Multilayer ceramic substrate

A multilayer ceramic substrate comprising:

• a plurality of base layers that are laminated containing a low-temperature co-fired ceramic material;
• a plurality of first constraint layers which contain a metal oxide that is not completely sintered at a sintering temperature of the low-temperature co-fired ceramic material and which are located between adjacent base layers of the plurality of laminated base layers;
• and
• a protective layer which contains the metal oxide and which is located outermost in a lamination direction of the plurality of laminated base layers and is in contact with an outermost base layer of the plurality of laminated base layers in the lamination direction,
• wherein X1>X2, where X1 is a first content of the metal oxide in a surface section of the protective layer and X2 is a second content of the metal oxide in a boundary section of the protective layer that is in contact with the outermost base layer, and
• wherein α11<α33<α22, where α11 is a first thermal expansion coefficient of the surface section of the protective layer, α22 is a second thermal expansion coefficient of the boundary section of the protective layer, and α33 is a third thermal expansion coefficient of the central section of the protective layer.

A multilayer ceramic substrate comprising:

• a plurality of base layers that are laminated containing a low-temperature co-fired ceramic material;
• a plurality of first constraint layers which contain a metal oxide that is not completely sintered at a sintering temperature of the low-temperature co-fired ceramic material and which are located between adjacent base layers of the plurality of laminated base layers;
• and
• a protective layer which contains the metal oxide and which is located outermost in a lamination direction of the plurality of laminated base layers and is in contact with an outermost base layer of the plurality of laminated base layers in the lamination direction,
• wherein X1>X2, where X1 is a first content of the metal oxide in a surface section of the protective layer and X2 is a second content of the metal oxide in a boundary section of the protective layer that is in contact with the outermost base layer, and
• wherein X2>Y, where Y is a third content of the metal oxide in the base layer.