where t0 ∈ ℜ, 0 < T < ∞, are studied. The result requires that the nonlocal conditions with integrals be introduced.

Słowa kluczowe

  • parabolic problems
  • semilinear equation
  • nonlocal initial condition with integral
  • cylindrical domain
  • uniqueness of solutions
Otwarty dostęp

CuO/TiO2 and Cu2O/TiO2 nanoparticles influence on parameters of bulk heterojunction organic solar cells

Data publikacji: 23 May 2020
Zakres stron: 165 - 171

Abstrakt

Abstract

This publication presents a study conducted for related pyrazoloquinoline doped nanoparticles CuO/TiO2 and Cu2O/TiO2. The mixtures were used as the active material for the construction of photovoltaic cells.

Słowa kluczowe

  • photovoltaic cells
  • pyrazoloquinoline
  • efficiency
  • light absorbing materials
Otwarty dostęp

AC low frequency conductivity in PZT PFS ferroelectric ceramics

Data publikacji: 23 May 2020
Zakres stron: 173 - 185

Abstrakt

Abstract

Low frequency AC conductivity has been studied in ferroelectric ceramics PZT + PFS (Pb[(Fe1/3Sb2/3)xTiyZrz]O3 with x = 0.1 and y = 0.43, 0.44, 0.47) using Fourier transformation of charging and discharging currents. The results are interpreted in terms of fractal structure of the randomly generated clusters formed by sequentially correlated hopping paths of charge carriers.

Słowa kluczowe

  • PZT
  • AC conductivity
  • fractal structure conduction clusters